BIOLOGY NOTES

BIOLOGY NOTES




FOR CIVIL SERVICER(PRELIM)-2008





















RAHUL DWIVEDI
2084,ACES,TYPE II
IIT KANPUR
MOB-9452529255



BIOLOGY NOTES
(Rahul Dwivedi)

Science (from Latin scientia - knowledge) refers to a system of acquiring knowledge – based on empiricism, experimentation, and methodological naturalism – aimed at finding out the truth. The basic unit of knowledge is the theory, which is a hypothesis that is predictive. The term science also refers to the organized body of knowledge humans have gained by such research.
Most scientists maintain that scientific investigation must adhere to the scientific method, a process for evaluating empirical knowledge under the working assumption of methodological materialism, which explains observable events in nature as a result of natural causes, rejecting supernatural notions. Less formally, the word science often describes any systematic field of study or the knowledge gained from it. Particular specialized studies that make use of empirical methods are often referred to as sciences as well. This article concentrates on the more specific definition.
Science as defined above is sometimes termed pure science to differentiate it from applied science, the application of research to human needs.

1.1 How to Define Life
A. Living Things Are Organized
1. Organization of living systems begins with atoms, which make up basic building blocks called elements.
2. The cell is the basic structural and functional unit of all living things.
3. Different cells combine to make up tissues (e.g., myocardial tissue).
4. Tissues combine to make up an organ (e.g., the heart).
5. Specific organs work together as a system (e.g., the heart, arteries, veins, etc.).
6. Multicellular organisms (each an "individual" within a particular species) contain organ systems (e.g., cardiovascular, digestive, respiratory, etc.).
7. A species in a particular area (e.g., gray squirrels in a forest) constitutes a population.
8. Interacting populations in a particular area comprise a community.
9. A community plus its physical environment is an ecosystem.
10. The biosphere is comprised of regions of the Earth's crust, waters, and atmosphere inhabited by organisms.
11. Each level of organization is more complex than the level preceding it.
12. Each level of organization has emergent properties due to interactions between the parts making up the whole; all emergent properties follow the laws of physics and chemistry.
B. Living Things Acquire Materials and Energy
1. Maintaining organization and conducting life-sustaining processes require an outside source of energy, defined as the capacity to do "work."
2. The ultimate source of energy for nearly all life on earth is the sun; plants and certain other organisms convert solar energy into chemical energy by the process of photosynthesis.
3. Food provides nutrient molecules used as building blocks for energy.
4. Metabolism is all the chemical reactions that occur in a cell.
5. All organisms must maintain a state of biological balance, or homeostasis. Temperature, moisture level, pH, etc. must be maintained within the tolerance range of the organism. Organisms have intricate feedback and control mechanisms to maintain homeostatic balance.
C. Living Things Respond
1. Living things interact with the environment and with other living things.
2. Response often results in movement of the organism (e.g., a plant bending toward the sun to capture solar energy, a turtle withdrawing into its shell for safety, etc.).
3. Responses help ensure survival of the organism and allow the organism to carry out its biological activities.
4. The collective responses of an organism constitute the behavior of the organism.
D. Living Things Reproduce and Develop
1. Reproduction is the ability of every type of organism to give rise to another organism like itself.
2. Bacteria, protozoans, and other unicellular organisms simply split in two (binary fission).
3. Multicellular organisms often unite sperm and egg, each from a different individual, resulting in an immature individual which develops into the adult.
4. The instructions for an organism's organization and development are encoded in genes.
5. Genes are comprised of long molecules of DNA (deoxyribonucleic acid); DNA is the genetic code in all living things.
E. Living Things Have Adaptations
1. Adaptations are modifications that make organisms suited to their way of life.
2. Natural selection is the process by which species become modified over time.
a. A species is a group of interbreeding individuals.
b. In natural selection, members of a species may inherit a genetic change that makes them better suited to a particular environment.
c. These members would be more likely to produce higher numbers of surviving offspring.
3. Evolution is defined as "descent with modification over time."
a. The fact that all life forms are composed of cells, contain genes comprised of DNA, and conduct the same metabolic reactions suggests all living things have a common ancestor.
b. One species can give rise to several species, each adapted to to a particular set of environmental conditions.
c. Evolution is responsible for the great diversity of life on Earth.
1.2 How the Biosphere is Organized
A. Levels of Complexity
1. The biosphere is the zone of air, land, and water where organisms exist.
2. A population consists of all members of one species in a particular area.
3. A community consists of all of the local interacting populations.
4. An ecosystem includes all aspects of a living community and the physical environment (soil, atmosphere, etc.).
5. Interactions between various food chains make up a food web.
6. Ecosystems are characterized by chemical cycling and energy flow.
7. Ecosystems stay in existence because of a constant input of solar energy and the ability of photosynthetic organisms to absorb it.
B. The Human Population
1. The human population modifies existing ecosystems for its own purposes.
2. Two biologically diverse ecosystems, rain forests and coral reefs, are severely threatened by the human population.
3. Human beings depend on healthy working ecosystems for food, medicines, and raw materials.
C. Biodiversity
1. Biodiversity is the total number of species, their variable genes, and their ecosystems.
2. Extinction is the death of a species or larger group; perhaps 400 species become extinct every day.
3. The continued existence of the human species is dependant on the preservation of ecosystems and the biosphere.
1.3 How Living Things Are Classified
A. Taxonomy is the discipline of identifying and classifying organisms according to certain rules.
1. Taxonomic classification changes as more is learned about living things, including the evolutionary relationships between species.
B. Categories of Classification
1. From smaller (least inclusive) categories to larger (more inclusive), the sequence of classification categories is: species, genus, family, order, class, phylum, kingdom, domain.
2. The species within one genus share many specific characteristics and are the most closely related.
3. Species in the same kingdom share only general characteristics with one another.
C. Domains
1. Biochemical evidence suggests that there are three domains: Bacteria, Archaea, and Eukarya.
2. The domains Bacteria and Archaea contain unicellular prokaryotes; organisms in the domain Eukarya have a membrane-bound nucleus.
3. The prokaryotes are structurally simple but are metabolically complex.
4. Archaea can live in water devoid of oxygen, and are able to survive harsh environmental conditions (temperatures, salinity, pH).
5. Bacteria are variously adapted to living almost anywhere (water, soil, atmosphere, in/on the human body, etc.).
D. Kingdoms
1. The domains Archaea and Bacteria are not yet categorized into kingdoms.
2. Eukarya contains four kingdoms: Protista, Fungi, Plantae, and Animalia.
3. Protists (kingdom Protista) range from unicellular forms to multicellular ones.
4. Fungi (kingdom Fungi) are the molds and mushrooms.
5. Plants (kingdom Plantae) are multicellular photosynthetic organisms.
6. Animals (kingdom Animalia) are multicellular organisms that ingest and process their food.
E. Scientific Name
1. A binomial name is a two-part scientific name: the genus (first word, capitalized) and the specific epithet of a species (second word, not capitalized).
2. Binomial names are based on Latin and are used universally by biologists.
3. Either the genus name or the specific epithet name may be abbreviated.
1.4 The Process of Science
A. Scientific Method
1. Biology is the scientific study of life, and it consists of many disciplines.
2. The scientific process differs from other ways of learning in that science follows the scientific method, which is characterized by observation, development of a hypothesis, experimentation and data collection, and forming a conclusion.
B. Observation
1. Scientists believe nature is orderly and measurable, and that natural laws (e.g., gravity) do not change with time.
2. Natural events, called, phenomena can therefore be understood from observations.
3. Scientists also use the knowledge and experiences of other scientists to expand their understanding of phenomena.
4. Chance alone can sometimes help a scientist get an idea (e.g., Alexander Fleming's discovery of penicillin).
C. Hypothesis
1. Inductive reasoning allows a person to combine isolated facts into a cohesive whole.
2. A scientist uses inductive reasoning to develop a possible explanation (a hypothesis) for a natural event; the scientist presents the hypothesis as an actual statement.
3. Scientists only consider hypotheses that can be tested (i.e., moral and religious beliefs may not be testable by the scientific method).
D. Experiments/Further Observations
1. Testing a hypothesis involves either conducting an experiment or making further observations.
2. Deductive reasoning involves "if, then" logic to make a prediction that the hypothesis can be supported by experimentation.
3. An experimental design is proposed to test the hypothesis in a meaningful way.
4. An experiment should include a control group which goes through all the steps of an experiment but lacks (or is not exposed to) the factor being tested.
5. Scientists may use a model (a representation of an actual object) in their experiments.
6. Results obtained from use of a model will remain a hypothesis in need of testing if it is impossible to test the actual phenomenon.
E. Data
1. Data are the results of an experiment, and are observable and objective rather than subjective.
2. Data are often displayed in a graph or table.
3. Many studies rely on statistical data which, among other things, determines the probability of error in the experiment.
F. Conclusion
1. Whether the data support or reject the hypothesis is the basis for the conclusion.
2. The conclusion of one experiment can lead to the hypothesis for another experiment.
3. Scientists report their findings in scientific journals so that their methodology and data are available to other scientists.
4. The experiments and observations must be repeatable or the research is suspect.
G. Scientific Theory
1. The ultimate goal is to understand the natural world in scientific theories, which are speculative ideas that join supported, related hypotheses, and are supported by a broad range of observations, experiments, and data.
2. Some basic theories of biology are:
a. Cell: all organisms are made of cells.
b. Homeostasis: the internal environment of an organism stays relatively constant.
c. Gene: organisms contain coded information that dictates their form, function, and behavior.
d. Ecosystem: organisms are members of populations which interact with each other and the physical environment.
e. Evolution: all living things have a common ancestor.
3. A principle or a law is a theory that is generally accepted by most scientists.
H. A Controlled Study
1. A controlled study ensures that the outcome is due to the experimental (independent) variable, the factor being tested.
2. The result is called the responding (dependent) variable because it is due to the dependent variable.
3. The Experiment
a. Hypothesis: pigeon pea/winter wheat rotation will increase winter wheat production as well as or better than the use of nitrogen fertilizer.
b. Prediction: wheat biomass following the growth of pigeon peas in the soil will surpass wheat biomass following nitrogen fertilizer treatment.
c. Control group: winter wheat that receives no fertilizer.
d. Test groups: winter wheat treated with different levels of fertilizer; winter wheat grown in soil into which pigeon pea plants had been tilled.
e. Environmental conditions and watering were identical in control and test groups.
f. Results: all test groups produced more biomass than control group, but high level of nitrogen fertilizer produced more biomass than pigeon pea test group. Thus, hypothesis is not supported.
4. Continuing the Experiment
a. To test the hypothesis that pigeon pea residues will build up over time and will increase winter wheat production compared to nitrogen fertilizer, the study is continued for another year.
b. The fertilizer-only treatment no longer exceeded biomass production with the use of pigeon peas; biomass in the pigeon pea-treated test group was highest.
c. Conclusion: at the end of two years, the yield of winter wheat is better in the pigeon pea-treated test group. Hypothesis supported.
d. Continuation of the study for another year showed that the soil was continuously improved by the pigeon peas compared to the nitrogen fertilizer test groups.
e. Results were reported in a scientific journal.
I. A Field Study
1. Hypothesis: aggression of the male mountain bluebird varies during the reproductive cycle.
2. Prediction: aggression will change after the nest is built, after the first egg is laid, and after hatching.
3. To test the hypothesis, a male bluebird model was placed near the nest while the male was gone and observations were made upon his return.
4. Control: a model of a male robin placed near certain nests.
5. Results: resident male bluebirds did not bother the control model but were aggressive toward the male bluebird model depending on the stage in the reproductive cycle.
6. Conclusion: hypothesis is supported.
7. Study was reported in scientific journal with evolutionary interpretation.
2.1 Chemical Elements
1. Matter is defined as anything that takes up space and has mass.
2. Matter exists in three states: solid, liquid, and gas.
3. All matter (both living and non-living) is composed of 92 naturally-occurring elements.
4. Elements, by definition, cannot be broken down to simpler substances with different chemical or physical properties.
5. Six elements (carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur—acronym CHNOPS) make up 98% of the body weight of organisms.
B. Atomic Structure
1. Elements consist of tiny particles called atoms.
2. An atom is the smallest unit of an element that displays the properties of the element.
3. One or two letters (e.g., H, Na) create the atomic symbol of the element.
4. The atomic mass of an atom depends on the presence of certain subatomic particles.
a. Atoms contain specific numbers of protons, neutrons, and electrons.
b. Protons and neutrons are in the nucleus of an atom; electrons move around the nucleus.
c. Protons are positively charged particles; neutrons have no charge; both have 1 atomic mass unit (amu) of weight.
d. Electrons are negatively charged particles located in orbitals outside the nucleus.
5. All atoms of an element have the same number of protons, called the atomic number of the element.
C. The Periodic Table
1. The periodic table shows how various characteristics of atoms of elements recur.
2. Groups are the vertical columns in the table, periods are the horizontal rows; atomic mass increases as you move down a group or across a period.
3. The atomic number is above the atomic symbol and the atomic mass is below the atomic symbol.
D. Isotopes
1. Isotopes are atoms of the same element that differ in the number of neutrons (and therefore have different atomic masses). For example, carbon-12 has 6 protons and 6 neutrons, carbon-14 has 6 protons and 8 neutrons.
2. A carbon atom with 8 rather than 6 neutrons is unstable; it releases energy and subatomic particles and is thus a radioactive isotope.
3. Because the chemical behavior of a radioactive isotope is the same as a stable isotope of a particular element, low levels of the radioactive isotope (e.g., radioactive iodine or glucose) allow researchers to trace the location and activity of the element in living tissues; these isotopes are called tracers.
4. High levels of radiation can destroy cells and cause cancer; careful use of radiation can sterilize products and kill cancer cells.
E. Electrons and Energy
1. Electrons occupy orbitals within various energy levels (or electron shells) near or distant from the nucleus of the atom. The farther the orbital from the nucleus, the higher the energy level.
2. An orbital is a volume of space where an electron is most likely to be found; an orbital can contain no more than 2 electrons.
3. When atoms absorb energy during photosynthesis, electrons are boosted to higher energy levels. When the electrons return to their original energy level, the released energy is converted into chemical energy. This chemical energy supports all life on Earth.
4. The innermost shell of an atom is complete with 2 electrons; all other shells are complete with 8 electrons. This is called the octet rule.
5. Atoms will give up, accept, or share electrons in order to have 8 electrons in a electron shell.
2.2 Elements and Compounds
1. When atoms of two or more different elements bond together, they form a compound (e.g., H2O).
2. A molecule is the smallest part of a compound that has the properties of the compound.
3. Electrons possess energy, and bonds that exist between atoms in molecules therefore contain energy.
B. Ionic Bonding
1. An ionic bond forms when electrons are transferred from one atom to another atom.
2. By losing or gaining electrons, atoms fill outer shells, and are more stable (the octet rule).
3. Example: sodium loses an electron and therefore has a positive charge; chlorine gains an electron to give it a negative charge. Such charged particles are called ions.
4. Attraction of oppositely charged ions holds the two atoms together in an ionic bond.
5. A salt (e.g., NaCl) is an example of an ionically-bonded compound.
C. Covalent Bonding
1. Covalent bonds result when two atoms share electrons so each atom has an octet of electrons in the outer shell (or, in the case of hydrogen, 2 electrons).
2. Hydrogen can give up an electron to become a hydrogen ion (H+) or share an electron with another atom to complete its shell with 2 electrons.
3. The structural formula of a compound indicates a shared pair of electrons by a line between the two atoms; e.g., single covalent bond (H–H), double covalent bond (O=O), and triple covalent bond (N = N). Each line between the atoms represents a pair of electrons.
4. The three-dimensional shapes of molecules are not represented by structural formulas, but shape is critical in understanding the biological action of molecules. Different molecules have different three-dimensional shapes, depending on the number of atoms in the molecule and the types of bonds (single , double, or triple covalent).
D. Nonpolar and Polar Covalent Bonds
1. In nonpolar covalent bonds, sharing of electrons is equal, i.e., the electrons are not attracted to either atom to a greater degree.
2. With polar covalent bonds, the sharing of electrons is unequal.
a. In a water molecule (H2O), sharing of electrons by oxygen and hydrogen is not equal; the oxygen atom with more protons attracts the electrons closer to it, and thus dominates the H2O association.
b. Attraction of an atom for electrons in a covalent bond is called the electronegativity of the atom; an oxygen atom is more electronegative than a hydrogen atom.
c. Oxygen in a water molecule, more attracted to the electron pair, assumes a partial negative charge.
E. Hydrogen Bonding
1. A hydrogen bond is a weak attractive force between the slightly positive charge of the hydrogen atom of one molecule and slightly negative charge of another atom (e.g., oxygen, nitrogen) in another or the same molecule.
2. Many hydrogen bonds taken together are relatively strong.
3. Hydrogen bonds between and within complex biological molecules (e.g., DNA, proteins) help maintain their proper structure and function.
2.3. Chemistry of Water
1. All living things are 70–90% water.
2. Because water is a polar molecule, water molecules are hydrogen bonded to one other.
3. Because of hydrogen bonding, water is liquid between 0° C and 100° C which is essential for the existence of life.
B. Properties of Water
1. Water has a high heat capacity
a. The temperature of liquid water rises and falls more slowly than that of most other liquids.
b. A calorie is the amount of heat energy required to raise the temperature of one gram of water 1° C.
c. Because the hydrogen bonds between water molecules hold more heat, water's temperature falls more slowly than other liquids; this protects organisms from rapid temperature changes and helps them maintain homeostatic temperature.
2. Water has a high heat of vaporization.
a. Hydrogen bonds between water molecules require a relatively large amount of heat to break.
b. This property moderates Earth's surface temperature; permits living systems to exist.
c. When animals sweat, evaporation of the sweat removes body heat, thus cooling the animal.
3. Water is a solvent.
a. Water dissolves a great number of substances (e.g., salts, large polar molecules).
b. Ionized or polar molecules attracted to water are hydrophilic ("water loving").
c. Nonionized and nonpolar molecules that cannot attract water are hydrophobic ("water fearing").
d. A solution contains dissolved substances called solutes.
4. Water molecules are cohesive and adhesive.
a. Cohesion allows water to flow freely without molecules separating.
b. Adhesion is ability to adhere to polar surfaces; water molecules have positive and negative poles.
c. Water rises up a tree from roots to leaves through small tubes.
i. Adhesion of water to walls of vessels prevents water column from breaking apart.
ii. Cohesion allows evaporation from leaves to pull water column from roots.
5. Water has a high surface tension.
a. Water is relatively difficult to break through at its surface.
b. This property permits a rock to be skipped across a pond surface, and supports insects walking on surface.
6. Unlike most substances, frozen water is less dense than liquid water.
a. Below 4° C, hydrogen bonding becomes more rigid but more open, causing expansion.
b. Because ice is less dense, it floats; therefore, bodies of water freeze from the top down.
c. If ice was heavier than water, ice would sink and bodies of water would freeze solid.
d. This property allows ice to act as an insulator on bodies of water, thereby protecting aquatic organisms during the winter.
C. Acids and Bases
1. When water ionizes or dissociates, it releases a small (107 moles/liter) but equal number of hydrogen (H+) ions and hydroxide (OH-) ions; H – O –H → H+ + OH-.
2. Acid molecules dissociate in water, releasing hydrogen (H+) ions: HCl → H+ + Cl-.
3. Bases are molecules that take up hydrogen ions or release hydroxide ions. NaOH → Na+ + OH-.
4. The pH scale indicates acidity and basicity (alkalinity) of a solution.
a. pH is the measurement of free hydrogen ions, expressed as a negative logarithm of the H+ concentration (-log [H+]).
b. pH values range from 0 (100 moles/liter; most acidic) to 14 (1014 moles/liter; most basic).
i. One mole of water has 107 moles/liter of hydrogen ions; therefore, has neutral pH of 7.
ii. An acid is a substance with pH less than 7; a base is a substance with pH greater than 7.
iii. Because it is a logarithmic scale, each lower unit has 10 times the amount of hydrogen ions as next higher pH unit; as move up pH scale, each unit has 10 times the basicity of previous unit.
5. Buffers keep pH steady and within normal limits in living organisms..
a. Buffers stabilize pH of a solution by taking up excess hydrogen (H+) or hydroxide (OH-) ions.
b. Carbonic acid helps keep blood pH within normal limits: H2CO3 → H+ + HCO3-.
.1 Organic Molecules
• Organic molecules contain carbon and hydrogen atoms bonded to other atoms.
1. Four types of organic molecules (biomolecules) exist in organisms: carbohydrates, lipids, proteins, and nucleic acids.
2. Organic molecules are a diverse group; even a simple bacterial cell contains some 5,000 organic molecules.
• The Carbon Atom
1. The chemictry of the carbon atom allows it to form covalent bonds with as many as four other elements (generally with the CHNOPS elements).
2. Hydrocarbons are chains of carbon atoms bonded exclusively to hydrogen atoms; hydrocarbons can be branched and they can form ringed (cyclic) compounds.
3. Carbon atoms can form double or triple bonds with certain atoms (carbon, nitrogen).
A. The Carbon Skeleton and Functional Groups
1. The carbon chain of an organic molecule is called its skeleton or backbone.
2. Functional groups are clusters of specific atoms bonded to the carbon skeleton with characteristic structure and functions.
a. As an example, the addition of an –OH (hydroxyl group) to a carbon skeleton turns the molecule into an alcohol.
b. Ethyl alcohol (ethanol) is hydrophilic (dissolves in water) because the hydroxyl group is polar.
c. Nonpolar organic molecules are hydrophobic (cannot dissolve in water) unless they contain a polar functional group. An example is ethane.
d. Depending on its functional groups, an organic molecule may be both acidic and hydrophilic. An example is a hydrocarbon that contains a carboxyl group; carboxyl groups ionize in solution by releasing hydrogen ions, becoming both polar and acidic.
e. Because cells are 70–90% water, the degree to which an organic molecule interacts with water affects its function.
3. Isomers are molecules with identical molecular formulas but different arrangements of their atoms (e.g., glyceraldehyde and dihydroxyacetone).
B. The Macromolecules of Cells
1. Carbohydrates, lipids, proteins, and nucleic acids are called macromolecules because of their large size.
2. The largest macromolecules are called polymers, constructed by linking many of the same type of small subunits, called monomers. Examples: amino acids (monomers) are linked to form a protein (polymer); many nucleotides (monomers) are linked to form a nucleic acid (polymer).
3. Cellular enzymes carry out dehydration reactions to synthesize macromolecules. In a dehydration reaction, a water molecule is removed and a covalent bond is made between two atoms of the monomers.
a. In a dehydration reaction, a hydroxyl (—OH) group is removed from one monomer and a hydrogen (—H) is removed from the other.
b. This produces water, and, because the water is leaving the monomers, it is a dehydration reaction.
4. Hydrolysis ("water breaking") reactions break down polymers in reverse of dehydration; a hydroxyl (—OH) group from water attaches to one monomer and hydrogen (—H) attaches to the other.
5. Enzymes are molecules that speed up chemical reactions by bringing reactants together; an enzyme may even participate in the reaction but is not changed by the reaction.
3.2 Carbohydrates
A. Monosaccharides: Ready Energy
1. Monosaccharides are simple sugars with a backbone of 3 to 7 carbon atoms.
a. Most monosaccharides of organisms have 6 carbons (hexose).
b. Glucose and fructose are hexoses, but are isomers of one another; each has the formula (C6H12O6) but they differ in arrangement of the atoms.
c. Glucoseis found in the blood of animals; it is the source of biochemical energy (ATP) in nearly all organisms.
2. Ribose and deoxyribose are five-carbon sugars (pentoses); they contribute to the backbones of RNA and DNA, respectively.
B. Disaccharides: Varied Uses
1. Disaccharides contain two monosaccharides joined by a dehydration reaction.
2. Lactose is composed of galactose and glucose and is found in milk.
3. Maltose is composed of two glucose molecules; it forms in the digestive tract of humans during starch digestion.
4. Sucrose (table sugar) is composed of glucose and fructose; it is used to sweeten food for human consumption.
C. Polysaccharides as Energy Storage Molecules
1. Polysaccharides are polymers of monosaccharides. They are not soluble in water and do not pass through the plasma membrane of the cell.
2. Starch, found in many plants, is a straight chain of glucose molecules with relatively few side branches. Amylose and amylopectin are the two forms of starch found in plants.
3. Glycogen is a highly branched polymer of glucose with many side branches. It is the storage form of glucose in animals.
D. Polysaccharides as Structural Molecules
1. Cellulose is a polymer of glucose which forms microfibrils, the primary constituent of plant cell walls.
a. Cotton is nearly pure cellulose.
b. Cellulose is indigestible by humans due to the unique bond between glucose molecules.
c. Grazing animals can digest cellulose due to special stomachs and bacteria.
d. Cellulose is the most abundant organic molecule on Earth.
2. Chitin is a polymer of glucose with an amino group attached to each glucose.
a. Chitin is the primary constituent of the exoskeleton of crabs and related animals (lobsters, insects, etc.).
b. Chitin is not digestible by humans.
3.3 Lipids
• Lipids are varied in structure.
1. Lipids are hydrocarbons that are insoluble in water because they lack polar groups.
2. Fat provides insulation and energy storage in animals.
3. Phospholipids form plasma membranes and steroids are important cell messengers.
4. Waxes have protective functions in many organisms.
A. Triglycerides: Long-Term Energy Storage
1. Fats and oils contain two molecular units: glycerol and fatty acids.
2. Glycerol is a water-soluble compound with three hydroxyl groups.
3. Triglycerides are glycerol joined to three fatty acids by dehydration reactions.
4. A fatty acid is a long hydrocarbon chain with a carboxyl (acid) group at one end.
a. Most fatty acids in cells contain 16 to 18 carbon atoms per molecule.
b. Saturated fatty acids have no double bonds between their carbon atoms.
c. Unsaturated fatty acids have double bonds in the carbon chain where there are less than two hydrogens per carbon atom.
5. Fats contain saturated fatty acids and are solid at room temperature (e.g., butter).
6. Oils contain unsaturated fatty acids and are liquid at room temperature.
7. Animals use fat rather than glycogen for long-term energy storage; fat stores more energy.
B. Phospholipids: Membrane Components
1. Phospholipids are constructed like neutral fats except that the third fatty acid is replaced by a polar (hydrophilic) phosphate group; the phosphate group usually bonds to another organic group (designated by R).
2. The hydrocarbon chains of the fatty acids become the nonpolar (hydrophobic) tails.
3. Phospholipids arrange themselves in a double layer in water, so the polar heads face toward water molecules and nonpolar tails face toward one other, away from water molecules.
4. This property enables phospholipids to form an interface or separation between two solutions (e.g., the interior and exterior of a cell); the plasma membrane is a phospholipid bilayer.
C. Steroids: Four Fused Rings
1. Steroids have skeletons of four fused carbon rings and vary according to attached functional groups; these functional groups determine the biological functions of the various steroid molecules.
2. Cholesterol is a component of an animal cell's plasma membrane, and is the precursor of the steroid hormone (aldosterone, testosterone, estrogen, calcitriol, etc.).
3. A diet high in saturated fats and cholesterol can lead to circulatory disorders.
D. Waxes
1. Waxes are long-chain fatty acids bonded to long-chain alcohols.
2. Waxes have a high melting point, are waterproof, and resist degradation.
3. Waxes form a protective covering in plants that retards water loss in leaves and fruits.
4. In animals, waxes maintain animal skin and fur, trap dust and dirt, and form the honeycomb.
3.4 Proteins
• Protein Functions
1. Support proteins include keratin, which makes up hair and nails, and collagen fibers, which support many of the body's structures (e.g., ligaments, tendons, skin).
2. Enzymes are proteins that act as organic catalysts to accelerate chemical reactions within cells.
3. Transport functions include channel and carrier proteins in the plasma membrane, and hemoglobin that transports oxygen in red blood cells.
4. Defense functions include antibodies that prevent infection.
5. Hormones are regulatory proteins that influence the metabolism of cells. For example, insulin regulates glucose content of blood and within cells.
6. Motion within cells and by muscle contraction is provided by the proteins myosin and actin.
A. Amino Acids: Building Blocks of Proteins
1. Amino acids contain an acidic group (— COOH) and an amino group (—NH2).
2. Amino acids differ according to their particular R group, ranging from single hydrogen to complicated ring compounds.
3. The R group of amino acid cystine ends with a sulfhydryl (—SH) that serves to connect one chain of amino acids to another by a disulfide bond (— S—S—).
4. There are 20 different amino acids commonly found in cells.
B. Peptides
1. A peptide bond is a covalent bond between two amino acids.
2. Atoms of a peptide bond share electrons unevenly (oxygen is more electronegative than nitrogen).
3. The polarity of the peptide bond permits hydrogen bonding between different amino acids in a polypeptide.
4. A peptide is two or more amino acids bonded together.
5. Polypeptides are chains of many amino acids joined by peptide bonds.
6. A protein may contain more than one polypeptide chain; it can thus have a very large number of amino acids.
a. The three-dimensional shape of a protein is critical; an abnormal sequence will have the wrong shape and will not function normally.
b. Frederick Sanger determined the first protein sequence (of the hormone insulin) in 1953.
C. Shape of Proteins
1. Protein shape determines the function of the protein in the organism; proteins can have up to four levels of structure (but not all proteins have four levels).
2. The primary structure is the protein's own particular sequence of amino acids.
a. Just as the English alphabet contains 26 letters, 20 amino acids can join to form a huge variety of "words."
3. The secondary structure results when a polypeptide coils or folds in a particular way.
a. The a (alpha) helix was the first pattern discovered.
i. In a peptide bond, oxygen is partially negative, hydrogen is partially positive.
ii. This allows for hydrogen bonding between the C=O of one amino acid and the N—H of another.
iii. Hydrogen bonding between every fourth amino acid holds the spiral shape of an a helix.
b. The b (beta) sheet was the second pattern discovered.
i. Pleated b sheet polypeptides turn back upon themselves.
ii. Hydrogen bonding occurs between extended lengths.
c. Fibrous proteins (e.g. keratin) are structural proteins with helices and/or pleated sheets that hydrogen bond to one another.
4. Tertiary structure results when proteins are folded, giving rise to the final three-dimensional shape of the protein. This is due to interactions among the R groups of the constituent amino acids.
a. Globular proteins tend to ball up into rounded shapes.
b. Strong disulfide linkages maintain the tertiary shape; hydrogen, ionic, and covalent bonds also contribute.
5. Quaternary structure results when two or more polypeptides combine.
a. Hemoglobin is globular protein with a quaternary structure of four polypeptides; each polypeptide has a primary, secondary, and tertiary structure.
D. Protein Folding Diseases
1. As proteins are synthesized, chaperone proteins help them fold into their correct shapes; chaperone proteins may also correct misfolding of a new protein and prevent them from making incorrect shapes.
2. Certain diseases (e.g., the transmissible spongiform encephalopathies, or TSEs) are likely due to misfolded proteins, called prions.
3.5 Nucleic Acids
1. Nucleic acids are polymers of nucleotides with very specific functions in cells.
2. DNA (deoxyribonucleic acid) stores the genetic code for its own replication and for the amino acid sequences in proteins.
3. RNA (ribonucleic acid) allows for translation of the genetic code of DNA into the amino acid sequence of proteins; other functions for RNA in the cell exist.
4. Some nucleotides have independent metabolic functions in cells.
a. Coenzymes are molecules which facilitate enzymatic reactions.
b. ATP (adenosine triphosphate) is a nucleotide used to supply energy for synthetic reactions and other energy-requiring metabolic activities in the cell.
B. Structure of DNA and RNA
1. Nucleotides are a molecular complex of three types of molecules: a phosphate (phosphoric acid), a pentose sugar, and a nitrogen-containing base.
2. DNA and RNA differ in the following ways:
a. Nucleotides of DNA contain deoxyribose sugar; nucleotides of RNA contain ribose.
b. In RNA, the base uracil occurs instead of the base thymine. Both RNA and DNA contain adenine, guanine, and cytosine.
c. DNA is double-stranded with complementary base pairing; RNA is single-stranded.
i. Complementary base pairing occurs where two strands of DNA are held together by hydrogen bonds between purine and pyrimidine bases.
ii. The number of purine bases always equals the number of pyrimidine bases.
iii. In DNA, thymine is always paired with adenine; cytosine is always paired with guanine. Thus, in DNA: A + G = C + T.
d. Two strands of DNA twist to form a double helix; RNA does not form helices.
C. ATP (Adenosine Triphosphate)
1. ATP (adenosine triphosphate) is a nucleotide in which adenosine is composed of ribose and adenine.
2. Triphosphate derives its name from three phosphate groups attached together and to the ribose.
3. ATP is a high-energy molecule because the last two phosphate bonds release energy when broken.
4. In cells, the terminal phosphate bond is hydrolyzed, leaving ADP (adenosine diphosphate); energy is released when this occurs.
5. The energy released from ATP breakdown is used in the energy-requiring processes of the cell, such as synthetic reactions, muscle contraction, and the transmission of nerve impulses.
4.1 Cellular Level of Organization
1. Detailed study of the cell began in the 1830s; some of the scientists contributing to the understanding of cell structure and function were Robert Brown, Matthais Schleiden, Theodor Schwann, and Rudolph Virchow.
2. The cell theory states that all organisms are composed of cells, that cells are the structural and functional unit of organisms, and that cells come only from preexisting cells.
B. Cell Size
1. Cells range in size from one millimeter down to one micrometer.
2. Cells need a surface area of plasma membrane large enough to adequately exchange materials.
3. The surface-area-to-volume ratio requires that cells be small.
a. As cells get larger in volume, surface area relative to volume decreases.
b. Size limits how large the actively metabolizing cells can become.
c. Cells needing greater surface area utilize membrane modifications such as folding, microvilli, etc.
C. Microscopy Today (Science Focus Box)
1. Compound light microscopes use light rays focused by glass lenses.
2. Transmission electron microscopes (TEM) use electrons passing through specimen and focused by magnets.
3. Scanning electron microscopes (SEM) use electrons scanned across metal-coated specimen; secondary electrons given off by metal are collected by a detector.
4. Magnification is a function of wavelength; the shorter wavelengths of electrons allow greater magnification than the longer wavelengths of light rays.
5. Resolution is the minimum distance between two objects at which they can still be seen as separate objects.
6. Immunofluorescence microscopy uses fluorescent antibodies to reveal proteins in cells.
7. Confocal microscopy uses laser beam to focus on a shallow plane within the cell; this forms a series of optical sections from which a computer creates a three dimensional image.
8. Video-enhanced contrast microscopy accentuates the light and dark regions and may use a computer to contrast regions with false colors.
9. Bright-field, phase contrast, differential interference, and darkfield are different types of light microscopes.
4.2 Prokaryotic Cells
1. Prokaryotic cells lack a nucleus and are smaller and simpler than eukaryotic cells (which have a nucleus).
2. Prokaryotic cells are placed in two taxonomic domains: Bacteria and Archaea. Organisms in these two domains are structurally similar but biochemically different.
B. The Structure of Bacteria
1. Bacteria are extremely small; average size is 1–1.5 μm wide and 2–6 μm long .
2. Bacteria occur in three basic shapes: spherical coccus, rod-shaped bacillus, and spiral spirillum (if rigid) or spirochete (if flexible).
3. Cell Envelope
a. Includes the plasma membrane, the cell wall, and the glycocalyx. The plasma membrane is a lipid bilayer with imbedded and peripheral proteins; it regulates the movement of substances into and out of the cell.
b. The plasma membrane can form internal pouches called mesosomes, which increase the internal surface area of the membrane for enzyme attachment.
c. The cell wall maintains the shape of the cell and is strengthened by peptidoglycan.
d. The glycocalyx is a layer of polysaccharides on the outside of the cell wall; it is called a capsule if organized and not easily removed, or a slime layer if it is not well-organized and is easily removed.
4. Cytoplasm
a. The cytoplasm is a semifluid solution containing water, inorganic and organic molecules, and enzymes.
b. The nucleoid is a region that contains the single, circular DNA molecule.
c. Plasmids are small accessory (extrachromosomal) rings of DNA; they are not part of the bacterial genetic material.
d. Ribosomes are particles with two RNA- and protein-containing subunits that synthesize proteins.
e. Inclusion bodies in the cytoplasm are granules of stored substances.
f. Cyanobacteria (also called blue-green bacteria) are bacteria that photosynthesize; they lack chloroplasts but have thylakoids containing chlorophyll and other pigments.
5. Appendages
a. Motile bacteria usually have flagella; the filament, hook, and basal body work to rotate the flagellum like a propeller to move through fluid medium.
b. Fimbriae are small, bristlelike fibers that attach to an appropriate surface.
c. Sex pili are tubes used by bacteria to pass DNA from cell to cell.
C. The Structure of Archaea
1. In addition to spheres, rods, and spirals, Archaea can be lobed, platelike, or irregular.
2. The cell wall contains various polysaccharides and proteins rather than peptidoglycan.
3. The membrane lipids are composed of glycerol bonded to hydrocarbons, not fatty acids.
4. The DNA and RNA base sequences are closer to eukaryotes than bacteria.
5. Many Archaea are found in extremely salty or hot environments; they may have been the first type of cell to evolve.
4.3 Eukaryotic Cells
1. Eukaryotic cells are members of the domain Eukarya, which includes the protists, fungi, plants, and animals.
2. A membrane-bounded nucleus houses DNA; the nucleus may have originated as an invagination of the plasma membrane.
3. Eukaryotic cells are much larger than prokaryotic cells, and therefore have less surface area per volume.
4. Eukaryotic cells are compartmentalized; they contain small structures called organelles that perform specific functions.
5. Some eukaryotic cells (e.g., plant cells) have a cell wall containing cellulose; plasmodesmata are channels in a cell wall that allow cytoplasmic strands to extend between adjacent cells.
B. The Structure of Eukaryotic Cells
1. The nucleus communicates with ribosomes in the cytoplasm.
2. The organelles of the endomembrane system communicate with one another; each organelle contains its own set of enzymes and produces its own products, which move from one organelle to another by transport vesicles.
3. The energy-related mitochondria (plant and animal cells) and chloroplasts (plant cells) do not communicate with other organelles; they contain their own DNA and are self-sufficient.
4. The cytoskeleton is a lattice of protein fibers that maintains the shape of the cell and assists in movement of the organelles.
C. Cell Fractionation and Differential Centrifugation (Science Focus Box)
1. Cell fractionation allows the researcher to isolate and individually study the organelles of a cell.
2. Differential centrifugation separates the cellular components by size and density.
3. Using these two techniques, researchers can obtain pure preparations of any cell component.
D. The Nucleus and Ribosomes
1. The nucleus has a diameter of about 5 μm.
2. Chromatin is a threadlike material that coils into chromosomes just before cell division occurs; contains DNA, protein, and some RNA.
3. Nucleoplasm is the semifluid medium of the nucleus.
4. Chromosomes are rodlike structures formed during cell division; composed of coiled or folded chromatin.
5. The nucleolus is a dark region of chromatin inside the nucleus; it is the site where ribosomal RNA (rRNA) joins with proteins to form ribosomes.
6. The nucleus is separated from the cytoplasm by the nuclear envelope, which contains nuclear pores to permit passage of substances (e.g., ribosomal subunits, messenger RNA, proteins, etc.) in and out of the nucleus
7. Ribosomes are the site of protein synthesis in the cell. In eukaryotic cells, ribosomes may occur freely or in groups called polyribosomes.
8. Ribosomes receive messenger RNA (mRNA) from the nucleus, which instructs the ribosomes of the correct sequence of amino acids in a protein to be synthesized.
E. The Endomembrane System
1. The endomembrane system is a series of intracellular membranes that compartmentalize the cell.
2. It consists of the nuclear envelope, the membranes of the endoplasmic reticulum, the Golgi apparatus, and several types of vesicles.
3. Endoplasmic Reticulum
a. The endoplasmic reticulum (ER) is a system of membrane channels and saccules (flattened vesicles) continuous with the outer membrane of the nuclear envelope.
b. Rough ER is studded with ribosomes on the cytoplasm side; it is the site where proteins are synthesized and enter the ER interior for processing and modification.
c. Smooth ER is continuous with rough ER but lacks ribosomes; it is a site of various synthetic processes, detoxification, and storage; smooth ER forms transport vesicles.
4. The Golgi Apparatus
a. It is named for Camillo Golgi, who discovered it in 1898.
b. The Golgi apparatus consists of a stack of slightly curved saccules.
c. The Golgi apparatus receives protein-filled vesicles that bud from the rough ER and lipid-filled vesicles from the smooth ER.
d. Enzymes within the Golgi apparatus modify the carbohydrates that were placed on proteins in the ER; proteins and lipids are sorted and packaged.
e. Vesicles formed from the membrane of the outer face of the Golgi apparatus move to different locations in a cell; at the plasma membrane they discharge their contents as secretions, a process called exocytosis because substances exit the cell.
5. Lysosomes
a. Lysosomes are membrane-bounded vesicles produced by the Golgi apparatus.
b. Lysosomes contain powerful digestive enzymes and are highly acidic.
c. Macromolecules enter a cell by vesicle formation; lysosomes fuse with vesicles and digest the contents of the vesicle.
d. White blood cells that engulf bacteria use lysosomes to digest the bacteria.
e. Autodigestion occurs when lysosomes digest parts of cells.
f. Lysosomes participate in apoptosis, or programmed cell death, a normal part of development.
6. Endomembrane System Summary
a. Proteins produced in rough ER and lipids from smooth ER are carried in vesicles to the Golgi apparatus.
b. The Golgi apparatus modifies these products and then sorts and packages them into vesicles that go to various cell destinations.
c. Secretory vesicles carry products to the membrane where exocytosis produces secretions.
d. Lysosomes fuse with incoming vesicles and digest macromolecules.
F. Peroxisomes and Vacuoles
1. Peroxisomes are membrane-bounded vesicles that contain specific enzymes.
a. Peroxisome action results in production of hydrogen peroxide.
b. Hydrogen peroxide (H2O2) is broken down to water and oxygen by catalase.
c. Peroxisomes in the liver produce bile salts from cholesterol and also break down fats.
d. Peroxisomes also occur in germinating seeds where they convert oils into sugars used as nutrients by the growing plant.
2. Vacuoles
a. Vacuoles are mebranous sacs and are larger than vesicles.
b. Contractile vacuoles in some protists rid the cell of excess water.
c. Digestive vacuoles digest nutrients.
d. Vacuoles generally store substances, e.g., plant vacuoles contain water, sugars, salts, pigments, and toxic molecules
e. The central vacuole of a plant cell maintins turgor pressure within the cell, stores nutrients and wastes, and degrades organelles as the cell ages.
G. Energy-Related Organelles
1. Chloroplasts are membranous organelles (a type of plastid) that serve as the site of photosynthesis.
a. Photosynthesis is represented by the equation:
b. solar energy + carbon dioxide + water → carbohydrate + oxygen
c. Only plants, algae, and certain bacteria are capable of conducting photosynthesis.
d. The chloroplast is bound by a double membrane organized into flattened disc-like sacs called thylakoids formed from a third membrane; a stack of thylakoids is a granum.
e. Chlorophyll and other pigments capture solar energy, and the enzymes which synthesize carbohydrates are located in the chloroplasts.
f. Chloroplasts have both their own DNA and ribosomes, supporting the endosymbiotic hypothesis.
g. Other types of plastids, which differ in color, form, and function from chloroplasts, include chromoplasts and leucoplasts.
2. Mitochondria are surrounded by a double membrane: the inner membrane surrounds the matrix and is convoluted to form cristae.
a. Mitochondria are smaller than chloroplasts, and often vary their shape.
b. Mitochondria also can be fixed in one location or form long, moving chains.
c. Mitochondria contain ribosomes and their own DNA.
d. The matrix of the mitochondria is concentrated with enzymes that break down carbohydrates.
e. ATP production occurs on the cristae.
f. More than forty different diseases involving mitochondria have been described.
H. The Cytoskeleton
1. The cytoskeleton is a network of connected filaments and tubules; it extends from the nucleus to the plasma membrane in eukaryotes.
a. Electron microscopy reveals an organized cytosol.
b. Immunofluorescence microscopy identifies protein fibers.
c. Elements of the cytoskeleton include: actin filaments, intermediate filaments, and microtubules.
2. Actin Filaments
a. Actin filaments are long, thin fibers (about 7 nm in diameter) that occur in bundles or meshlike networks.
b. The actin filament consists of two chains of globular actin monomers twisted to form a helix.
c. Actin filaments play a structural role, forming a dense complex web just under the plasma membrane; this accounts for the formation of pseudopods in amoeboid movement.
d. Actin filaments in microvilli of intestinal cells likely shorten or extend cell into intestine.
e. In plant cells, they form tracks along which chloroplasts circulate.
f. Actin filaments move by interacting with myosin; myosin combines with and splits ATP, binding to actin and changing configuration to pull actin filament forward.
g. Similar action accounts for pinching off cells during cell division.
3. Intermediate Filaments
a. Intermediate filaments are 8–11 nm in diameter, between actin filaments and microtubules in size.
b. They are rope-like assemblies of fibrous polypeptides.
c. Some support the nuclear envelope; others support plasma membrane and form cell-to-cell junctions.
4. Microtubules
a. Microtubules are small hollow cylinders (25 nm in diameter and from 0.2–25 μm in length).
b. Microtubules are composed of a globular protein tubulin that occurs as α tubulin and β tubulin.
c. Assembly brings these two together as dimers and the dimers arrange themselves in rows.
d. Regulation of microtubule assembly is under control of a microtubule organizing center (MTOC): the main MTOC is called a centrosome.
e. Microtubules radiate from the MTOC, helping maintain the shape of cells and acting as tracks along which organelles move.
f. Similar to actin-myosin, the motor molecules kinesin and dynein are associated with microtubules.
g. Different kinds of kinesin proteins specialize to move one kind of vesicle or cell organelle.
h. Cytoplasmic dynein is similar to the molecule dynein found in flagella.
5. Centrioles
a. Centrioles are short cylinders with a ring pattern (9 + 0) of microtubule triplets.
b. In animal cells and most protists, centrosome contains two centrioles lying at right angles to each other.
c. Plant and fungal cells have the equivalent of a centrosome, but they do not contain centrioles.
d. Centrioles serve as basal bodies for cilia and flagella.
6. Cilia and Flagella
a. Cilia are short, usually numerous hairlike projections that can move in an undulating fashion (e.g., Paramecium; lining of human upper respiratory tract).
b. Flagella are longer, usually fewer, projections that move in whip-like fashion (e.g., sperm cells).
c. Both have similar construction, but differ from prokaryotic flagella.
i. Membrane-bounded cylinders enclose a matrix containing a cylinder of nine pairs of microtubules encircling two single microtubules (9 + 2 pattern of microtubules).
ii. Cilia and flagella move when the microtubules slide past one another.
iii. Cilia and flagella have a basal body at base with the same arrangement of microtubule triples as centrioles.
iv. Cilia and flagella grow by the addition of tubulin dimers to their tips.
5.1 Membrane Models
1. In the early 1900s, researchers noted that lipid-soluble molecules entered cells more rapidly than water-soluble molecules, suggesting lipids are component of plasma membrane.
2. Later, chemical analysis revealed that the membrane contained phospholipids.
3. Gorter and Grendel (1925) found that the amount of phospholipid extracted from a red blood cell was just enough to form one bilayer; they also suggested the nonpolar tails were directed inward and polar heads outward.
4. To account for the permeability of membrane to nonlipid substances, Danielli and Davson (1940s) proposed the "sandwich" model, with a phospholipid bilayer between layers of protein.
5. Robertson (1950s) proposed that proteins were embedded in an outer membrane and that all membranes in cells had similar compositions—the "unit membrane" model.
6. Additional research showed great diversity in membrane structure and function.
B. Fluid-Mosaic Model
1. In 1972, Singer and Nicolson introduced the currently accepted fluid-mosaic model.
a. The plasma membrane is a phospholipid bilayer, in which protein molecules are embedded.
b. Embedded proteins are scattered throughout membrane in an irregular pattern; this varies among membranes.
5.2 Plasma Membrane Structure and Function
1. The plasma membrane is a phospholipid bilayer with embedded proteins.
2. Phospholipids have both hydrophilic and hydrophobic regions; nonpolar tails (hydrophobic) are directed inward, polar heads (hydrophilic) are directed outward to face both extracellular and intracellular fluid.
3. The proteins form a mosaic pattern on the membrane.
4. Cholesterol is a lipid found in animal plasma membranes; it stiffens and strengthens the membrane.
5. Glycolipids have a structure similar to phospholipids except the hydrophilic head is a variety of sugar; they are protective and assist in various functions.
6. Glycoproteins have an attached carbohydrate chain of sugar that projects externally.
7. The plasma membrane is asymmetrical; glycolipids and proteins occur only on outside and cytoskeletal filaments attach to proteins only on the inside surface.
B. Carbohydrate Chains
1. In animal cells, the glycocalyx is a "sugar coat" of carbohydrate chains; it has several functions.
2. Cells are unique in that they have highly varied carbohydrate chains (a "fingerprint").
3. The immune system recognizes foreign tissues that have inappropriate carbohydrate chains.
4. Carbohydrate chains are the basis for A, B, and O blood groups in humans.
C. Fluidity of the Plasma Membrane
1. At body temperature, the phospholipid bilayer has the consistency of olive oil.
2. The greater the concentration of unsaturated fatty acid residues, the more fluid the bilayer.
3. In each monolayer, the hydrocarbon tails wiggle, and entire phospholipid molecules can move sideways.
4. Phospholipid molecules rarely "flip-flop" from one layer to the other.
5. Fluidity of the phospholipid bilayer allows cells to be pliable.
6. Some proteins are held in place by cytoskeletal filaments; most drift in the fluid bilayer.
D. The Functions of the Proteins
1. Plasma membrane and organelle membranes have unique proteins; red blood cells (RBC) plasma membrane contains 50+ types of proteins.
2. Membrane proteins determine most of the membrane's functions.
3. Channel proteins allow a particular molecule to cross membrane freely (e.g., Cl-channels).
4. Carrier proteins selectively interact with a specific molecule so it can cross the plasma membrane (e.g., Na+-K+ pump).
5. Cell recognition proteins are glycoproteins that allow the body's immune system to distinguish between foreign invaders and body cells.
6. Receptor proteins are shaped so a specific molecule (e.g., hormone) can bind to it.
7. Enzymatic proteins carry out specific metabolic reactions.
5.3 Permeability of the Plasma Membrane
1. The plasma membrane is differentially (selectively) permeable; only certain molecules can pass through.
a. Small non-charged lipid molecules (alcohol, oxygen) pass through the membrane freely.
b. Small polar molecules (carbon dioxide, water) move "down" a concentration gradient, i.e., from high to low concentration.
c. Ions and charged molecules cannot readily pass through the hydrophobic component of the bilayer and usually combine with carrier proteins.
2. Both passive and active mechanisms move molecules across membrane.
a. Passive transport moves molecules across membrane without expenditure of energy; includes diffusion and facilitated transport.
b. Active transport requires a carrier protein and uses energy (ATP) to move molecules across a plasma membrane; includes active transport, exocytosis, endocytosis, and pinocytosis.
B. Diffusion and Osmosis
1. Diffusion is the movement of molecules from higher to lower concentration (i.e., "down" the concentration gradient).
a. A solution contains a solute, usually a solid, and a solvent, usually a liquid.
b. In the case of a dye diffusing in water, the dye is a solute and water is the solvent.
c. Once a solute is evenly distributed, random movement continues but with no net change.
d. Membrane chemical and physical properties allow only a few types of molecules to cross by diffusion.
e. Gases readily diffuse through the lipid bilayer; e.g., the movement of oxygen from air sacs (alveoli) to the blood in lung capillaries depends on the concentration of oxygen in alveoli.
f. Temperature, pressure, electrical currents, and molecular size influence the rate of diffusion.
2. Osmosis is the diffusion of water across a differentially (selectively) permeable membrane.
a. Osmosis is illustrated by the thistle tube example:
i. A differentially permeable membrane separates two solutions.
ii. The beaker has more water (lower percentage of solute) and the thistle tube has less water (higher percentage of solute).
iii. The membrane does not permit passage of the solute; water enters but the solute does not exit.
iv. The membrane permits passage of water with a net movement of water from the beaker to the inside of the thistle tube.
b. Osmotic pressure is the pressure that develops in such a system due to osmosis.
c. Osmotic pressure results in water being absorbed by the kidneys and water being taken up from tissue fluid.
3. Tonicity is strength of a solution with respect to osmotic pressure.
a. Isotonic solutions occur where the relative solute concentrations of two solutions are equal; a 0.9% salt solution is used in injections because it is isotonic to red blood cells (RBCs).
b. A hypotonic solution has a solute concentration that is less than another solution; when a cell is placed in a hypotonic solution, water enters the cell and it may undergo cytolysis ("cell bursting").
c. Swelling of a plant cell in a hypotonic solution creates turgor pressure; this is how plants maintain an erect position.
d. A hypertonic solution has a solute concentration that is higher than another solution; when a cell is placed in a hypertonic solution, it shrivels (a condition called crenation).
e. Plasmolysis is shrinking of the cytoplasm due to osmosis in a hypertonic solution; as the central vacuole loses water, the plasma membrane pulls away from the cell wall.
C. Transport by Carrier Proteins
1. The plasma membrane impedes passage of most substances but many molecules enter or leave at rapid rates.
2. Carrier proteins are membrane proteins that combine with and transport only one type of molecule or ion; they are believed to undergo a change in shape to move the molecule across the membrane.
3. Facilitated transport is the transport of a specific solute "down" or "with" its concentration gradient (from high to low), facilitated by a carrier protein; glucose and amino acids move across the membrane in this way.
4. Active transport is transport of a specific solute across plasma membranes "up" or "against" (from low to high) its concentration gradient through use of cellular energy (ATP).
a. Iodine is concentrated in cells of thyroid gland, glucose is completely absorbed into lining of digestive tract, and sodium is mostly reabsorbed by kidney tubule lining.
b. Active transport requires both carrier proteins and ATP; therefore cells must have high number of mitochondria near membranes where active transport occurs.
c. Proteins involved in active transport are often called "pumps"; the sodium-potassium pump is an important carrier system in nerve and muscle cells.
d. Salt (NaCl) crosses a plasma membrane because sodium ions are pumped across, and the chloride ion is attracted to the sodium ion and simply diffuses across specific channels in the membrane.
5. Membrane-Assisted Transport
a. In exocytosis, a vesicle formed by the Golgi apparatus fuses with the plasma membrane as secretion occurs; insulin leaves insulin-secreting cells by this method.
b. During endocytosis, cells take in substances by vesicle formation as plasma membrane pinches off by either phagocytosis, pinocytosis, or receptor-mediated endocytosis.
c. In phagocytosis, cells engulf large particles (e.g., bacteria), forming an endocytic vesicle.
i. Phagocytosis is commonly performed by ameboid-type cells (e.g., amoebas and macrophages).
ii. When the endocytic vesicle fuses with a lysosome, digestion of the internalized substance occurs.
d. Pinocytosis occurs when vesicles form around a liquid or very small particles; this is only visible with electron microscopy.
e. Receptor-mediated endocytosis, a form of pinocytosis, occurs when specific macromolecules bind to plasma membrane receptors.
i. The receptor proteins are shaped to fit with specific substances (vitamin, hormone, lipoprotein molecule, etc.), and are found at one location in the plasma membrane.
ii. This location is a coated pit with a layer of fibrous protein on the cytoplasmic side; when the vesicle is uncoated, it may fuse with a lysosome.
iii. Pits are associated with exchange of substances between cells (e.g., maternal and fetal blood).
iv. This system is selective and more efficient than pinocytosis; it is important in moving substances from maternal to fetal blood.
v. Cholesterol (transported in a molecule called a low-density lipoprotein, LDL) enters a cell from the bloodstream via receptors in coated pits; in familial hypocholesterolemia, the LDL receptor cannot bind to the coated pit and the excess cholesterol accumulates in the circulatory system.
5.4 Modification of Cell Surfaces
A. Cell Surfaces in Animals
1. Junctions Between Cells are points of contact between cells that allow them to behave in a coordinated manner.
a. Anchoring junctions mechanically attach adjacent cells.
b. In adhesion junctions, internal cytoplasmic plaques, firmly attached to cytoskeleton within each cell are joined by intercellular filaments; they hold cells together where tissues stretch (e.g., in heart, stomach, bladder).
c. c. In desmosomes, a single point of attachment between adjacent cells connects the cytoskeletons of adjacent cells.
d. d. In tight junctions, plasma membrane proteins attach in zipper-like fastenings; they hold cells together so tightly that the tissues are barriers (e.g., epithelial lining of stomach, kidney tubules, blood-brain barrier).
e. A gap junction allows cells to communicate; formed when two identical plasma membrane channels join.
i. They provide strength to the cells involved and allow the movement of small molecules and ions from the cytoplasm of one cell to the cytoplasm of the other cell.
ii. Gap junctions permit flow of ions for heart muscle and smooth muscle cells to contract.
2. The extracellular matrix is a meshwork of polysaccharides and proteins produced by animal cells.
a. Collagen gives the matrix strength and elastin gives it resilience.
b. Fibronectins and laminins bind to membrane receptors and permit communication between matrix and cytoplasm; these proteins also form "highways" that direct the migration of cells during development.
c. Proteoglycans are glycoproteins that provide a packing gel that joins the various proteins in matrix and most likely regulate signaling proteins that bind to receptors in the plasma protein.
B. Plant Cell Walls
1. Plant cells are surrounded by a porous cell wall; it varies in thickness, depending on the function of the cell.
2. Plant cells have a primary cell wall composed of cellulose polymers united into threadlike microfibrils that form fibrils.
3. Cellulose fibrils form a framework whose spaces are filled by non-cellulose molecules.
4. Pectins allow the cell wall to stretch and are abundant in the middle lamella that holds cells together.
5. Non-cellulose polysaccharides harden the wall of mature cells.
6. Lignin adds strength and is a common ingredient of secondary cell walls in woody plants.
7. Plasmodesmata are narrow membrane-lined channels that pass through cell walls of neighboring cells and connect their cytoplasms, allowing direct exchange of molecules and ions between neighboring plant cells.
6.1 Cells and the Flow of Energy
A. Forms of Energy
1. Energy is capacity to do work; cells continually use energy to develop, grow, repair, reproduce, etc.
2. Kinetic energy is energy of motion; all moving objects have kinetic energy.
3. Potential energy is stored energy.
4. Food is chemical energy; it contains potential energy.
5. Chemical energy can be converted into mechanical energy, e.g., muscle movement.
B. Two Laws of Thermodynamics
1. First law of thermodynamics (also called the law of conservation of energy)
a. Energy cannot be created or destroyed, but it can be changed from one form to another.
b. In an ecosystem, solar energy is converted to chemical energy by the process of photosynthesis; some of the chemical energy in the plant is converted to chemical energy in an animal, which in turn can become mechanical energy or heat loss.
c. Neither the plant nor the animal create energy, they convert it from one form to another.
d. Likewise, energy is not destroyed; some becomes heat that dissipates into the environment.
2. Second law of thermodynamics
a. Energy cannot be changed from one form into another without a loss of usable energy.
b. Heat is a form of energy that dissipates into the environment; heat can never be converted back to another form of energy.
C. Cells and Entropy
1. Every energy transformation makes the universe less organized and more disordered; entropy is the term used to indicate the relative amount of disorganization.
2. When ions distribute randomly across a membrane, entropy has increased.
3. Organized/usable forms of energy (as in the glucose molecule) have relatively low entropy; unorganized/less stable forms have relatively high entropy.
4. Energy conversions result in heat; therefore, the entropy of the universe is always increasing.
5. Living things depend on a constant supply of energy from the sun, because the ultimate fate of all solar energy in the biosphere is to become randomized in the universe as heat; the living cell is a temporary repository of order purchased at the cost of a constant flow of energy.
6.2 Metabolic Reactions and Energy Transformations
1. Metabolism is the sum of all the biochemical reactions in a cell.
2. In the reaction A + B = C + D, A and B are reactants and C and D are products.
3. Free energy (DG) is the amount of energy that is free to do work after a chemical reaction.
4. Change in free energy is noted as DG; a negative DG means that products have less free energy than reactants; the reaction occurs spontaneously.
5. Exergonic reactions have a negative DG and energy is released.
6. Endergonic reactions have a positive DG; products have more energy than reactants; such reactions can only occur with an input of energy.
B. ATP: Energy for Cells
1. Adenosine triphosphate (ATP) is the energy currency of cells; when cells need energy, they "spend" ATP.
2. ATP is an energy carrier for many different types of reactions.
3. When ATP is converted into ADP + P, the energy released is sufficient for biological reactions with little wasted.
4. ATP breakdown is coupled to endergonic reactions in a way that minimizes energy loss.
5. ATP is a nucleotide composed of the base adenine and the 5-carbon sugar ribose and three phosphate groups.
6. When one phosphate group is removed, about 7.3 kcal of energy is released per mole.
C. Coupled Reactions
1. A coupled reaction occurs when energy released by an exergonic reaction is used to drive an endergonic reaction.
2. ATP breakdown is often coupled to cellular reactions that require energy.
3. ATP supply is maintained by breakdown of glucose during cellular respiration.
4. Only 39% of the chemical energy of glucose is transformed into ATP; 61% is lost as heat.
D. ATP can have any of three functions.
1. Chemical Work: ATP supplies energy to synthesize molecules that make up the cell.
2. Transport Work: ATP supplies energy to pump substances across the plasma membrane.
3. Mechanical Work: ATP supplies energy needed to perform mechanical processes (e.g., muscle contraction, propel cilia, etc.).
6.3 Metabolic Pathways and Enzymes
1. A metabolic pathway is an orderly sequence of linked reactions; each step is catalyzed by a specific enzyme.
2. Metabolic pathways begin with a particular reactant, end with a particular end product(s), and may have many intermediate steps.
3. In many instances, one pathway leads to the next; since pathways often have one or more molecules in common, one pathway can lead to several others.
4. Metabolic energy is captured more easily if it is released in small increments.
5. A reactant is the substance that is converted into a product by the reaction; often many intermediate steps occur.
6. Each step in a series of chemical reactions requires a specific enzyme.
7. Enzymes are catalysts that speed chemical reactions without the enzyme being affected by the reaction.
8. Every enzyme is specific in its action and catalyzes only one reaction or one type of reaction.
9. A substrate is a reactant for an enzymatic reaction.
B. Energy of Activation
1. Molecules often do not react with each other unless activated in some way.
2. For metabolic reactions to occur in a cell, an enzyme must usually be present.
3. The energy of activation (Ea) is the energy that must be added to cause molecules to react; without an enzyme (i.e., in a reaction vessel in the laboratory) this energy may be provided by heat, which causes an increase in the number of molecular collisions.
C. Enzyme-Substrate Complex
1. Enzymes speed chemical reactions by lowering the energy of activation (Ea) by forming a complex with their substrate(s) at the active site.
a. An active site is a small region on the surface of the enzyme where the substrate(s) bind.
b. When a substrate binds to an enzyme, the active site undergoes a slight change in shape that facilitates the reaction. This is called the induced fit model of enzyme catalysis.
2. Only a small amount of enzyme is needed in a cell because enzymes are not consumed during catalysis.
3. Some enzymes (e.g., trypsin) actually participate in the reaction.
4. A particular reactant(s) may produce more than one type of product(s).
a. Presence or absence of enzyme determines which reaction takes place.
b. If reactants can form more than one product, the enzymes present determine which product is formed.
5. Every cell reaction requires its specific enzyme; enzymes are sometimes named for substrates by adding "-ase."
D. Factors Affecting Enzymatic Speed
1. Substrate concentration.
a. Because molecules must collide to react, enzyme activity increases as substrate concentration increases; as more substrate molecules fill active sites, more product is produced per unit time.
2. Temperature and pH
a. As temperature rises, enzyme activity increases because there are more enzyme-substrate collisions.
b. Enzyme activity declines rapidly when enzyme is denatured at a certain temperature, due to a change in shape of the enzyme.
c. Every enzyme has optimal pH at which its rate of reaction is optimal.
d. A change in pH can alter the ionization of the R groups of the amino acids in the enzyme, thereby disrupting the enzyme's activity.
3. Enzyme concentration
a. The amount of active enzyme can regulate the rate of an enzymatic reaction.
b. Cells can activate specific genes when certain enzymes are needed.
c. Enzyme Cofactors
i. Many enzymes require an inorganic ion or non-protein cofactor to function.
ii. Inorganic cofactors are ions of metals.
iii. A coenzyme is an organic cofactor, which assists the enzyme (i.e., it may actually contribute atoms to the reaction).
iv. Vitamins are small organic molecules required in trace amounts for synthesis of coenzymes; they become part of a coenzyme's molecular structure; vitamin deficiency causes a lack of a specific coenzyme and therefore a lack of its enzymatic action.
v. Phosphorylation of enzymes occurs when signal proteins turn on kinases, which then activate specific enzymes; some hormones use this mechanism.
d. Enzyme inhibition occurs when a substance (called an inhibitor) binds to an enzyme and decreases its activity; normally, enzyme inhibition is reversible.
i. In competitive inhibition, the substrate and the inhibitor are both able to bind to the enzyme's active site.
ii. In noncompetitive inhibition, the inhibitor binds to the enzyme at a location other than the active site (the allosteric site), changing the shape of the enzyme and rendering it unable to bind to its substrate.
iii. Competitive and noncompetitive inhibition are both examples of feedback inhibition.
iv. In irreversible inhibition, the inhibitor permanently inactivates or destroys the enzyme; cyanide, mercury, and lead are irreversible inhibitors for several specific enzymes.
6.4 Oxidation-Reduction and the Flow of Energy
1. In oxidation-reduction (redox) reactions, electrons pass from one molecule to another.
2. Oxidation is the loss of electrons.
3. Reduction is the gain of electrons.
4. Both reactions occur at the same time because one molecule accepts electrons given up by another molecule.
B. Photosynthesis
1. Photosynthesis uses energy to combine carbon dioxide and water to produce glucose in the formula:
6 CO2 + 6 H2O + energy = C6H12O6 + 6 O2
2. When hydrogen atoms are transferred to carbon dioxide from water, water has been oxidized and carbon dioxide has been reduced.
3. Input of energy is needed to produce the high-energy glucose molecule.
4. Chloroplasts capture solar energy and convert it by way of an electron transport system into the chemical energy of ATP.
5. ATP is used along with hydrogen atoms to reduce glucose; when NADP+(nicotinamide adenine dinucleotide phosphate) donates hydrogen atoms (H+ + e-) to a substrate during photosynthesis, the substrate has accepted electrons and is therefore reduced.
6. The reaction that reduces NADP+ is:
NADP+ + 2e- + H+ = NADPH
C. Cellular Respiration
1. The overall equation for cellular respiration is opposite that of photosynthesis:
C6H12O6 + 6 O2 = 6 CO2 + 6 H2O + energy
2. When NAD removes hydrogen atoms (H+ + e-) during cellular respiration, the substrate has lost electrons and is therefore oxidized.
3. At the end of cellular respiration, glucose has been oxidized to carbon dioxide and water and ATP molecules have been produced.
4. In metabolic pathways, most oxidations involve the coenzyme NAD+ (nicotinamide adenine dinucleotide); the molecule accepts two electrons but only one hydrogen ion: NAD+ + 2e- + H+ = NADH
D. Electron Transport Chain
1. Both photosynthesis and respiration use an electron transport chain consisting of membrane-bound carriers that pass electrons from one carrier to another.
2. High-energy electrons are delivered to the system and low-energy electrons leave it.
3. The overall effect is a series of redox reactions; every time electrons transfer to a new carrier, energy is released for the production of ATP.
E. ATP Production
1. ATP synthesis is coupled to the electron transport system.
2. Peter Mitchell received the 1978 Nobel Prize for his chemiosmotic theory of ATP production.
3. In both mitochondria and chloroplasts, carriers of electron transport systems are located within a membrane.
4. H+ ions (protons) collect on one side of the membrane because they are pumped there by specific proteins.
5. The electrochemical gradient thus established across the membrane is used to provide energy for ATP production.
6. Enzymes and their carrier proteins, called ATP synthase complexes, span the membrane; each complex contains a channel that allows H+ ions to flow down their electrochemical gradient.
7. In photosynthesis, energized electrons lead to the pumping of hydrogen ions across the thylakoid membrane; as hydrogen ions flow through the ATP synthase complex, ATP is formed.
8. During cellular respiration, glucose breakdown provides energy for a hydrogen ion gradient on the inner membrane of the mitochondria that also couples hydrogen ion flow with ATP formation.
7.1 Photosynthetic Organisms
1. Photosynthetic organisms (algae, plants, and cyanobacteria) transform solar energy into carbohydrates.
2. Photosynthetic organisms (plants, algae, cyanobacteria) are called autotrophs because they produce their own food.
3. Organisms that must take in preformed organic molecules are called heterotrophs.
4. Both autotrophs and heterotrophs use organic molecules produced by photosynthesis as chemical building blocks and as a source of energy.
B. Flowering Plants as Photosynthesizers
1. Raw materials for photosynthesis are carbon dioxide and water.
2. Roots absorb water that moves up vascular tissue in the stem until it reaches the leaf veins.
3. Carbon dioxide enters a leaf through small openings called stomata.
4. Carbon dioxide and water diffuse into the chloroplasts, the organelles that carry on photosynthesis.
5. In chloroplasts, a double membrane encloses a fluid-filled space called the stroma.
6. An internal membrane system within the stroma forms flattened sacs called thylakoids, which in some cases are organized into stacks to form grana.
7. Spaces within all thylakoids are connected to form an inner compartment, the thylakoid space.
8. Chlorophyll and other pigments involved in absorption of solar energy reside within thylakoid membranes; these pigments absorb solar energy, and energize electrons prior to reduction of CO2 to a carbohydrate.
7.2 Plants as Solar Energy Converters
1. Only 42% of the solar radiation that hits the Earth's atmosphere reaches surface; most is visible light.
2. Higher energy wavelengths are screened out by the ozone layer in the upper atmosphere.
3. Lower energy wavelengths are screened out by water vapor and CO2.
4. Both the organic molecules within organisms and certain processes (e.g., vision, photosynthesis) are adapted to visible light, the radiation that is most prevalent in the environment.
B. Photosynthetic Pigments
1. Photosynthetic pigments use primarily the visible light portion of the electromagnetic spectrum.
2. Pigments found in chlorophyll absorb various portions of visible light; this is called their absorption spectrum.
3. Two major photosynthetic pigments are chlorophyll a and chlorophyll b.
4. Both chlorophylls absorb violet, blue, and red wavelengths best.
5. Very little green light is absorbed; most is reflected (this is why leaves appear green).
6. Carotenoids are yellow-orange pigments that absorb light in violet, blue, and green regions.
7. When chlorophyll breaks down in the fall, the yellow-orange pigments in leaves show through.
8. Absorption and action spectrum
a. A spectrophotometer measures the amount of light that passes through a sample.
i. As light is shone on a sample, some wavelengths are absorbed and others pass through the sample.
ii. A graph of percent of light absorbed at each wavelength is a compound's absorption spectrum.
b. Action spectrum
i. Photosynthesis produces oxygen; the production rate of oxygen is used to measure the rate of photosynthesis.
ii. Oxygen production and therefore photosynthetic activity is measured for plants under each specific wavelength; when plotted on a graph, this gives an action spectrum for a compound.
iii. The action spectrum for chlorophyll resembles its absorption spectrum, thus indicating that chlorophyll contributes to photosynthesis.
C. Photosynthetic Reaction
1. In 1930, van Niel showed that O2 given off by photosynthesis comes from water and not from CO2.
2. The net equation of photosynthesis reads: 6CO2 + 6H2O = C6 H12O6 + 6O2.
D. Two Sets of Reactions
1. In 1905, Blackman proposed two sets of reactions for photosynthesis.
2. Light reactions take place only in the presence of light.
a. Light reactions are the energy-capturing reactions.
b. Chlorophyl within thylakoid membranes absorbs solar energy and energizes electrons.
c. When energized electrons move down an electron transport chain, energy is captured and used for ATP production.
d. Energized electrons are also taken up by NADP+, converting it to NADPH.
3. Calvin cycle reactions
a. These reactions take place in the stroma; the reactions can occur in either the presence or the absence of light.
b. These are synthetic reactions that use NADPH and ATP to reduce CO2.
7.3 Light Reactions
1. Two electron pathways operate in the thylakoid membrane: the noncyclic pathway and the cyclic pathway.
2. Both pathways produce ATP; only the noncyclic pathway also produces NADPH.
3. ATP production during photosynthesis is called photophosphorylation; therefore these pathways are also known as cyclic and noncyclic photophosphorylation.
B. Noncyclic Electron Pathway
1. This pathway occurs in the thylakoid membranes and requires participation of two light-gathering units: photosystem I (PS I) and photosystem II (PS II).
2. A photosystem is a photosynthetic unit comprised of a pigment complex and an electron acceptor; solar energy is absorbed and high-energy electrons are generated.
3. Each photosystem has a pigment complex of chlorophyll a, chlorophyll b, carotenoid, and electron acceptor molecules.
4. Absorbed energy is passed from one pigment molecule to another until concentrated in reaction-centerchlorophyll a molecules.
5. Electrons in reaction-center chlorophyll a become excited, and escape to the electron-acceptor molecule.
6. The noncyclic pathway begins with PSII; electrons move from H2O through PS II to PS I and then on to NADP+.
7. The PS II pigment complex absorbs solar energy; high-energy electrons (e-) leave the reaction-center chlorophyll a molecule.
8. PS II takes replacement electrons from H2O, which splits, releasing O2 and H+ ions: H2O = 2 H+ + 2 e- + ½ O2.
9. Oxygen is released as oxygen gas (O2).
10. The H+ ions temporarily stay within the thylakoid space and contribute to a H+ ion gradient.
11. As H+ flow down electrochemical gradient through ATP synthase complexes, chemiosmosis occurs.
12. Low-energy electrons leaving the electron transport system enter PS I.
13. When the PS I pigment complex absorbs solar energy, high-energy electrons leave reaction-center chlorophyll a and are captured by an electron acceptor.
14. The electron acceptor passes them on to NADP+.
15. NADP+ takes on an H+ to become NADPH: NADP+ + 2 e- + H+ = NADPH.
16. NADPH and ATP (produced by noncyclic-flow electrons in the thylakoid membrane) are used by enzymes in the stroma during the light-independent (dark) reactions.
C. Cyclic Electron Pathway
1. The cyclic electron pathway begins when the PS I antenna complex absorbs solar energy.
2. High-energy electrons leave PS I reaction-center chlorophyll a molecule.
3. Before they return, the electrons enter and travel down an electron transport chain.
a. Electrons pass from a higher to a lower energy level.
b. Energy released is stored in the form of a hydrogen (H+) gradient.
c. When hydrogen ions flow down their electrochemical gradient through ATP synthase complexes, ATP production occurs.
d. The electrons return to PSI rather than move on to NADP+--this is why it is called cyclic and also why no NADPH is produced.
4. It is possible that in plants, the cyclic flow of electrons is utilized only when CO2 is in such limited supply that carbohydrate is not being produced.
D. The Organization of the Thylakoid Membrane
1. PS II consists of a pigment complex and electron-acceptor molecules; it oxidizes H2O and produces O2.
2. The electron transport system consists of cytochrome complexes and transports electrons and pumps H+ ions into the thylakoid space.
3. PS I has a pigment complex and electron-acceptor molecules; it is associated with an enzyme that reduces NADP+ to NADPH.
4. ATP synthase complex has an H+ channel and ATP synthase; it produces ATP.
E. ATP Production
1. The thylakoid space acts as a reservoir for H+ ions; each time H2O is split, two H+ remain.
2. Electrons move carrier-to-carrier, giving up energy used to pump H+ from the stroma into the thylakoid space.
3. Flow of H+ from high to low concentration across thylakoid membrane provides energy to produce ATP from ADP + P by using an ATP synthase enzyme.
4. This is called chemiosmosis because ATP production is tied to an electrochemical (H+) gradient.
7.4 Calvin Cycle Reactions
1. The Calvin cycle is a series of reactions producing carbohydrates; these reactions follow the light reactions.
2. The cycle is named for Melvin Calvin who used a radioactive isotope of carbon to trace the reactions.
3. The Calvin cycle includes carbon dioxide fixation, carbon dioxide reduction, and regeneration of ribulose 1,5-bisphosphate (RuBP).
B. Fixation of Carbon Dioxide
1. CO2 fixation is the attachment of CO2 to an organic compound called RuBP.
2. RuBP (ribulose bisphosphate) is a five-carbon molecule that combines with carbon dioxide; the resulting 6-carbon molecule then splits into two 3-carbon molecules.
3. The enzyme RuBP carboxylase (rubisco) speeds this reaction; this enzyme comprises 20–50% of the protein content of chloroplasts--it is an unusually slow enzyme.
C. Reduction of Carbon Dioxide
1. With the reduction of carbon dioxide, a 3PG (3-phosphoglycerate) molecule forms.
2. Each of two 3PG molecules undergoes reduction to G3P (glyceraldehyde-3-phosphate) in two steps.
3. Light-dependent reactions provide NADPH (electrons) and ATP (energy) to reduce 3PG to G3P.
D. Regeneration of RuBP
1. For every three turns of the Calvin cycle, five molecules of G3P are used to re-form three molecules of RuBP.
2. This reaction also uses ATP produced by the light reactions.
E. The Importance of the Calvin Cycle
1. G3P, the product of the Calvin Cycle can be converted into many other molecules.
2. Glucose phosphate is one result of G3P metabolism; it is a common energy molecule.
3. Glucose phosphate can bond with fructose to form sucrose.
4. Glucose phosphate is the starting point for synthesis of starch and cellulose.
5. The hydrocarbon skeleton of G3P is used to form fatty acids and glycerol; the addition of nitrogen forms various amino acids.
7.5 Other Types of Photosynthesis
1. In C3 plants, the Calvin cycle fixes CO2 directly; the first molecule following CO2 fixation is 3PG.
2. In hot weather, stomata close to save water; CO2 concentration decreases in leaves; O2 increases.
3. This is called photorespiration since oxygen is taken up and CO2 is produced; this produces only one 3PG.
B. C4 Photosynthesis
1. In a C3 plant, mesophyll cells contain well-formed chloroplasts, arranged in parallel layers.
2. In C4 plants, bundle sheath cells as well as the mesophyll cells contain chloroplasts.
3. In C4 leaf, mesophyll cells are arranged concentrically around the bundle sheath cells.
4. C3 plants use RuBP carboxylase to fix CO2 to RuBP in mesophyll; the first detected molecule is G3P.
5. C4 plants use the enzyme PEP carboxylase (PEPCase) to fix CO2 to PEP (phosphoenolpyruvate, a C3 molecule); the end product is oxaloacetate (a C4 molecule).
6. In C4 plants, CO2 is taken up in mesophyll cells and malate, a reduced form of oxaloacetate, is pumped into the bundle-sheath cells; here CO2 enters Calvin cycle.
7. In hot, dry climates, net photosynthetic rate of C4 plants (e.g., corn) is 2–3 times that of C4 plants.
8. Photorespiration does not occur in C4 leaves because PEPCase does not combine with O2; even when stomates are closed, CO2 is delivered to the Calvin cycle in bundle sheath cells.
9. C4 plants have advantage over C3 plants in hot and dry weather because photorespiration does not occur; e.g., bluegrass (C3) dominates lawns in early summer, whereas crabgrass (C4) takes over in the hot midsummer.
C. CAM Photosynthesis
1. CAM (crassulacean-acid metabolism) plants form a C4 molecule at night when stomates can open without loss of water; found in many succulent desert plants including the family Crassulaceae.
2. At night, CAM plants use PEPCase to fix CO2 by forming C4 molecule stored in large vacuoles in mesophyll.
3. C4 formed at night is broken down to CO2 during the day and enters the Calvin cycle within the same cell, which now has NADPH and ATP available to it from the light-dependent reactions.
4. CAM plants open stomates only at night, allowing CO2 to enter photosynthesizing tissues; during the day, stomates are closed to conserve water but CO2 cannot enter photosynthesizing tissues.
5. Photosynthesis in a CAM plant is minimal, due to limited amount of CO2 fixed at night; but this does allow CAM plants to live under stressful conditions.
D. Photosynthesis and Adaptation to the Environment
1. Each method of photosynthesis has its advantages, depending on the environment.
2. C4 plants are adapted to areas of high light intensities, high temperatures, and limited rainfall.
3. C3 plants do better in cooler climates.
4. CAM plants do well in an arid environment.
8.1 Cellular Respiration
1. Cellular respiration involves various metabolic pathways that break down carbohydrates and other metabolites with the concomitant buildup of ATP.
2. Cellular respiration consumes oxygen and produces CO2; because oxygen is required, cellular respiration is aerobic.
3. Cellular respiration usually involves the complete breakdown of glucose into CO2 and H2O.
4. The net equation for glucose breakdown is: C6H12O6 + 6 O2 = 6 CO2 + 6 H2O + energy
5. Glucose is a high-energy molecule; CO2 and H2O are low-energy molecules; cellular respiration is thus exergonic because it releases energy.
6. Electrons are removed from substrates and received by oxygen, which combines with H+ to become water.
7. Glucose is oxidized and O2 is reduced.
8. The buildup of ATP is an endergonic reaction (i.e., requires energy).
9. The reactions of cellular respiration allow energy in glucose to be released slowly; therefore ATP is produced gradually.
10. In contrast, if glucose were broken down rapidly, most of its energy would be lost as non-usable heat.
11. The breakdown of glucose yields synthesis of 36 or 38 ATP (depending on certain conditions); this preserves about 39% of the energy available in glucose.
12. This is relatively efficient compared to, for example, the 25% efficiency of a car burning gasoline.
B. NAD+ and FAD
1. Each metabolic reaction in cellular respiration is catalyzed by a specific enzyme.
2. As a metabolite is oxidized, NAD+ (nicotinamide adenine dinucleotide) accepts two electrons and a hydrogen ion (H+); this results in NADH + H+.
3. Electrons received by NAD+ and FAD are high-energy electrons and are usually carried to the electron transport chain.
4. NAD+ is a coenzyme of oxidation-reduction since it both accepts and gives up electrons; thus, NAD+ is sometimes called a redox coenzyme
5. Only a small amount of NAD+ is needed in cells because each NAD+ molecule is used repeatedly.
6. FAD coenzyme of oxidation-reduction can replace NAD+; FAD accepts two electrons and two hydrogen ions to become FADH2.
C. Phases of Cellular Respiration
1. Cellular respiration includes four phases:
a. Glycolysis is the breakdown of glucose in the cytoplasm into two molecules of pyruvate.
i. Enough energy is released for an immediate yield of two ATP.
ii. Glycolysis takes place outside the mitochondria and does not utilize oxygen; it is therefore an anaerobic process.
b. In the preparatory (prep) reaction, pyruvate enters a mitochondrion and is oxidized to a two-carbon acetyl group and CO2 is removed; this reaction occurs twice per glucose molecule.
c. The citric acid cycle:
i. occurs in the matrix of the mitochondrion and produces NADH and FADH2;
ii. is a series of reactions that gives off CO2 and produces one ATP;
iii. turns twice because two acetyl-CoA molecules enter the cycle per glucose molecule;
iv. produces two immediate ATP molecules per glucose molecule.
d. The electron transport chain:
i. is a series of carriers in the inner mitochondrial membrane that accept electrons from glucose--electrons are passed from carrier to carrier until received by oxygen;
ii. passes electrons from higher to lower energy states, allowing energy to be released and stored for ATP production;
iii. accounts for 32 or 34 ATP, depending on certain cell conditions.
2. Pyruvate is a pivotal metabolite in cellular respiration.
a. If O2 is not available to the cell, fermentation, an anaerobic process, occurs in the cytoplasm.
b. During fermentation, glucose is incompletely metabolized to lactate, or to CO2 and alcohol (depending on the organism).
c. Fermentation results in a net gain of only two ATP per glucose molecule.
8.2 Outside the Mitochondria: Glycolysis
1. Glycolysis occurs in the cytoplasm outside the mitochondria.
2. Glycolysis is the breakdown of glucose into two pyruvate molecules.
3. Glycolysis is universally found in organisms; therefore, it likely evolved before the citric acid cycle and electron transport chain.
B. Energy-Investment Steps
1. Glycolysis begins with the activation of glucose with two ATP; the glucose splits into two C3 molecules known as G3P, each of which carries a phosphate group.
C. Energy-Harvesting Steps
1. Oxidation of G3P occurs by removal of electrons and hydrogen ions.
2. Two electrons and one hydrogen ion are accepted by NAD+, resulting in two NADH; later, when the NADH molecules pass two electrons to another electron carrier, they become NAD+ again.
3. The oxidation of G3P and subsequent substrates results in four high-energy phosphate groups, which are used to synthesize four ATP molecules; this process is called substrate-level phosphorylation.
4. Two of four ATP molecules produced are required to replace two ATP molecules used in the initial phosphorylation of glucose; therefore there is a net gain of two ATP from glycolysis.
5. Pyruvate enters a mitochondrion (if oxygen is available) and cellular respiration ensues.
6. If oxygen is not available, fermentation occurs and pyruvate undergoes reduction.
8.3 Inside the Mitochondria
1. The next reactions of cellular respiration involve the preparatoryreaction, the citric acid cycle, and the electron transport chain.
2. In these reactions, the pyruvate from glycolysis is broken down completely to CO2 and H2O.
3. CO2 and ATP are transported out of the mitochondria into the cytoplasm.
4. The H2O can remain in the mitochondria or within the cell, or it can enter the blood and be excreted by the kidneys.
5. A mitochondrion has a double membrane with an intermembrane space (between the outer and inner membrane).
6. Cristae are the inner folds of membrane that jut into the matrix, the innermost compartment of a mitochondrion that is filled with a gel-like fluid.
7. The prep reaction and citric acid cycle enzymes are in the matrix; the electron transport chain is in the cristae.
8. Most of the ATP produced in cellular respiration is produced in the mitochondria; therefore, mitochondria are often called the "powerhouses" of the cell.
B. Preparatory Reaction
1. The preparatory reaction connects glycolysis to the citric acid cycle.
2. In this reaction, pyruvate is converted to a two-carbon acetyl group, and is attached to coenzyme A, resulting in the compound acetyl-CoA.
3. This redox reaction removes electrons from pyruvate by a dehydrogenase enzyme, using NAD+ as a coenzyme.
4. This reaction occurs twice for each glucose molecule.
C. Citric Acid Cycle
1. The citric acid cycle occurs in the matrix of mitochondria.
2. The cycle is sometimes called the Krebs cycle, named for Sir Hans Krebs, who described the fundamentals of the reactions in the 1930s.
3. The cycle begins by the addition of a two-carbon acetyl group to a four-carbon molecule, forming a six-carbon citrate (citric acid) molecule.
4. In the subsequent reactions, at three different times two electrons and one hydrogen ion are accepted by NAD+, forming NADH.
5. At one time, two electrons and one hydrogen ion are accepted by FAD, forming FADH2.
6. NADH and FADH2 carry these electrons to the electron transport chain.
7. Some energy is released and is used to synthesize ATP by substrate-level phosphorylation.
8. One high-energy metabolite accepts a phosphate group and transfers it to convert ADP to ATP.
9. The citric acid cycle turns twice for each original glucose molecule.
10. The products of the citric acid cycle (per glucose molecule) are 4 CO2, 2 ATP, 6 NADH and 2 FADH2.
11. The six carbon atoms in the glucose molecule have now become the carbon atoms of six CO2 molecules, two from the prep reaction and four from the citric acid cycle.
D. The Electron Transport Chain
1. The electron transport chain is located in the cristae of mitochondria and consists of carriers that pass electrons successively from one to another.
2. Some of the protein carriers are cytochrome molecules, complex carbon rings with a heme (iron) group in the center.
3. NADH and FADH2 carry the electrons to the electron transport system..
4. NADH gives up its electrons and becomes NAD+; the next carrier then gains electrons and is thereby reduced.
5. At each sequential redox reaction, energy is released to form ATP molecules.
6. Because O2 must be present for the proteins to work, this process is also called oxidative phosphorylation.
7. Oxygen serves as the terminal electron acceptor and combines with hydrogen ions to form water.
8. By the time electrons are received by O2, three ATP have been made.
9. When FADH2 delivers electrons to the electron transport system, two ATP are formed by the time the electrons are received by O2.
10. Coenzymes and ATP undergo recycling.
a. Cell needs a limited supply of coenzymes NAD+ and FAD because they constantly recycle.
b. Once NADH delivers electrons to the electron transport chain, it can accept more hydrogen atoms.
c. ADP and phosphate also recycle.
d. Efficiency of recycling NAD+, FAD, and ADP eliminates the need to continuously synthesize them anew.
E. The Cristae of a Mitochondrion
1. The electron transport chain consists of three protein complexes and two protein mobile carriers that transport electrons.
2. The three protein complexes include NADH-Q reductase complex, the cytochrome reductasecomplex, and the cytochrome oxidase complex; the two protein mobile carriers are coenzyme Q and cytochrome c.
3. Energy released from the flow of electrons down the electron transport chain is used to pump H+ ions, which are carried by NADH and FADH2, into intermembrane space.
4. Accumulation of H+ ions in this intermembrane space creates a strong electrochemical gradient.
5. ATP synthase complexes are channel proteins that serve as enzymes for ATP synthesis.
6. As H+ ions flow from high to low concentration, ATP synthase synthesizes ATP by the reaction: ADP + P = ATP.
7. Chemiosmosis is the term used for ATP production tied to an electrochemical (H+) gradient across a membrane.
8. Respiratory poisons confirm the chemiosmotic nature of ATP synthesis (i.e., a poison that inhibits ATP synthesis increases the H+ gradient).
9. Once formed, ATP molecules diffuse out of the mitochondrial matrix through channel proteins.
10. ATP is the energy currency for all living things; all organisms must continuously produce high levels of ATP to survive.
F. Energy Yield From Glucose Metabolism
1. Substrate-Level Phosphorylation
a. Per glucose molecule, there is a net gain of two ATP from glycolysis in cytoplasm.
b. The citric acid cycle in the matrix of the mitochondria produces two ATP per glucose.
c. Thus, a total of four ATP are formed by substrate-level phosphorylation outside of the electron transport chain.
2. Electron Transport Chain and Chemiosmosis
a. Most of the ATP is produced by the electron transport chain and chemiosmosis.
b. Per glucose, ten NADH and two FADH2 molecules provide electrons and H+ ions to the electron transport chain.
c. For each NADH formed within the mitochondrion, three ATP are produced.
d. For each FADH2 formed by the citric acid cycle, two ATP are produced.
e. For each NADH formed outside mitochondria by glycolysis, two ATP are produced as electrons are shuttled across the mitochondrial membrane by an organic molecule and delivered to FAD.
3. Efficiency of Cellular Respiration
a. The energy difference between total reactants (glucose and O2) and products (CO2 and H2O) is 686 kcal.
b. An ATP phosphate bond has an energy of 7.3 kcal; 36 to 38 ATP are produced during glucose breakdown for a total of at least 263 kcal.
c. This efficiency is 263/686, or 39% of the available energy in glucose is transferred to ATP; the rest of the energy is lost as heat.
8.4 Fermentation
1. Fermentation is an anaerobic (i.e., occurs in the absence of oxygen) process which consists of glycolysis plus reduction of pyruvate to either lactate or to alcohol and CO2 (depending on the organism).
2. NADH passes its electrons to pyruvate instead of to an electron transport chain; NAD+ is then free to return and pick up more electrons during earlier reactions of glycolysis.
3. Alcoholic fermentation, carried out by yeasts, produces carbon dioxide and ethyl alcohol; this process is used in the production of alcoholic spirits and breads.
4. Lactic acid fermentation, carried out by certain bacteria and fungi, produces lactic acid (lactate); this process is used commercially in the production of cheese, yogurt, and sauerkraut.
5. Other bacteria produce chemicals anaerobically, including isopropanol, butyric acid, proprionic acid, and acetic acid.
B. Advantages and Disadvantages of Fermentation
1. Despite a low yield of two ATP molecules, fermentation provides a quick burst of ATP energy for muscular activity.
2. Lactate is toxic to cells.
a. When blood cannot remove all lactate from muscles, lactate changes pH and causes muscles to fatigue.
b. The individual is in oxygen debt because oxygen is needed to restore ATP levels and rid the body of lactate.
c. Recovery occurs after lactate is sent to the liver where it is converted into pyruvate; some pyruvate is then respired or converted back into glucose.
C. Efficiency of Fermentation
1. Two ATP produced per glucose molecule during fermentation is equivalent to 14.6 kcal.
2. Complete glucose breakdown to CO2 and H2O during cellular respiration represents a potential yield of 686 kcal of energy.
3. Efficiency of fermentation is 14.6/686 or about 2.1%, far less efficient than complete breakdown of glucose.
8.5 Metabolic Pool
1. Degradative reactions (catabolism) break down molecules; they tend to be exergonic.
2. Synthetic reactions (anabolism) build molecules; they tend to be endergonic.
B. Catabolism
1. Just as glucose is broken down in cellular respiration, other molecules in the cell undergo catabolism.
2. Fat breaks down into glycerol and three fatty acids.
a. Glycerol is converted to G3P, a metabolite in glycolysis.
b. An 18-carbon fatty acid is converted to nine acetyl-CoA molecules that enter the citric acid cycle.
c. Respiration of fat products can produce 108 kcal in ATP molecules; fats are an efficient form of stored energy.
3. Amino acids break down into carbon chains and amino groups.
a. Hydrolysis of proteins results in amino acids.
b. R-group size determines whether carbon chain is oxidized in glycolysis or the citric acid cycle.
c. A carbon skeleton is produced in the liver by removal of the amino group, by the process of deamination.
d. The amino group becomes ammonia (NH3), which enters the urea cycle and ultimately becomes part of excreted urea.
e. The size of the R-group determines the number of carbons left after deamination.
C. Anabolism
1. ATP produced during catabolism drives anabolism.
2. Substrates making up pathways can be used as starting materials for synthetic reactions.
3. The molecules used for biosynthesis constitute the cell's metabolic pool.
4. Carbohydrates can result in fat synthesis: G3P converts to glycerol, acetyl groups join to form fatty acids.
5. Some metabolites can be converted to amino acids by transamination, the transfer of an amino acid group to an organic acid.
6. Plants synthesize all the amino acids they need; animals lack some enzymes needed to make some amino acids.
7. Humans synthesize 11 of 20 amino acids; the remaining 9 essential amino acids must be provided by the diet.
9.1 The Cell Cycle
1. The cell cycle is an orderly set of stages from the first division to the time the daughter cells divide.
2. When a cell is preparing for division, it grows larger, the number of organelles doubles, and the DNA replicates.
B. Interphase
1. Most of a cell's life is spent in interphase, in which the cell performs its usual functions.
2. Time spent in interphase varies by cell type: nerve and muscle cells do not complete the cell cycle and remain in the G0 stage while embryonic cells complete the cycle every few hours.
3. The G1 stage is just prior to DNA replication; a cell grows in size, organelles increase in number, and material accumulates for DNA synthesis.
4. The S stage is the DNA synthesis (replication) period; proteins associated with DNA are also synthesized; at the end of the S stage, each chromosome has two identical DNA double helix molecules, called sister chromatids.
5. The G2 stage occurs just prior to cell division; the cell synthesizes proteins needed for cell division, such as proteins in microtubules.
6. Interphase therefore consists of G1, S, and G2.
C. M (Mitotic) Stage
1. M stage (M = mitosis) is the entire cell division stage, including both mitosis and cytokinesis.
2. Mitosis is nuclear division, cytokinesis is division of the cytoplasm.
3. When division of the cytoplasm is complete, two daughter cells are produced.
D. Control of the Cell Cycle
1. The cell cycle is controlled by both internal and external signals.
2. A signal is a molecule that either stimulates or inhibits a metabolic event.
3. Growth factors are external signals received at the plasma membrane.
4. Cell Cycle Checkpoints
a. There appear to be three checkpoints where the cell cycle either stops or continues onward, depending on the internal signals it receives.
b. Researchers have identified a family of proteins called cyclins, internal signals that increase or decrease during the cell cycle.
c. Cyclin must be present for the cell to move from the G1 stage to the S stage, and from the G2 stage to the M stage.
d. The cell cycle stops at the G2 stage if DNA has not finished replicating; stopping the cell cycle at this stage allows time for repair of possible damaged DNA.
e. Also, the cycle stops if chromosomes are not distributed accurately to daughter cells.
f. DNA damage also stops the cycle at the G1 checkpoint by the protein p53; if the DNA is not repaired, p53 triggers apoptosis.
E. Apoptosis
1. Apoptosis is programmed cell death and involves a sequence of cellular events involving:
a. fragmenting of the nucleus,
b. blistering of the plasma membrane, and
c. engulfing of cell fragments by macrophages and/or neighboring cells.
2. Apoptosis is caused by enzymes called caspases.
3. Cells normally hold caspases in check with inhibitors.
4. Caspases are released by internal or external signals.
5. Apoptosis and cell division are balancing processes that maintain the normal level of somatic (body) cells.
6. Cell death is a normal and necessary part of development: frogs, for example, must destroy tail tissue they used as tadpoles, and the human embryo must eliminate webbing found between fingers and toes.
7. Death by apoptosis prevents a tumor from developing.
9.2 Mitosis and Cytokinesis
A. Eukaryotic Chromosomes
1. DNA in chromosomes of eukaryotic cells is associated with proteins; histone proteins organize chromosomes.
2. When a cell is not undergoing division, DNA in the nucleus is a tangled mass of threads called chromatin.
3. At cell division, chromatin becomes highly coiled and condensed and is now visible as individual chromosomes.
4. Each species has a characteristic number of chromosomes.
a. The diploid (2n) number includes two sets of chromosomes of each type.
i. The diploid number is found in all the non-sex cells of an organism's body (with a few exceptions).
ii. Examples include humans (46), crayfish (200), etc.
b. The haploid (n) number contains one of each kind of chromosome.
i. In the life cycle of many animals, only sperm and egg cells have the haploid number.
ii. Examples include humans (23), crayfish (100), etc.
5. Cell division in eukaryotes involves nuclear division and cytokinesis.
a. Somatic cells undergo mitosis for development, growth, and repair.
i. This nuclear division leaves the chromosome number constant.
ii. A 2n nucleus replicates and divides to provide daughter nuclei that are also 2n.
b. A chromosome begins cell division with two sister chromatids.
i. Sister chromatids are two strands of genetically identical chromosomes.
ii. At the beginning of cell division, they are attached at a centromere, a region of constriction on a chromosome.
B. Stages of Mitosis
1. The centrosome, the main microtubule organizing center of the cell, divides before mitosis begins.
2. Each centrosome contains a pair of barrel-shaped organelles called centrioles.
3. The mitotic spindle contains many fibers, each composed of a bundle of microtubules.
4. Microtubules are made of the protein tubulin.
a. Microtubules assemble when tubulin subunits join, disassemble when tubulin subunits become free, and form interconnected filaments of cytoskeleton.
b. Microtubules disassemble as spindle fibers form.
5. Mitosis is divided into five phases: prophase, prometaphase, metaphase, anaphase, and telophase.
6. Prophase
a. Nuclear division is about to occur: chromatin condenses and chromosomes become visible.
b. The nucleolus disappears and the nuclear envelope fragments.
c. Duplicated chromosomes are composed of two sister chromatids held together by a centromere; chromosomes have no particular orientation in the cell at this time.
d. The spindle begins to assemble as pairs of centrosomes migrate away from each other.
e. An array of microtubules called asters radiates toward the plasma membrane from the centrosomes.
7. Prometaphase (Late Prophase)
a. Specialized protein complexes (kinetochores) develop on each side of the centromere for future chromosome orientation.
b. An important event during prometaphase is attachment of the chromosomes to the spindle and their movement as they align at the metaphase plate (equator) of the spindle.
c. The kinetochores of sister chromatids capture kinetochore spindle fibers.
d. Chromosomes move back and forth toward alignment at the metaphase plate.
8. Metaphase
a. Chromosomes, attached to kinetochore fibers, are now aligned at the metaphase plate.
b. Non-attached spindle fibers, called polar spindle fibers, can reach beyond the metaphase plate and overlap.
9. Anaphase
a. The two sister chromatids of each duplicated chromosome separate at the centromere.
b. Daughter chromosomes, each with a centromere and single chromatid, move to opposite poles.
i. Polar spindle fibers lengthen as they slide past each other.
ii. Kinetochore spindle fibers disassemble at the kinetochores; this pulls daughter chromosomes to poles.
iii. The motor molecules kinesin and dynein are involved in this sliding process.
iv. Anaphase is the shortest stage of mitosis.
10. Telophase
a. Spindle disappears in this stage.
b. The nuclear envelope reforms around the daughter chromosomes.
c. The daughter chromosomes diffuse, again forming chromatin.
d. The nucleolus reappears in each daughter nucleus.
C. Cytokinesis in Animal and Plant Cells
1. Cytokinesis in Animal Cells
a. A cleavage furrow indents the plasma membrane between the two daughter nuclei at a midpoint; this deepens to divide the cytoplasm during cell division.
b. Cytoplasmic cleavage begins as anaphase draws to a close and organelles are distributed.
c. The cleavage furrow deepens as a band of actin filaments, called the contractile ring, constricts between the two daughter cells.
d. A narrow bridge exists between daughter cells during telophase until constriction completely separates the cytoplasm.
2. Cytokinesis in Plant Cells
a. The rigid cell wall that surrounds plant cells does not permit cytokinesis by furrowing.
b. The Golgi apparatus produces vesicles, which move along the microtubules to a small flattened disc that has formed.
c. Vesicles fuse forming a cell plate; their membranes complete the plasma membranes of the daughter cells.
d. The new membrane also releases molecules from the new plant cell walls; the cell walls are strengthened by the addition of cellulose fibrils.
D. The Functions of Mitosis
1. Mitosis permits growth and repair.
2. In flowering plants, the meristematic tissue retains the ability to divide throughout the life of the plant; this accounts for the continued growth, both in height and laterally, of a plant.
3. In mammals, mitosis is necessary as a fertilized egg becomes an embryo and as the embryo becomes a fetus; throughout life, mitosis allows a cut to heal or a broken bone to mend.
E. Stem Cells
1. Many mammalian organs contain stem cells (or adult stem cells), which retain the ability to divide.
2. Red bone marrow stem cells repeatedly divide to produce the various types of blood cells.
3. Therapeutic cloning to produce human tissues can begin with either adult stem cells or embryonic stem cells.
4. Embryonic stem cells can be used for reproductive cloning, the production of a new individual.
9.3 The Cell Cycle and Cancer
1. A neoplasm is an abnormal growth of cells.
2. A benign neoplasm is not cancerous; a malignant neoplasm is cancerous.
3. Cancer is a cellular growth disorder that results from the mutation of genes that regulate the cell cycle; i.e., cancer results from the loss of control and a disruption of the cell cycle.
4. Carcinogenesis, the development of cancer is gradual—it may take decades before a cell has the characteristics of a cancer cell.
B. Characteristics of Cancer Cells
1. Cancer cells lack differentiation.
a. Unlike normal cells that differentiate into muscle or nerves cells, cancer cells have an abnormal form and are nonspecialized.
b. Normal cells enter the cell cycle only about 50 times; cancer cells are immortal in that they can enter the cell cycle repeatedly.
2. Cancer cells have abnormal nuclei.
a. The nuclei may be enlarged and may have an abnormal number of chromosomes.
b. The chromosomes have mutated; some chromosomes may be duplicated or deleted.
c. Gene amplification, extra copies of genes, is more frequent in cancerous cells.
d. Whereas ordinary cells with DNA damage undergo apoptosis, cancer cells do not.
3. Cancer cells form tumors.
a. Normal cells are anchored and stop dividing when in contact with other cells; i.e., they exhibit contact inhibition.
b. Cancer cells invade and destroy normal tissue and their growth is not inhibited.
c. Cancer cells pile on top of each other to form a tumor.
4. Cancer cells undergo metastasis and angiogenesis.
a. A benign tumor is encapsulated and does not invade adjacent tissue.
b. Cancer in situ is a tumor in its place of origin but is not encapsulated—it will invade surrounding tissues.
c. Many types of cancer can undergo metastasis, in which new tumors form which are distant from the primary tumor.
d. Angiogenesis, the formation of new blood vessels, is required to bring nutrients and oxygen to the tumor.
e. A cancer patient's prognosis depends on whether the tumor has invaded surrounding tissue, whether there is lymph node involvement, and whether there are metastatic tumors elsewhere in the body.
C. Origin of Cancer
1. A DNA repair system corrects mutations during replication; mutations in genes encoding the various repair enzymes can cause cancer.
2. Proto-oncogenes specify proteins that stimulate the cell cycle while tumor-suppressor genes specify proteins that inhibit the cell cycle; mutations of either of these genes can cause cancer.
3. DNA segments called telomeres form the ends of chromosomes and shorten with each replication, eventually signaling the cell to end division; cancer cells produce telomerase that keeps telomeres at a constant length and thus the cells to continue dividing.
D. Regulation of the Cell Cycle
1. Proto-oncogenes are at the end of a stimulatory pathway from the plasma membrane to the nucleus; a growth factor binding at the plasma membrane can result in turning on an oncogene.
2. Tumor-suppressor genes are at the end of an inhibitory pathway; a growth-inhibitory factor can result in turning on a tumor suppressor gene that inhibits the cell cycle.
3. The balance between stimulatory and inhibitory signals determines whether proto-oncogenes or tumor-suppressor genes are active, and therefore whether or not cell division occurs.
E. Oncogenes
1. Proto-oncogenes can undergo mutation to become oncogenes (cancer-causing genes).
2. An oncogene may code for a faulty receptor in the stimulatory pathway, or,
3. An oncogene can specify an abnormal protein product or abnormally high levels of a normal product that stimulates the cell cycle.
4. About 100 oncogenes have been described; the ras gene family includes variants associated with lung, colon, pancreatic cancers as well as leukemias, lymphomas, and thyroid cancers; the BRCA1gene is associated with certain forms of breast and ovarian cancer.
F. Tumor-suppressor Genes
1. Mutation of a tumor-suppressor gene results in unregulated cell growth.
2. Researchers have identified about a half dozen tumor-suppressor genes.
3. The RB tumor-suppressor gene prevents retinoblastoma, a cancer of the retina, and has been found to malfunction in cancers of the breast, prostate, bladder, and small-cell lung carcinoma.
4. The p53 tumor-suppressor gene is more frequently mutated in human cancers than any other known gene; it normally functions to trigger cell cycle inhibitors and stimulate apoptosis.
9.4 Prokaryotic Cell Division
1. Unicellular organisms reproduce via asexual reproduction, in which the offspring are genetically identical to the parent.
B. The Prokaryotic Chromosome
1. Prokaryotic cells (bacteria and archaea) lack a nucleus and other membranous organelles.
2. The prokaryotic chromosome is composed of DNA and associated proteins, but much less protein than eukaryotic chromosomes.
3. The chromosome appears as a nucleoid, an irregular-shaped region that is not enclosed by a membrane.
4. The chromosome is a circular loop attached to the inside of the plasma membrane; it is about 1,000 times the length of the cell.
C. Binary Fission
1. Binary fission of prokaryotic cells produces two genetically identical daughter cells.
2. Before cell division, DNA is replicated--both chromosomes are attached to a special site inside the plasma membrane.
3. The two chromosomes separate as a cell lengthens and pulls them apart.
4. When the cell is approximately twice its original length, the plasma membrane grows inward, a septum (consisting of new cell wall and plasma membrane) forms, dividing the cell into two daughter cells.
5. The generation time of bacteria depends on the species and environmental conditions; Escherichia coli's generation time is about 20 minutes.
D. Comparing Prokaryotes and Eukaryotes
1. Both binary fission and mitosis ensure that each daughter cell is genetically identical to the parent.
2. Bacteria and protists use asexual reproduction to produce identical offspring.
3. In multicellular fungi, plants, and animals, cell division is part of the growth process that produces and repairs the organism.
4. Prokaryotes have a single chromosome with mostly DNA and some associated protein; there is no spindle apparatus.
5. Eukaryotic cells have chromosomes with DNA and many associated proteins; histone proteins organize the chromosome.
6. The spindle is involved in distributing the daughter chromosomes to the daughter nuclei.
11.1 Gregor Mendel
1. Mendel was an Austrian monk.
2. Mendel formulated two fundamental laws of heredity in the early 1860s.
3. He had previously studied science and mathematics at the University of Vienna.
4. At time of his research, he was a substitute science teacher at a local technical high school.
5. Prior to Mendel's work, investigators had been trying to support a "blending" concept of inheritance.
B. Blending Concept of Inheritance
1. This theory stated that offspring would have traits intermediate between those of the parents.
2. Red and white flowers produce pink flowers; any return to red or white offspring was considered instability in the genetic material.
3. Charles Darwin wanted to develop a theory of evolution based on hereditary principles; blending theory was of no help.
a. A blending theory did not account for variation (differences) and could not explain species diversity.
b. The particulate theory of inheritance proposed by Mendel can account for presence of differences among members of a population generation after generation.
c. Mendel's work was unrecognized until 1900; Darwin was never able to use it to support his theory of evolution.
C. Mendel's Experimental Procedure
1. Because Mendel had a mathematical background, he used a statistical basis for his breeding experiments.
2. Mendel prepared his experiments carefully and conducted preliminary studies.
a. He chose the garden pea, Pisum sativum, because peas were easy to cultivate, had a short generation time, and could be cross-pollinated by hand.
b. From many varieties, Mendel chose 22 true-breeding varieties for his experiments.
c. True-breeding varieties had all offspring like the parents and like each other.
d. Mendel studied simple traits (e.g., seed shape and color, flower color, etc.).
3. Mendel traced inheritance of individual traits and kept careful records of numbers.
4. He used his understanding of mathematical principles of probability to interpret results.
5. He arrived at a particulate theory of inheritance because it is based on the existence of minute particles—now called genes.
11.2 Mendel's Law of Segregation
1. Mendel confirmed that his tall plants always had tall offspring, i.e., were true-breeding, before crossing two different strains that differed in only one trait—this is called a monohybrid cross.
2. A monohybrid cross is between two parent organisms true-breeding for two distinct forms of one trait.
3. Mendel tracked each trait through two generations.
a. P generation is the parental generation in a breeding experiment.
b. F1 generation is the first-generation offspring in a breeding experiment.
c. F2 generation is the second-generation offspring in a breeding experiment.
4. He performed reciprocal crosses, i.e. pollen of tall plant to stigma of short plant and viceversa.
5. His results were contrary to those predicted by a blending theory of inheritance.
6. He found that the F1 plants resembled only one of the parents.
7. Characteristics of other parent reappeared in about 1/4 of F2 plants; 3/4 of offspring resembled the F1 plants.
8. Mendel saw that these 3:1 results were possible if:
a. F1 hybrids contained two factors for each trait, one being dominant and the other recessive;
b. factors separated when gametes were formed; a gamete carried one copy of each factor;
c. and random fusion of all possible gametes occurred upon fertilization.
9. Results of his experiments led Mendel to develop his first law of inheritance—the law of segregation:
a. Each organism contains two factors for each trait.
b. Factors segregate in the formation of gametes.
c. Each gamete contains one factor for each trait.
d. Fertilization gives each new individual two factors for each trait.
B. As Viewed by Modern Genetics
1. Each trait in a pea plant is controlled by two alleles, alternate forms of a gene that occur at the same gene locus on homologous chromosomes.
2. A dominant allele masks or hides expression of a recessive allele; it is represented by an uppercase letter.
3. A recessive allele is an allele that exerts its effect only in the homozygous state; its expression is masked by a dominant allele; it is represented by a lowercase letter.
4. The gene locus is the specific location of alleles on homologous chromosomes.
5. The process of meiosis explains Mendel's law of segregation.
6. In Mendel's cross, the parents were true-breeding; each parent had two identical alleles for a trait–they were homozygous, indicating they possess two identical alleles for a trait.
7. Homozygous dominant genotypes possess two dominant alleles for a trait.
8. Homozygous recessive genotypes possess two recessive alleles for a trait.
9. After cross-pollination, all individuals of the F1 generation had one of each type of allele.
10. Heterozygous genotypes possess one of each allele for a particular trait.
11. The allele not expressed in a heterozygote is a recessive allele.
C. Genotype Versus Phenotype
1. Two organisms with different allele combinations can have the same outward appearance (e.g., TT and Tt pea plants are both tall; therefore, it is necessary to distinguish between alleles present and the appearance of the organism).
2. Genotype refers to the alleles an individual receives at fertilization (dominant, recessive).
3. Phenotype refers to the physical appearance of the individual (tall, short, etc.).
D. One-trait Genetics Problems
1. First determine which characteristic is dominant; then code the alleles involved.
2. Determine the genotype and gametes for both parents; an individual has two alleles for each trait; each gamete has only one allele for each trait.
3. Each gamete is haploid; each has a 50% chance of receiving either allele.
E. Laws of Probability
1. Probability is the likely outcome a given event will occur from random chance.
a. For example, with every coin flip there is a 50% chance of heads and 50% chance of tails.
b. Chance of inheriting one of either two alleles from a parent is also 50%.
2. The multiplicative law of probability states that the chance of two or more independent events occurring together is the product of the probability of the events occurring separately.
a. The chance of inheriting a specific allele from one parent and a specific allele from another is ½ x ½ or 1/4.
b. Possible combinations for the alleles Ee of heterozygous parents are the following: EE = ½ x ½ = 1/4 eE = ½ x ½ = 1/4 Ee = ½ x ½ = 1/4 ee = ½ x ½ = 1/4
3. The additive law of probability calculates the probability of an event that occurs in two or more independent ways; it is the sum of individual probabilities of each way an event can occur; in the above example where unattached earlobes are dominant (EE,Ee, and eE), the chance for unattached earlobes is 1/4 + 1/4 + 1/4 = 3/4.
F. The Punnett Square
1. The Punnett square was introduced by R. C. Punnett (early 1900s) and provides a simple method to calculate the probable results of a genetic cross.
2. In a Punnett square, all possible types of sperm alleles are lined up vertically and all possible egg alleles are lined up horizontally; every possible combination is placed in squares.
3. The larger the sample size examined, the more likely the outcome will reflect predicted ratios; a large number of offspring must be counted to observe the expected results; only in that way can all possible genetic types of sperm fertilize all possible types of eggs.
4. Specific crosses in humans cannot be done in order to count many offspring; therefore in humans, the phenotypic ratio is used to estimate the probability of any child having a particular characteristic.
5. Punnett square uses laws of probability; it does not dictate what the next child will inherit.
6. "Chance has no memory": if two heterozygous parents have a first child with attached earlobes (likely in 1/4th of children), a second child born still has 1/4 chance of having attached earlobes.
G. One-Trait Testcross
1. To confirm that the F1 was heterozygous, Mendel crossed his F1 plants with homozygous recessive plants.
2. Results indicated the recessive factor was present in the F1 plants; they were thus heterozygous.
3. A monohybrid testcross is used between an individual with dominant phenotype and an individual with a recessive phenotype to see if the individual with dominant phenotype is homozygous or heterozygous.
11.3 Mendel's Law of Independent Assortment
1. This two-trait (dihybrid) cross is between two parent organisms that are true-breeding for different forms of two traits; it produces offspring heterozygous for both traits.
2. Mendel observed that the F1 individuals were dominant in both traits.
3. 3.. He further noted four phenotypes among F2 offspring; he deduced second law of heredity.
4. Mendel's law of independent assortment states that members of one pair of factors assort independently of members of another pair, and that all combinations of factors occur in gametes.
B. Two-trait Genetics Problems
1. Laws of probability indicate a 9:3:3:1 phenotypic ratio of F2 offspring resulting in the following:
a. 9/16 of the offspring are dominant for both traits;
b. 3/16 of the offspring are dominant for one trait and recessive for the other trait;
c. 3/16 of the offspring are dominant and recessive opposite of the previous proportions; and
d. 1/16 of the offspring are recessive for both traits.
2. The Punnett Square for two-trait crosses
a. A larger Punnett square is used to calculate probable results of this cross.
b. A phenotypic ratio of 9:3:3:1 is expected when heterozygotes for two traits are crossed and simple dominance is present for both genes.
c. Independent assortment during meiosis explains these results.
C. Two-Trait Testcross
1. A two-trait testcross tests if individuals showing two dominant characteristics are homozygous for both or for one trait only, or heterozygous for both.
2. If an organism heterozygous for two traits is crossed with another recessive for both traits, the expected phenotypic ratio is 1:1:1:1.
3. In dihybrid genetics problems, the individual has four alleles, two for each trait.
11.4 Human Genetic Disorders
A. Patterns of Inheritance
1. Genetic disorders are medical conditions caused by alleles inherited from parents.
2. An autosome is any chromosome other than a sex (X or Y) chromosome.
3. In a pedigree chart, males are designated by squares, females by circles; shaded circles and squares are affected individuals; line between square and circle represents a union; vertical line leads to offspring.
4. A carrier is a heterozygous individual with no apparent abnormality but able to pass on an allele for a recessively-inherited genetic disorder.
5. Autosomal dominant and autosomal recessive alleles have different patterns of inheritance.
a. Characteristics of autosomal dominant disorders
i. Affected children usually have an affected parent.
ii. Heterozygotes are affected.: two affected parents can produce unaffected child; two unaffected parents will not have affected children.
b. Characteristics of autosomal recessive disorders
i. Most affected children have normal parents since heterozygotes have a normal phenotype.
ii. Two affected parents always produce an affected child.
iii. Close relatives who reproduce together are more likely to have affected children.
B. Autosomal Recessive Disorders
1. Tay-Sachs Disease
a. Usually occurs among Jewish people in the U.S. of central and eastern European descent.
b. Symptoms are not initially apparent; infant's development begins to slow between four to eight months, neurological and psychomotor difficulties become apparent, child gradually becomes blind and helpless, develops seizures, eventually becomes paralyzed and dies by age of three or four.
c. This results from lack of enzyme hexosaminidase A (Hex A) and the subsequent storage of its substrate, glycosphingolipid, in lysosomes.
d. Primary sites of storage are cells of the brain; accounts for progressive deterioration.
e. There is no treatment or cure.
f. Prenatal diagnosis is possible by amniocentesis or chorionic villi sampling.
g. The gene is located on chromosome 15.
2. Cystic Fibrosis
a. This is the most common lethal genetic disease in Caucasians in the U.S.
b. About 1 in 20 Caucasians is a carrier, and about 1 in 3,000 newborns has this disorder.
c. An increased production of a viscous form of mucus in the lungs and pancreatic ducts is seen.
i. The resultant accumulation of mucus in the respiratory tract interferes with gas exchange.
ii. Digestive enzymes must be mixed with food to supplant the pancreatic juices.
d. New treatments have raised the average life expectancy to up to 35 years.
e. Chloride ions (Cl–) fail to pass plasma membrane proteins.
f. Since water normally follows Cl–, lack of water in the lungs causes thick mucus.
g. The cause is a gene on chromosome 7; attempts to insert the gene into nasal epithelium has had little success.
h. Genetic testing for adult carriers and fetuses is possible.
3. Phenylketonuria (PKU)
a. PKU occurs once in every 5,000 births; it is the most common inherited disease of the nervous system.
b. It is caused by a lack of an enzyme needed to metabolize amino acid phenylalanine; this results in accumulation of the amino acid in nerve cells of the brain and impairs nervous system development.
c. PKU is caused by a gene on chromosome 12.
d. Newborns are routinely tested in the hospital for high levels of phenylalanine in the blood.
e. If an infant has PKU, the child is placed on a diet low in phenylalanine until the brain is fully developed, near age seven.
4. Sickle-Cell Disease
a. This disease is the most common inherited disorder in blacks, affecting about 1 in 500 African Americans.
b. The gene is on chromosome 11.
c. In affected individuals, the red blood cells are shaped like sickles—an abnormal hemoglobin molecule, Hbs, causes the defect.
i. Normal hemoglobin, HbA, differs from Hbs by one amino acid in the protein globin.
d. The disease is an example of pleiotropy, describing a gene that affects more than one characteristic of an individual.
e. Sickling of the red blood cells occurs when the oxygen content of the person's blood is low, thereby slowing down blood flow and clogging small vessels.
f. Signs and symptoms include anemia, weakness, fever, pain, rheumatism, low resistance to disease, kidney and heart failure.
g. Treatment includes pain management, blood transfusions, and bone marrow transplants.
h. The disease can be diagnosed prenatally.
i. Individuals with the sickle cell trait (carriers), who normally do not have any sickle-shaped cells unless they experience dehydration or mild oxygen deprivation, are resistant to the disease malaria.
C. Autosomal Dominant Disorders
1. Neurofibromatosis
a. This is an autosomal dominant disorder that affects one in 3,500 newborns and is distributed equally around the world.
b. Affected individuals have tan skin spots at birth, which develop into benign tumors.
c. Neurofibromas are lumps under the skin comprised of fibrous coverings of nerves.
d. In most cases, symptoms are mild and patients live a normal life; sometimes symptoms are severe:
i. skeletal deformities, including a large head;
ii. eye and ear tumors that can lead to blindness and hearing loss; and
iii. learning disabilities and hyperactivity.
iv. Such variation is called variable expressivity.
e. The gene that codes for neurofibromatosis was discovered in 1990 to be on chromosome 17.
i. The gene controls production of neurofibromin protein that normally blocks growth signals for cell division.
ii. Many types of mutations result in this effect.
iii. Some mutations are caused by a gene that moves from another location in the genome.
2. Huntington Disease
a. This leads to progressive degeneration of brain cells, which in turn causes severe muscle spasm, personality disorders, and death in 10–15 years after onset.
b. Most appear normal until they are of middle age and already have had children who might carry the gene; occasionally, first signs of the disease are seen in teenagers or even younger.
c. The gene for Huntington disease is located on chromosome 4.
d. This gene contains many repeats of a base triplet that codes for glutamine in the huntingtin protein; normal persons have 10–15 glutamines; affected persons have 36 or more.
e. A huntingtin protein with over 36 glutamines changes shape and forms large clumps inside neurons; it also attracts other proteins to clump with it.
3. Achondroplasia
a. This disease is a common form of dwarfism, associated with a defect in the growth of long bones.
b. Affected individuals have short arms and legs, a sway back, and a normal torso and head.
c. About 1 in 25,000 people have the disease.
d. Individuals with the disease are heterozygotes (Aa); the homozygous recessive (aa) condition yields normal-length limbs, while the homozygous dominant (AA) condition is lethal.
11.5 Beyond Mendelian Genetics
A. Incomplete Dominance
1. Incomplete dominance: offspring show traits intermediate between two parental phenotypes.
a. True-breeding red and white-flowered four-o'clocks produce pink-flowered offspring.
b. Incomplete dominance has a biochemical basis; the level of gene-directed protein production may be between that of the two homozygotes.
c. One allele of a heterozygous pair only partially dominates expression of its partner.
d. This does not support a blending theory; parental phenotypes reappear in F2 generation.
B. Human Examples of Incomplete Dominance
1. Curly versus Straight Hair
2. A curly-haired Caucasian and a straight-haired Caucasian will have wavy-haired offspring.
3. Two wavy-haired parents will produce a 1:2:1 ratio of curly-wavy-straight hair children.
4. Sickle-cell disease, Tay Sachs disease, and cystic fibrosis are considered examples of incomplete dominance.
C. Multiple Allelic Traits
1. This occurs when a gene has many allelic forms or alternative expressions.
2. ABO Blood Types
a. The ABO system of human blood types is a multiple allele system.
b. Two dominant alleles (IAand IB) code for presence of A and B glycoproteins on red blood cells.
c. This also includes a recessive allele (i°) coding for no A or B glycoproteins on red blood cells.
d. As a result, there are four possible phenotypes (blood types): A, B, AB, and O
e. This is a case of codominance, where both alleles are fully expressed.
3. The Rh factor is inherited independently from the ABO system; the Rh+ allele is dominant.
D. Polygenic Inheritance
1. Polygenic inheritance occurs when one trait is governed by two or more sets of alleles.
2. Dominant alleles have a quantitative effect on the phenotype: each adds to the effect.
3. The more genes involved, the more continuous is the variation in phenotypes, resulting in a bell-shaped curve.
4. Crosses of white and dark-red wheat seeds produce seeds with seven degrees of intermediate colors due to genes at three separate loci.
5. Human Examples of Polygenic Inheritance
a. A hybrid cross for skin color provides a range of intermediates.
b. Parents with intermediate skin color can produce children with the full range of skin colors.
c. Albinism, where one gene interferes with the expression of others, is an example of epistasis.
E. Polygenic Disorders
1. This includes cleft lip, clubfoot, congenital dislocations of the hip, hypertension, diabetes, schizophrenia, allergies and cancers.
2. Behavioral traits including suicide, phobias, alcoholism, and homosexuality may be associated with particular genes but are not likely completely predetermined.
3. Environment and the Phenotype
a. In water buttercups, the aquatic environment dramatically influences the structure of the plant.
b. Temperature triggers a primrose to develop white flowers when grown above 32°C and red flowers when grown at 24°C.
c. The coats of Siamese cats and Himalayan rabbits have darker tipped ears, nose, paws, etc. due to the enzyme encoded by an allele which is only active at the extremities at low temperatures.
F. Environment and the Phenotype
1. Both genotype and the environment affect the phenotype.
2. Water and temperature can have profound influence on the phenotype.
a. A flower might be one color at one temperature and another color at another temperature.
b. The coat color of certain animals can change with temperature.
12.1 Chromosomal Inheritance
1. Genes are located on chromosomes; this is called the chromosome theory of inheritance.
2. Chromosomes can be categorized as two types:
a. Autosomes are non-sex chromosomes that are the same number and kind between sexes.
b. Sex chromosomes determine if the individual is male or female.
3. Sex chromosomes in the human female are XX; those of the male are XY.
4. Males produce X-containing and Y-containing gametes; therefore males determine the sex of offspring.
5. Besides genes that determine sex, sex chromosomes carry many genes for traits unrelated to sex.
6. An X-linked gene is any gene located on X chromosome; used to describe genes on X chromosome that are missing on the Y chromosome.
B. X-Linked Alleles
1. Work with fruit flies (Drosophila) by Thomas Hunt Morgan (early 1900s) confirmed genes were on chromosomes.
a. Fruit flies are easily and inexpensively raised in common laboratory glassware.
b. Females only mate once and lay hundreds of eggs.
c. The fruit fly generation time is short, allowing rapid experiments.
2. Fruit flies have an XY sex chromosome system similar to the human system; experiments can be correlated to the human situation.
a. Newly discovered mutant male fruit flies had white eyes.
b. Cross of the hybrids from the white-eyed male crossed with a dominant red-eyed female yielded the expected 3:1 red-to-white ratio; however, all of the white-eyed flies were males.
c. An allele for eye color on the X but not on the Y chromosome supports the results of this cross.
d. Behavior of this allele corresponds to the behavior of the chromosome; this confirmed the chromosomal theory of inheritance.
3. X-Linked Problems
a. X-linked alleles are designated as superscripts to the X chromosome.
b. Heterozygous females are carriers; they do not show the trait but can transmit it.
c. Males are never carriers but express the one allele on the X chromosome; the allele could be dominant or recessive.
d. One form of color-blindness is X-linked recessive.
C. Human X-Linked Disorders
1. More males have X-linked traits because recessive alleles on the X chromosome in males are expressed in males.
2. Color Blindness
a. Color blindness can be an X-linked recessive disorder involving mutations of genes coding for green or red sensitive cone cells, resulting in the inability to perceive green or red, respectively; the pigment for blue-sensitive protein is autosomal.
b. About 8% of Caucasian men have red-green color blindness.
3. Muscular Dystrophy
a. Duchenne muscular dystrophy is the most common form and is characterized by wasting away of muscles, eventually leading to death; it affects one out of every 3,600 male births.
b. This X-linked recessive disease involves a mutant gene that fails to produce the protein dystrophin.
c. Signs and symptoms (e.g., waddling gait, toe walking, frequent falls, difficulty in rising) soon appear.
d. Muscles weaken until the individual is confined to a wheelchair; death usually occurs by age 20.
e. Affected males are rarely fathers; the gene passes from carrier mother to carrier daughter.
f. Lack of dystrophin protein causes calcium ions to leak into muscle cells; this promotes action of an enzyme that dissolves muscle fibers.
g. As the body attempts to repair tissue, fibrous tissue forms and cuts off blood supply to the affected muscles.
h. A test now detects carriers of Duchenne muscular dystrophy; treatments are being attempted.
4. Hemophilia
a. About one in 10,000 males is a hemophiliac with impaired ability of blood to clot.
b. The two common types: Hemophilia A, due to the absence of clotting factor IX; Hemophilia B, due to the absence of clotting factor VIII.
c. Hemophiliacs bleed externally after an injury and also suffer internal bleeding around joints.
d. Hemorrhages stop with transfusions of blood (or plasma) or concentrates of clotting protein.
e. Factor VIII is now available as a genetically-engineered product.
f. Of Queen Victoria's 26 offspring, five grandsons had hemophilia and four granddaughters were carriers.
5. Fragile X Syndrome (See Health and Focus box)
a. In this case, the X chromosome is nearly broken; most often found in males.
b. This affects one in 1,500 males and one in 2,500 females.
c. As children, they are often hyperactive or autistic with delayed or repetitive speech.
d. As adults, males usually have larger testes, unusually protruding ears, and other symptoms.
e. About one-fifth of males with fragile X do not show symptoms.
f. Fragile X passes from a symptomless male carrier to grandson.
g. It has been traced to excessive repeats of base triplet CGG (cytosine-guanine-guanine); up to 230 copies compared to normal 6–to–50 copies.
12.2 Gene Linkage
1. Fruit flies have four pairs of chromosomes to hold thousands of genes; therefore each chromosome must hold many genes.
2. All alleles on one chromosome form a linkage group that are inherited together except when crossing over occurs.
3. Crossing-over causes recombinant gametes and at fertilization, recombinant phenotypes.
4. Linked alleles do not obey Mendel's laws because they tend to go into the gametes together.
5. A linkage map (or chromosome map) tells the relative distances between gene loci on a chromosome.
B. Constructing a Chromosome Map
1. The percentage of recombinant phenotypes, which is statistically related to the frequency of crossing over between specific loci, is used to measure the distance between genes.
2. Crosses involving linked genes do not give the same results as unlinked genes.
3. A heterozygote forms only two types of gametes and produces offspring with only two phenotypes.
C. Linkage Data
1. Linked genes indicate the distance between genes on the chromosomes.
2. If 1% of crossing-over equals one map unit, then 6% recombinants reveal 6 map units between genes.
3. If crosses are performed for three alleles on a chromosome, only one map order explains map units.
4. Humans have few offspring and a long generation time, and it is impossible to designate matings; therefore biochemical methods are used to map human chromosomes.
12.3 Changes in Chromosome Numbers
1. Chromosomal mutations are changes in chromosome number or structure.
2. Mutations, along with crossing-over, recombination of chromosomes during meiosis, and gamete fusion during fertilization, increase the amount of variation among offspring.
3. The correct number of chromosomes in a species is called euploidy; changes in chromosome number include polyploidy and aneuploidy.
B. Polyploidy
1. A polyploid is a eukaryote with three or more complete sets of chromosomes.
2. Polyploid organisms are named according to the number of sets of chromosomes they have: triploids (3n), tetraploids (4n), etc.
3. Polyploidy is not often seen in animals.
4. Polyploidy is a major evolutionary mechanism in plants; it is probably involved in 47% of flowering plants including some important crops (wheat, corn, fruits, etc.).
5. Polyploidy generally arises following hybridization (reproduction between two different species); a hybrid may have an odd number of chromosomes and thus be sterile, but if the chromosomes in the hybrid double in number, the now-even number of chromosomes can undergo synapsis during meiosis; successful polyploidy thus results in a new species.
C. Aneuploidy
1. Aneuploidy is the condition in which an organism gains or loses one or more chromosomes.
2. Monosomy (2n – 1) occurs when an individual has only one of a particular type of chromosome.
3. Trisomy (2n + 1)occurs when an individual has three of a particular type of chromosome.
4. Nondisjunction is the failure of chromosomes to separate at meiosis—both members of the homologous pair go into the same gamete.
5. Monosomy and trisomy occur in plants and animals; in autosomes of animals, it is generally lethal.
6. Trisomy 21 is the most common autosomal trisomy.
a. Trisomy 21 (also called Down syndrome) occurs when three copies of chromosome 21 are present.
b. Usually two copies of chromosome 21 are contributed by the egg; in 23% of the cases, the sperm had the extra chromosome 21.
c. Over 90% of individuals with Down syndrome have three copies of chromosome 21.
d. Chances of a woman having a Down syndrome child increase with age, starting at age 40.
e. Chorionic villi sampling testing or amniocentesis and karyotyping (see the Science Focus box for a detailed description of this procedure) detects a Down syndrome child; however, risks for young women exceed likelihood of detection.
f. A Down syndrome child has many characteristic signs and symptoms, including a tendency for leukemia, cataracts, faster aging, mental retardation, and an increased chance of developing Alzheimer disease later in life.
g. The Gart gene, located on the bottom third of chromosome 21, leads to a high level of purines and is associated with the signs and symptoms of Down syndrome; future research may lead to suppression of this gene.
D. Changes in Sex Chromosome Number
1. Nondisjunction during oogenesis can result in too few or too many X chromosomes; nondisjunction during spermatogenesis can result in missing or too many Y chromosomes.
2. Turner syndrome females have only one sex chromosome, an X; thus, they are XO, with O signifying the absence of a second sex chromosome.
a. Turner females are short, have a broad chest and folds of skin on back of neck.
b. Ovaries of Turner females never become functional; therefore, females do not undergo puberty.
c. They usually have normal intelligence and can lead fairly normal lives with hormone supplements.
3. Klinefelter syndrome males have one Y chromosome and two or more X chromosomes (e.g., XXY).
a. Affected individuals are sterile males; the testes and prostate are underdeveloped.
b. Individuals have large hands and feet, long arms and legs, and lack facial hair.
c. Presence of the Y chromosome drives male formation but more than two X chromosomes may result in mental retardation.
d. A Barr body, usually only seen in the nuclei of a female's cells, is seen in this syndrome due to the two X chromosomes.
4. Poly-X females (or superfemale) have three or more X chromosomes and therefore extra Barr bodies in the nucleus.
a. There is no increased femininity; most lack any physical abnormalities.
b. XXX individuals are not mentally retarded but may have delayed motor and language development; XXXX females are usually tall and severely mentally retarded.
c. Some experience menstrual irregularities but many menstruate regularly and are fertile.
5. Jacobs syndrome (XYY) are males with two Y chromosomes instead of one.
a. This results from nondisjunction during spermatogenesis.
b. Males are usually taller than average, suffer from persistent acne, and tend to have speech and reading problems.
c. Earlier claims that XYY individuals were likely to be aggressive were not correct.
12.4 Changes in Chromosome Structure
1. Environmental factors including radiation, chemicals, and viruses, can cause chromosomes to break; if the broken ends do not rejoin in the same pattern, this causes a change in chromosomal structure.
2. Deletion: a type of mutation in which an end of a chromosome breaks off or when two simultaneous breaks lead to the loss of a segment.
3. Translocation: a chromosomal segment is removed from one chromosome and inserted into another nonhomologous chromosome; in Down syndrome, 5% of cases are due to a translocation between chromosome 21 and 14, a situation that runs in the family of the father or mother.
4. Duplication: the presence of a chromosomal segment more than once on the same chromosome.
a. A broken segment from one chromosome can simply attach to its homologue or unequal crossing-over may occur.
b. A duplication may also involve an inversion where a segment that has become separated from the chromosome is reinserted at the same place but in reverse; the position and sequence of genes are altered.
B. Human Syndromes
1. Deletion Syndromes
a. Williams syndrome occurs when chromosome 7 loses an end piece: children look like pixies, have poor academic skills but good verbal and musical skills; lack of elastin causes cardiovascular problems and skin aging.
b. Cri du chat syndrome ("cry of the cat") is a deletion in which an individual has a small head, is mentally retarded, has facial abnormalities, and an abnormal glottis and larynx resulting in a cry resembling that of a cat.
2. Translocation Syndromes
a. If a translocation results in the normal amount of genetic material, the person will remain healthy; if a person inherits only one of the translocated chromosomes, that person may have only one allele or three alleles rather than the normal two.
b. In Alagille syndrome, chromosomes 2 and 20 exchange segments, causing a small deletion on chromosome 20 that may produce some abnormalities.
13.1 The Genetic Material
• Early researchers knew that the genetic material must be:
1. able to store information used to control both the development and the metabolic activities of cells;
2. stable so it can be replicated accurately during cell division and be transmitted for generations; and,
3. able to undergo mutations providing the genetic variability required for evolution.
• Previous Knowledge About DNA
1. Understanding the chemistry of DNA was essential to the discovery that DNA is genetic material.
2. Friedrich Miescher (1869) removed nuclei from pus cells and isolated DNA "nuclein"; it was rich in phosphorus and lacked sulfur.
3. Subsequent analysis of nuclein found that it contained an acidic substance: named it nucleic acid.
4. Two types of nucleic acids were soon discovered: DNA (deoxyribonucleic acid) and RNA (ribonucleic acid).
5. In the early twentieth century, it was shown that nucleic acids contain four types of nucleotides.
a. DNA was composed of repeating units, each of which always had just one of each of four different nucleotides (a nitrogenous base, a phosphate, and a pentose).
b. In this model, DNA could not vary between species and therefore could not be the genetic material; therefore some other protein component was expected to be the genetic material.
A. Transformation of Bacteria
1. Bacteriologist Frederick Griffith (1931) experimented with Streptococcus pneumoniae (a pneumococcus that causes pneumonia in mammals).
2. Mice were injected with two strains of pneumococcus: an encapsulated (S) strain and a non-encapsulated (R) strain.
a. The S strain is virulent (the mice died); it has a mucous capsule and forms "shiny" colonies.
b. The R strain is not virulent (the mice lived); it has no capsule and forms "dull" colonies.
3. In an effort to determine if the capsule alone was responsible for the virulence of the S strain, he injected mice with heat-killed S strain bacteria; the mice lived.
4. Finally, he injected mice with a mixture of heat-killed S strain and live R strain bacteria.
a. The mice died; living S strain pneumococcus were recovered from their bodies.
b. Griffith concluded that some substance necessary for synthesis of the capsule--and therefore for virulence--must pass from dead S strain bacteria to living R strain bacteria so the R strain were transformed.
c. This change in phenotype of the R strain must be due to a change in the bacterial genotype, suggesting that the transforming substance passed from S strain to R strain.
B. DNA: The Transforming Substance
1. Oswald Avery et al. (1944) reported that the transforming substance was DNA.
2. Purified DNA is capable of bringing about the transformation. Evidence:
a. DNA from S strain pneumococcus causes R strain bacteria to be transformed.
b. Enzymes that degrade proteins cannot prevent transformation, nor can enzymes that digest RNA.
c. Digestion of the transforming substance with enzyme that digests DNA prevents transformation.
d. The molecular weight of the transforming substance is great enough for some genetic variability.
3. Avery's experimental results demonstrated DNA is genetic material and DNA controls biosynthetic properties of a cell.
C. Transformation Experiments Today
1. Transformation experiments today are common in teaching and research labs.
2. Transformation occurs when organisms receive foreign DNA and thereby receive a new trait.
3. Studies with bacteria show some can take up DNA from the medium and thereby gain penicillin resistance.
D. Reproduction of Viruses
1. Bacteriophages are viruses that infect bacteria; they consist of a protein coat surrounding a nucleic acid.
2. Bacteriophage T2 is a virus that infects the Escherichia coli (E. coli), a species of bacteria that normally lives within the human gut.
3. Alfred Hershey and Martha Chase (1952) studied bacteriophage T2.
a. The purpose of their experiments was to see which of the bacteriophage components—the protein coat or the DNA—entered bacterial cells and directed reproduction of the virus.
b. They labeled the protein coat with 35S and the DNA with 32P.
c. Viral coats were removed from the bacterial cells and separated by centrifugation.
d. Results: radioactive 32P alone is taken up by bacterial host and incorporated in virus reproduction.
e. This result reinforced the notion that DNA (and not the protein) is the genetic material.
13.2 The Structure of DNA
A. Nucleotide Data
1. Erwin Chargaff (1940s) analyzed the base content of DNA.
2. It was known DNA contained four different nucleotides:
a. two with purine bases, adenine (A) and guanine(G); a purine is a type of nitrogen-containing base having a double-ring structure.
b. two with pyrimidine bases, thymine (T) and cytosine (C); a pyrimidine is a type of nitrogen-containing base having a single-ring structure.
3. Results: DNA does have the variability necessary for the genetic material, and,
4. For a species, DNA has the constancy required of genetic material.
5. This constancy is given in Chargaff's rules:
a. The amount of A, T, G, and C in DNA varies from species to species.
b. In each species, the amount of A = T and the amount of G = C (A +G = T +C).
6. The tetranucleotide hypothesis (proposing DNA was repeating units of one of four bases) was disproved: each species has its own constant base composition.
B. Variation in Base Sequence
1. The variability is staggering; a human chromosome contains about 140 million base pairs.
2. Since any of the four possible nucleotides can be present at each nucleotide position, the total number of possible nucleotide sequences is 4140 x 106 = 4140,000,000.
C. Diffraction Data
1. Rosalind Franklin produced X-ray diffraction photographs.
2. Franklin's work provided evidence that DNA had the following features:
a. DNA is a helix.
b. Some portion of the helix is repeated.
D. The Watson and Crick Model
1. American James Watson joined with Francis H. C. Crick in England to work on the structure of DNA.
2. Watson and Crick received the Nobel Prize in 1962 for their model of DNA.
3. Using information generated by Chargaff and Franklin, Watson and Crick constructed a model of DNA as a double helix with sugar-phosphate groups on the outside, and paired bases on the inside.
4. Their model was consistent with both Chargaff's rules and Franklin's X-ray diffraction studies.
5. Complementary base pairing is the paired relationship between purines and pyrimidines in DNA: A is hydrogen-bonded to T and G is hydrogen-bonded to C.
13.3 Replication of DNA
• DNA replication is the process of copying a DNA molecule. Replication is semiconservative, with each strand of the original double helix (parental molecule) serving as a template (mold or model) for a new strand in a daughter molecule. This process consists of:
1. Unwinding: old strands of the parent DNA molecule are unwound as weak hydrogen bonds between the paired bases are "unzipped" and broken by the enzyme helicase.
2. Complementary base pairing: free nucleotides present in the nucleus bind with complementary bases on unzipped portions of the two strands of DNA; this process is catalyzed by DNA polymerase.
3. Joining: complementary nucleotides bond to each other to form new strands; each daughter DNA molecule contains an old strand and a new strand; this process is also catalyzed by DNA polymerase.
4. DNA replication must occur before a cell can divide; in cancer, drugs with molecules similar to the four nucleotides are used to stop replication.
A. Replication is Semiconservative
1. DNA replication is semiconservative: each daughter double helix has one parental strand and one new strand.
2. Matthew Meselson and Franklin Stahl (1958) confirmed the process of DNA replication.
a. They grew bacteria in a medium with heavy nitrogen (15N), then switched to light nitrogen (14N).
b. The density of DNA following replication is intermediate as measured by centrifugation of molecules.
c. After one division, only "hybrid" DNA molecules were in the cells.
d. After two divisions, half the DNA molecules were "light" and half were "hybrid."
3. These were exactly the results to be expected if DNA replication is semiconservative.
B. Prokaryotic Versus Eukaryotic Replication
1. Prokaryotic Replication
a. Bacteria have a single loop of DNA that must replicate before the cell divides.
b. Replication in prokaryotes may be bidirectional from one point of origin or in only one direction.
c. Replication only proceeds in one direction, from 5' to 3'.
d. Bacterial cells are able to replicate their DNA at a rate of about 106 base pairs per minute.
e. Bacterial cells can complete DNA replication in 40 minutes; eukaryotes take hours.
2. Eukaryotic Replication
a. Replication in eukaryotes starts at many points of origin and spreads with many replication bubbles—places where the DNA strands are separating and replication is occurring.
b. Replication forks are the V-shape ends of the replication bubbles; the sites of DNA replication.
c. Eukaryotes replicate their DNA at a slower rate – 500 to 5,000 base pairs per minute.
d. Eukaryotes take hours to complete DNA replication.
C. Replication Errors
1. A genetic mutation is a permanent change in the sequence of bases.
2. Base changes during replication are one way mutations occur.
3. A mismatched nucleotide may occur once per 100,000 base pairs, causing a pause in replication.
4. Proofreading is the removal of a mismatched nucleotide; DNA repair enzymes perform this proofreading function and reduce the error rate to one per billion base pairs.
5. Incorrect base pairs that survive the proofreading process contribute to gene mutations.
14.1 The Function of Genes
1. Sir Archibald Garrod (early 1900s) introduced the phrase inborn error of metabolism.
a. Garrod proposed that inherited defects could be caused by the lack of a particular enzyme.
b. Knowing that enzymes are proteins, Garrod suggested a link between genes and proteins.
B. Genes Specify Enzymes
1. George Beadle and Edward Tatum (1940) X-rayed spores of the red bread mold, Neurospora crassa.
2. They observed that some resulting cultures lacked a particular enzyme for growth on minimal medium.
3. They found that a single gene was mutated, which resulted in the lack of a single enzyme.
4. They proposed the one gene–one enzyme hypothesis: one gene specifies the synthesis of one enzyme.
C. Genes Specify a Polypeptide
1. Linus Pauling and Harvey Itano (1949) compared hemoglobin in red blood cells of persons with sickle-cell disease and normal individuals.
2. They discovered that the chemical properties of a protein chain of sickle-cell hemoglobin differed from that of normal hemoglobin.
3. Vernon Ingram subsequently showed that the biochemical difference in the protein chain of sickle-cell hemoglobin is the substitution of a nonpolar amino acid valine for the negatively charged amino acid glutamate.
4. Pauling and Itano formulated the one gene–one polypeptide hypothesis: each gene specifies one polypeptide of a protein, a molecule that may contain one or more different polypeptides.
D. From DNA to RNA to Protein
1. A gene is a sequence of DNA nucleotide bases that codes for a sequence of nucleotides in an RNA molecule.
2. DNA is restricted to nucleus; protein synthesis occurs at ribosomes in the cytoplasm.
3. Ribonucleic acid (RNA) is found in both regions of the cell.
E. Types of RNA
1. Like DNA, RNA is a polymer of nucleotides.
2. Unlike DNA, RNA is single-stranded, contains the sugar ribose, and the base uracil instead of thymine (in addition to cytosine, guanine, and adenine).
3. There are three major classes of RNA.
a. Messenger RNA (mRNA) takes a message from DNA in the nucleus to ribosomes in the cytoplasm.
b. Ribosomal RNA (rRNA) and proteins make up ribosomes where proteins are synthesized.
c. Transfer RNA (tRNA) transfers a particular amino acid to a ribosome.
F. Gene Expression
1. DNA undergoes transcription to mRNA, which is translated to a protein.
2. DNA is a template for RNA formation during transcription.
3. Transcription is the first step in gene expression; it is the process whereby a DNA strand serves as a template for the formation of mRNA.
4. During translation, an mRNA transcript directs the sequence of amino acids in a polypeptide.
14.2 The Genetic Code
1. The central dogma of molecular biology states that the sequence of nucleotides in DNA specifies the order of amino acids in a polypeptide.
2. The genetic code is a triplet code, comprised of three-base code words (e.g., AUG).
3. A codon consists of 3 nucleotide bases of DNA.
4. Four nucleotides based on 3-unit codons allows up to 64 different amino acids to the specified.
B. Finding the Genetic Code
1. Marshall Nirenberg and J. Heinrich Matthei (1961) found that an enzyme that could be used to construct synthetic RNA in a cell-free system; they showed the codon UUU coded for phenylalanine.
2. By translating just three nucleotides at a time, they assigned an amino acid to each of the RNA codons, and discovered important properties of the genetic code.
3. The code is degenerate: there are 64 triplets to code for 20 naturally occurring amino acids; this protects against potentially harmful mutations.
4. The genetic code is unambiguous; each triplet codon specifies one and only one amino acid.
5. The code has start and stop signals: there are one start codon and three stop codons.
C. The Code Is Universal
1. The few exceptions to universality of the genetic code suggests the code dates back to the very first organisms and that all organisms are related.
2. Once the code was established, changes would be disruptive.
14.3 First Step: Transcription
A. Messenger RNA is Formed
1. A segment of the DNA helix unwinds and unzips.
2. Transcription begins when RNA polymerase attaches to a promoter on DNA. A promoter is a region of DNA which defines the start of the gene, the direction of transcription, and the strand to be transcribed.
3. As RNA polymerase (an enzyme that speeds formation of RNA from a DNA template) moves along the template strand of the DNA, complementary RNA nucleotides are paired with DNA nucleotides of the codingstrand. The strand of DNA not being transcribed is called the noncodingstrand.
4. RNA polymerase adds nucleotides to the 3'-end of the polymer under construction. Thus, RNA synthesis is in the 5'-to-3' direction.
5. The RNA/DNA association is not as stable as the DNA double helix; therefore, only the newest portion of the RNA molecule associated with RNA polymerase is bound to DNA; the rest dangles off to the side.
6. Elongation of mRNA continues until RNA polymerase comes to a stop sequence.
7. The stop sequence causes RNA polymerase to stop transcribing DNA and to release the mRNA transcript.
8. Many RNA polymerase molecules work to produce mRNA from the same DNA region at the same time.
9. Cells produce thousands of copies of the same mRNA molecule and many copies of the same protein in a shorter period of time than if a single copy of RNA were used to direct protein synthesis.
B. RNA Molecules Are Processed
1. Newly formed primary mRNA transcript is processed before leaving the nucleus.
2. Primary mRNA transcript is the immediate product of transcription; it contains exons and introns.
3. The ends of the mRNA molecule are altered: a cap is put on the 5' end and a poly-A tail is put on the 3' end.
a. The "cap" is a modified guanine (G) where a ribosome attaches to begin translation.
b. The "poly-A tail" consists of a 150–200 adenine (A) nucleotide chain that facilitates transport of mRNA out of the nucleus and inhibits enzymatic degradation of mRNA.
4. Portions of the primary mRNA transcript, called introns, are removed.
a. An exon is a portion of the DNA code in the primary mRNA transcript eventually expressed in the final polypeptide product.
b. An intron is a non-coding segment of DNA removed by spliceosomes before the mRNA leaves the nucleus.
5. Ribozymes are RNAs with an enzymatic function restricted to removing introns from themselves.
a. RNA could have served as both genetic material and as the first enzymes in early life forms.
6. Spliceosomes are complexes that contains several kinds of ribonucleoproteins.
a. Spliceosomes cut the primary mRNA transcript and then rejoin adjacent exons.
C. Function of Introns
1. In humans, introns comprise 95% of the average protein-coding gene.
2. Possibly introns divide a gene into regions that can be joined in different combinations for different products.
3. Introns may function to determine which genes are to be expressed and how they should be spliced.
14.4 Second Step: Translation
1. Translation takes place in the cytoplasm of eukaryotic cells.
2. Translation is the second step by which gene expression leads to protein synthesis.
3. One language (nucleic acids) is translated into another language (protein).
B. The Role of Transfer RNA
1. transfer RNA (tRNA) molecules transfer amino acids to the ribosomes.
2. The tRNA is a single-stranded ribonucleic acid that doubles back on itself to create regions where complementary bases are hydrogen-bonded to one another.
3. The amino acid binds to the 3' end; the opposite end of the molecule contains an anticodon that binds to the mRNA codon in a complementary fashion.
4. There is at least one tRNA molecule for each of the 20 amino acids found in proteins.
5. There are fewer tRNAs than codons because some tRNAs pair with more than one codon; if an anticodon contains a U in the third position, it will pair with either an A or G–this is called the wobble hypothesis.
6. The tRNA synthetases are amino acid-activating enzymes that recognize which amino acid should join which tRNA molecule, and covalently joins them. This requires ATP.
7. An amino acid–tRNA complex forms, which then travels to a ribosome to "transfer" its amino acid during protein synthesis.
C. The Role of Ribosomal RNA
1. Ribosomal RNA (rRNA) is produced from a DNA template in the nucleolus of the nucleus.
2. The rRNA is packaged with a variety of proteins into ribosomal subunits, one larger than the other.
3. Subunits move separately through nuclear envelope pores into the cytoplasm where they combine when translation begins.
4. Ribosomes can float free in cytosol or attach to endoplasmic reticulum.
5. Prokaryotic cells contain about 10,000 ribosomes; eukaryotic cells contain many times more.
6. Ribosomes have a binding site for mRNA and binding sites for two tRNA molecules.
7. They facilitate complementary base pairing between tRNA anticodons and mRNA codons; rRNA acts as an enzyme (ribozyme) that joins amino acids together by means of a peptide bond.
8. A ribosome moves down the mRNA molecule, new tRNAs arrive, the amino acids join, and a polypeptide forms.
9. Translation terminates once the polypeptide is formed; the ribosome then dissociates into its two subunits.
10. Polyribosomes are clusters of several ribosomes synthesizing the same protein.
11. To get from a polypeptide to a function protein requires correct bending and twisting; chaperonemolecules assure that the final protein develops the correct shape.
12. Some proteins contain more than one polypeptide; they must be joined to achieve the final three-dimensional shape.
D. Translation Requires Three Steps
1. During translation, mRNA codons base-pair with tRNA anticodons carrying specific amino acids.
2. Codon order determines the order of tRNA molecules and the sequence of amino acids in polypeptides.
3. Protein synthesis involves initiation, elongation, and termination.
4. Enzymes are required for all three steps; energy (ATP) is needed for the first two steps.
5. Chain Initiation
a. A small ribosomal subunit attaches to mRNA in the vicinity of the start codon (AUG).
b. First or initiator tRNA pairs with this codon; then the large ribosomal subunit joins to the small subunit.
c. Each ribosome contains three binding sites: the P (for peptide) site, the A (for amino acid) site, and the E (for exit) site.
d. The initiator tRNA binds to the P site although it carries one amino acid, methionine.
e. The A site is for the next tRNA carrying the next amino acid.
f. The E site is to discharge tRNAs from the ribosome.
g. Initiation factor proteins are required to bring together the necessary translation components: the small ribosomal subunit, mRNA, initiator tRNA, and the large ribosomal subunit.
6. Chain Elongation
a. The tRNA with attached polypeptide is at the P site; a tRNA-amino acid complex arrives at the A site.
b. Proteins called elongation factors facilitate complementary base pairing between the tRNA anticodon and the mRNA codon.
c. The polypeptide is transferred and attached by a peptide bond to the newly arrived amino acid in the A site.
d. This reaction is catalyzed by a ribozyme, which is part of the larger subunit.
e. The tRNA molecule in the P site is now empty.
f. Translocation occurs with mRNA, along with peptide-bearing tRNA, moving to the P site and the spent tRNA moves from the P site to the E site and exits the ribosome.
g. As the ribosome moves forward three nucleotides, there is a new codon now located at the empty A site.
h. The complete cycle is rapidly repeated, about 15 times per second in Escherichia coli.
7. Chain Termination
a. Termination of polypeptide synthesis occurs at a stop codon that does not code for amino acid.
b. The polypeptide is enzymatically cleaved from the last tRNA by a release factor.
c. The tRNA and polypeptide leave the ribosome, which dissociates into its two subunits.
8. Definition of a Gene and a Genetic Mutation
a. Originally a gene was defined as a locus on the chromosome.
b. The one gene-one polypeptide concept connected inborn errors of metabolism with a sequence of DNA bases.
c. A gene could also be defined as a sequence of DNA bases coding for a single polypeptide or a single RNA.
d. These concepts can allow us to define a mutation as a permanent change in the sequence of DNAbases.
e. Current definitions: a protein-coding gene is one that is transcribed into mRNA, while a noncoding gene is one that is transcribed into any other type of RNA.
E. Protein Synthesis and the Eukaryotic Cell
1. The first few amino acids of a polypeptide act as a signal peptide that indicates where the polypeptide belongs in the cell or if it is to be secreted by the cell.
2. After the polypeptide enters the lumen of the ER, it is folded and further processed by addition of sugars, phosphates, or lipids.
3. Transport vesicles carry the proteins between organelles and to the plasma membrane.
15.1 Prokaryotic Regulation
1. Bacteria do not require the same enzymes all the time; they produce just those needed at the moment.
2. Francois Jacob and Jacques Monod (1961) proposed the operon model to explain regulation of gene expression in prokaryotes.
a. In the operon model, several genes code for an enzyme in the same metabolic pathway and are located in a sequence on a chromosome; expression of structural genes is controlled by the same regulatory genes.
b. An operon is a group of structural and regulatory genes that function as a single unit; it includes the following:
i. A regulator gene, located outside the operon, codes for a repressor protein molecule that controls whether the operon is active or not.
ii. A promotor is the sequence of DNA where RNA polymerase attaches when a gene is to be transcribed.
iii. An operator is a short sequence of DNA where an active repressor binds, preventing RNA polymerase from attaching to the promotor--transcription therefore does not occur.
iv. Structural genes are one to several genes coding for enzymes of a metabolic pathway that are transcribed as a unit.
B. The trp Operon
1. Some operons in E. coli usually exist in the "on" rather than the "off" condition.
2. E. coli produces five enzymes as part of the anabolic pathway to synthesize the amino acid tryptophan.
3. If tryptophan is already present in medium, these enzymes are not needed and the operon is turned off .
a. The regulator codes for a repressor that usually is unable to attach to the operator.
b. The repressor has a binding site for tryptophan (if tryptophan is present, it binds to the repressor).
c. This changes the shape of the repressor that now binds to the operator.
4. The entire unit is called a repressible operon; tryptophan is the corepressor.
5. Repressible operons are involved in anabolic pathways that synthesize substances needed by cells.
C. The lac Operon
1. If E. coli is denied glucose and given lactose instead, it makes three enzymes to metabolize the lactose.
2. These three enzymes are encoded by three genes.
a. One gene codes for beta-galactosidase that breaks lactose to glucose and galactose.
b. A second gene codes for a permease that facilitates entry of lactose into the cell.
c. A third gene codes for enzyme transacetylase, which is an accessory in lactose metabolism.
3. The three genes are adjacent on a chromosome and under control of one promoter and one operator.
4. The regulator gene codes for a lac operon repressor protein that binds to the operator and prevents transcription of the three genes.
5. When E. coli is switched to medium containing an allolactose, this lactose binds to the repressor and the repressor undergoes a change in shape that prevents it from binding to the operator.
6. Because the repressor is unable to bind to the operator, the promoter is able to bind to RNA polymerase, which carries out transcription and produces the three enzymes.
7. An inducer is any substance (lactose in the case of the lac operon) that can bind to a particular repressor protein, preventing the repressor from binding to a particular operator; consequently, RNA polymerase can bind to the promoter and transcribe the structural genes.
D. Further Control of the lac Operon
1. Since E. coli prefers to break down glucose, how does E. coli know how to turn on when glucose is absent?
2. When glucose is absent, cyclic AMP (cAMP) accumulates; cAMP has only one phosphate group and attaches to ribose at two locations.
a. CAP is a catabolite activator protein (CAP) in the cytoplasm.
b. When cAMP binds to CAP, the complex attaches to a CAP binding site next to the lac promoter.
c. When CAP binds to DNA, DNA bends, exposing the promoter to RNA polymerase.
d. Only then does RNA polymerase bind to the promoter; this allows expression of the lac operon structural genes.
3. When glucose is present, there is little cAMP in the cell.
a. CAP is inactive and the lactose operon does not function maximally.
b. CAP affects other operons when glucose is absent.
c. This encourages metabolism of lactose and provides a backup system for when glucose is absent.
E. Negative Versus Positive Control
1. Active repressors shut down the activity of an operon—this is negative control..
2. CAP is an example of positive control; when the molecule is active, it promotes the activity of the operon.
3. Use of both positive and negative controls allows the cell to fine-tune control of its metabolism.
4. If both glucose and lactose are present, the cell preferentially metabolizes glucose.
15.2 Eukaryotic Regulation
1. Different cells in the human body turn on different genes that code for different protein products.
2. Eukaryotes have four levels of regulatory mechanisms to control gene expression; two in the nucleus and two in the cytoplasm.
3. There are several levels of control that can modify the amount of gene product.
a. Chromatin structure: if genes are not accessible to RNA polymerase, they cannot be transcribed.
i. Chromatin structure is part of epigenetic inheritance, the transmission of genetic information outside the coding sequences of a gene.
b. Transcriptional control in the nucleus determines which structural genes are transcribed and the rate of transcription; it includes transcription factors initiating transcription and transposons (DNA sequences that move between chromosomes and shut down genes).
c. Posttranscriptional control occurs in the nucleus after DNA is transcribed and preliminary mRNA forms.
i. This may involve differential processing of mRNA before it leaves the nucleus.
ii. The speed that mature mRNA leaves nucleus affects the ultimate amount of gene product.
d. Translational control occurs in cytoplasm after mRNA leaves the nucleus but before there is a protein product.
i. The life expectancy of mRNA molecules can vary, as well as their ability to bind ribosomes.
ii. Some mRNAs may need additional changes before they are translated at all.
e. Posttranslational control occurs in the cytoplasm after protein synthesis.
i. Polypeptide products may undergo additional changes before they are biologically functional.
ii. A functional enzyme is subject to feedback control; binding of an end product can change the shape of an enzyme so it no longer carries out its reaction.
B. Chromatin Structure
1. Eukaryotic DNA is in the form of chromatin, a stringy material associated with proteins.
2. The DNA is wound around a core of eight protein molecules ("beads on a string"); the proteins are called histones and each "bead" is called a nucleosome.
3. During interphase, some chromatin is highly compact, darkly stained, and genetically inactive heterochromatin.
4. The rest is diffuse, lightly colored euchromatin thought to be genetically active.
5. Barr bodies are an example of heterochromatin.
a. Since human males have only one X chromosome, it might be supposed that they produce half the gene product of a female with two X chromosomes.
b. However, females have in each nucleated cell a darkly staining Barr body, a condensed, inactive X chromosome.
c. Which X chromosome is condensed in each cell is determined by chance.
d. Thus, the body of heterozygous females is "mosaic"; half her cells express alleles on one X chromosome and half of her cells express the alleles on the other X chromosome.
e. Female gonads do not show Barr bodies; both X chromosomes are needed in development.
f. Only one active X chromosome in the female zygote means that lower X-coded gene products are normal.
g. Other examples of this mosaic effect include: ocular albinism, Duchenne muscular dystrophy, and female calico cat coat color.
6. Euchromatin activity is related to the extent nucleosomes are coiled and condensed.
a. A nucleosome is a bead-like unit made of a segment of DNA wound around a complex of histone proteins.
b. When DNA is transcribed, activators called remodeling proteins are able to push aside the histone proteins so transcription can begin.
7. Epigenetic inheritance is the term used to describe inheritance patterns that do not depend on the genes themselves.
a. Histones proteins have different chemical modifications in heterochromatin and euchromatin.
b. Methylation of DNA accounts for a phenomenon called genomic imprinting: gene expression is dependent on whether the chromosome carrying the gene is inherited from the mother or the father.
c. Epigenetic inheritance explains unusual inheritance patterns, and may have implications in growth, aging and cancer.
C. Transcriptional Control
1. Transcriptional control is the most critical level of genetic control.
a. Transcription is controlled by DNA-binding proteins called transcription factors.
b. Each cell contains many different types of transcription factors.
c. A group of transcription factors binds to a promoter adjacent to a gene; the complex attracts and binds RNA polymerase, but transcription may still not begin.
d. Transcription activators are often involved in controlling transcription in eukaryotes.
i. Different combinations may regulate different genes.
ii. Transcription activators bind to DNA regions called enhancers.
iii. Enhancers can be quite a distance from the promoter, but a hairpin loop in the DNA brings the activator attached to an enhancer into contact with the promotor.
iv. Mediator proteins act as a bridge between transcription factors and transcription factors at the promotor.
e. Transcription factors are always present in the cell and most likely they have to be activated in some way (e.g., regulatory pathways involving kinases or phosphatases) before they bind to DNA.
2. Transposons are specific DNA sequences that move with and between chromosomes.
a. Their movement may increase or decrease the expression of neighboring genes.
b. They are among the 40% of the human genome consisting of the same short sequence of DNA continuously repeated.
c. They are noncoding sequences that play regulatory functions, and could thus be cansidered part of epigenetic inheritance.
D. Posttranscriptional Control
1. Posttranscriptional control includes mRNA processing and the speed at which mRNA leaves the nucleus.
2. Messenger RNA molecules are processed before they leave the nucleus and enter the cytoplasm.
3. Differential excision of introns and splicing of mRNA can vary the type of mRNA that leaves nucleus.
a. The hypothalamus and thyroid glands produce calcitonin but the mRNA that leaves the nucleus is not the same in both types of cells.
b. Radioactive labeling shows they vary because of a difference in mRNA splicing.
c. Evidence of different patterns of mRNA splicing is found in cells that produce neurotransmitters, muscle regulatory proteins, and antibodies.
4. Speed of transport of mRNA from nucleus into cytoplasm affects the amount of gene product realized per unit time following transcription; there is a difference in the length of time it takes various mRNA molecules to pass through nuclear pores.
E. Translational Control
1. Translational control begins when the processed mRNA molecule reaches the cytoplasm and before there is a protein product.
a. The longer an active mRNA molecule remains in the cytoplasm, the more product is produced.
b. Mature red blood cells eject their nucleus but synthesize hemoglobin for several months; the mRNAs must persist during this time.
c. Ribonucleases are enzymes associated with ribosomes that degrade mRNA.
d. Mature mRNA has non-coding segments at 3' cap and 5' poly-A tail ends; differences in these segments influence how long the mRNA avoids being degraded.
e. MicroRNAs are small, processed pieces of intron; after microRNAs are degraded, they combine with protein and the complex binds to mRNAs, destroying them.
F. Posttranslational Control
1. Posttranslational control begins once a protein has been synthesized and has become active.
a. Some proteins are not active after synthesis; the polypeptide product has to undergo additional changes before it is biologically functional.
b. Bovine proinsulin, for example, is inactive when first produced; a single long polypeptide folds into a three-dimensional structure, a sequence of 30 amino acids is removed from the middle, and the two polypeptide chains are bonded together by disulfide bonds resulting in an active protein.
c. Many proteins are short-lived in cells and degraded or destroyed so they are no longer active.
d. Giant protein complexes called proteasomes carry out this task.
e. One example is the cyclins that control the cell cycle; they are only temporarily present..
15.3 Genetic Mutations
• A genetic mutation is a permanent change in the sequence of bases in DNA; mutations range from having no effect to total inactivity.
A. Effect of Mutations on Protein Activity
1. Point mutations change a single nucleotide and therefore change a single specific codon.
a. The effect of the point mutation depends on the specific base change in the codon.
b. Changes to codons that code for the same amino acid have no effect; e.g., UAU to UAC both code for tyrosine.
c. A change from UAC to UAG (a stop codon) results in a shorter protein, and a change from UAC to CAC incorporates histidine instead of tyrosine.
d. Sickle cell disease results from a single base change in DNA where the beta-chain of hemoglobin contains valine instead of glutamate at one location and the resulting distorted hemoglobin causes red blood cells to clog vessels and die off sooner.
2. Frameshift Mutations
a. The reading frame depends on the sequence of codons from the starting point: THE CAT ATE THE RAT.
b. If, for example, C is deleted, the reading frame is shifted: THE ATA TET HER AT.
c. Frameshift mutations occur when one or more nucleotides are inserted or deleted from DNA.
d. The result of a frameshift mutation is a new sequence of codons and nonfunctional proteins.
B. Nonfunctional Proteins
1. A single nonfunctioning protein can cause dramatic effects.
2. The human transposon Alu is responsible for hemophilia when it places a premature stop codon in the gene for clotting factor IX.
3. PKU results when a person cannot convert phenylalanine to tyrosine; phenylalanine builds up in the system, leading to mental retardation.
4. A faulty code for an enzyme in the same pathway results in an albino individual.
5. Cystic fibrosis is due to inheriting a faulty code for a chloride transport protein in the plasma membrane.
6. Androgen insensitivity is due to a faulty receptor for male sex hormones; body cells cannot respond to testosterone and the individual develops as a female (even though all of the body cells are XY).
C. Carcinogenesis
1. The development of cancer involves a series of various types of mutations.
2. Tumor-suppressor genes normally act as brakes on cell division when it begins to occur abnormally.
3. When proto-oncogenes mutate, they become oncogenes.
4. Tumor-suppressor genes and proto-oncogenes often code for transcription factors or proteins that control transcription factors.
5. P53, a major tumor-suppressor gene, is more frequently mutated in human cancers than any other known gene.
a. The p53 protein acts as a transcription factor to turn on the expression of genes whose products are cell cycle inhibitors.
b. The p53 can also stimulate apoptosis (programmed cell death).
6. Other proto-oncogenes code for Ras proteins, which are needed for normal cell growth and DNA synthesis.
7. Inheritance of Cancer
a. Genes called BRCA1and BRCA2 are tumor-suppressor genes that behave as outosomal recessive alleles.
b. A mutated RB allele causes cancer of the eye; it takes another mutated allele to increase the chance of cancer.
c. The inherited RET gene predisposes a person to thyroid cancer.
D. Cause of Mutations
1. Some mutations are spontaneous, others are due to environmental mutagens.
2. Mutations due to replication errors are very rare.
3. DNA polymerase constantly proofreads new DNA against the old, and repairs any irregularities, thereby reducing mistakes to one out of every one billion nucleotide pairs replicated.
4. Environmental mutagens are environmental substances that increase the chances of mutation.
a. Common mutagens are radiation and organic chemicals.
b. Carcinogens are mutagens that increase the chances of cancer.
i. The Ames test is commonly-used to determine if a chemical is carcinogenic.
ii. A histidine-requiring strain of bacteria is exposed to a chemical.
iii. If the chemical is mutagenic, the bacteria regain the ability to grow without histidine.
c. Tobacco smoke contains a number of known carcinogenic chemicals.
d. X rays and gamma rays are ionizing radiation that creates free radicals, ionized atoms with unpaired electrons.
e. Ultraviolet (UV) radiation is easily absorbed by pyrimidines in DNA.
i. Where two thymine molecules are near each other, UV may bond them together as thymine dimers.
ii. Usually dimers are removed from damaged DNA by special enzymes called repair enzymes.
16.1 DNA Cloning
1. Cloning is the production of identical copies of DNA through some asexual means.
a. An underground stem or root sends up new shoots that are clones of the parent plant.
b. Members of a bacterial colony on a petri dish are clones because they all came from division of the same cell.
c. Human identical twins are clones; the original single embryo separate to become two individuals.
2. Gene cloning is production of many identical copies of the same gene.
a. If the inserted gene is replicated and expressed, we can recover the cloned gene or protein product.
b. Cloned genes have many research purposes: determining the base sequence between normal and mutated genes, altering the phenotype, obtaining the protein coded by a specific gene, etc.
c. Humans can be treated with gene therapy: alteration of the phenotype in a beneficial way.
B. Recombinant DNA Technology
1. Recombinant DNA (rDNA) contains DNA from two or more different sources.
2. To make rDNA, technician selects a vector.
3. A vector is a plasmid or a virus used to transfer foreign genetic material into a cell.
4. A plasmid is a small accessory ring of DNA in the cytoplasm of some bacteria.
5. Plasmids were discovered in research on reproduction of intestinal bacteria Escherichia coli.
6. Introduction of foreign DNA into vector DNA to produce rDNA requires two enzymes.
a. Restriction enzyme is a bacterial enzyme that stops viral reproduction by cleaving viral DNA.
i. The restriction enzyme is used to cut DNA at specific points during production of rDNA.
ii. It is called a restriction enzyme because it restricts growth of viruses but it acts as a molecular scissors to cleave any piece of DNA at a specific site.
iii. Restriction enzymes cleave vector (plasmid) and foreign (human) DNA.
iv. Cleaving DNA makes DNA fragments ending in short single-stranded segments with "sticky ends."
v. The "sticky ends" allow insertion of foreign DNA into vector DNA.
b. DNA ligase seals the foreign gene into the vector DNA
i. Treated cells take up plasmids, and then bacteria and plasmids reproduce.
ii. Eventually, there are many copies of the plasmid and many copies of the foreign gene.
iii. When DNA splicing is complete, an rDNA (recombinant DNA) molecule is formed.
7. If the human gene is to express itself in a bacterium, the gene must be accompanied by the regulatory regions unique to bacteria and meet other requirements.
a. The gene cannot contain introns because bacteria do not have introns.
b. An enzyme called reverse transcriptase can be used to make a DNA copy of mRNA.
c. This DNA molecule is called complementary DNA (cDNA) and does not contain introns.
d. A laboratory DNA synthesizer can produce small pieces of DNA without introns.
C. The Polymerase Chain Reaction (PCR)
1. PCR can create millions of copies of a single gene or a specific piece of DNA in a test tube.
2. PCR is very specific—the targeted DNA sequence can be less than one part in a million of the total dxcDNA sample; therefore a single gene can be amplified using PCR.
3. PCR uses the enzyme DNA polymerase to carry out multiple replications (a chain reaction) of target DNA.
4. PCR automation is possible because heat-resistant DNA polymerase from Thermus aquaticus, which grows in hot springs, is an enzyme that withstands the temperature necessary to separate double-stranded DNA.
5. Analyzing DNA Segments
a. Mitochondria DNA sequences in modern living populations can decipher the evolutionary history of human populations.
b. DNA fingerprinting is the technique of using DNA fragment lengths, resulting from restriction enzyme cleavage and amplified by PCR, to identify particular individuals.
c. DNA is treated with restriction enzymes to cut it into different sized fragments.
d. During gel electrophoresis, fragments separate according to length, resulting in a pattern of bands.
e. DNA fingerprinting can identify deceased individuals from skeletal remains, perpetrators of crimes from blood or semen samples, and genetic makeup of long-dead individuals or extinct organisms.
6. PCR amplification and DNA analysis is used to:
a. detect viral infections, genetic disorders, and cancer;
b. determine the nucleotide sequence of human genes, and, because it is inherited,
c. associate samples with DNA of parents.
16.2 Biotechnology Products
1. Genetically engineered organisms can produce biotechnology products.
2. Organisms that have had a foreign gene inserted into them are transgenic.
B. Transgenic Bacteria
1. Bacteria are grown in large vats called bioreactors.
a. Foreign genes are inserted and the product is harvested.
b. Products on the market include insulin, hepatitis B vaccine, t-PA, and human growth hormone.
2. Transgenic bacteria have been produced to protect and improve the health of plants.
a. Frost-minus bacteria protect the vegetative parts of plants from frost damage.
b. Root-colonizing bacteria receive genes from bacteria for insect toxin, protecting the roots.
c. Bacteria that colonize corn roots can be endowed with genes for insect toxin.
3. Transgenic bacteria can degrade substances.
a. Bacteria selected for ability to degrade oil can be improved by genetic engineering.
b. Bacteria can be bio-filters to prevent airborne chemical pollutants from being vented into the air.
c. Bacteria can also remove sulfur from coal before it is burned and help clean up toxic dumps.
d. Bacteria can also be given"suicide genes" that caused them to die after they have done their job.
4. Transgenic bacteria can produce chemical products.
a. Genes coding for enzymes can be manipulated to catalyze synthesis of valuable chemicals.
b. Phenylalanine used in artificial sweetener can be grown by engineered bacteria.
5. Transgenic bacteria process minerals.
a. Many major mining companies already use bacteria to obtain various metals.
b. Genetically engineered "bio-leaching" bacteria extract copper, uranium, and gold from low-grade ore.
C. Transgenic Plants
1. Plant cells that have had the cell wall removed are called protoplasts.
2. Electric current makes tiny holes in the plasma membrane through which genetic material enters.
3. The protoplasts then develop into mature plants.
4. Foreign genes now give cotton, corn, and potato strains the ability to produce an insect toxin and soybeans are now resistant to a common herbicide.
5. Plants are being engineered to produce human proteins including hormones, clotting factors, and antibodies in their seeds; antibodies made by corn, deliver radioisotopes to tumor cells and a soybean engineered antibody can treat genital herpes.
D. Transgenic Animals
1. Animal use requires methods to insert genes into eggs of animals.
a. It is possible to microinject foreign genes into eggs by hand.
b. Vortex mixing places eggs in an agitator with DNA and silicon-carbide needles that make tiny holes through which the DNA can enter.
c. Using this technique, many types of animal eggs have been injected with bovine growth hormone (bGH) to produce larger fishes, cows, pigs, rabbits, and sheep.
2. Gene pharming is the use of transgenic farm animals to produce pharmaceuticals; the product is obtainable from the milk of females.
a. Genes for therapeutic proteins are inserted into animal's DNA; animal's milk produces proteins.
b. Drugs obtained through gene pharming are planned for the treatment of cystic fibrosis, cancer, blood diseases, and other disorders.
E. Cloning Transgenic Animals
1. For many years, it was believed that adult vertebrate animals could not be cloned; the cloning of Dolly in 1997 demonstrated this can be done.
2. Cloning of an adult vertebrate would require that all genes of an adult cell be turned on again.
3. Cloning of mammals involves injecting a 2n nucleus adult cell into an enucleated egg.
4. The cloned eggs begin development in vitro and are then returned to host mothers until the clones are born.
16.3 Genomics
• Genetics in the 21st century concerns genomics: the study of genomes of humans and other organisms.
A. Sequencing the Bases
1. The Human Genome Project has produced a working draft of all the base pairs in all our chromosomes.
2. The task took 13 years to learn the sequence of the three billion base pairs along the length of our chromosomes.
B. Genome Comparisons
1. There is little difference between the sequence of our bases and other organisms whose DNA sequences are known.
2. We share a large number of genes with simpler organisms (e.g., bacteria, yeast, mice); perhaps our uniqueness is due to regulation of these genes.
3. Researchers found that certain genes on chromosome 22 differed in humans and chimpanzees: those for speech development, hearing, and smell.
4. Many genes found were responsible for human diseases.
C. The HapMap Project
1. This project will catalog sequence differences, called haplotypes, in humans.
2. The goal of the project is to link haplotypes to the risk for specific illnesses.
D. The Genetic Profile
1. An important aspect of genomics is to determine how genes work together to control the phenotype.
2. DNA chips (or DNA microarrays) will soon be available that will rapidly identify a person's complete genotype; this is called the genetic profile.
3. DNA profiles can determine if a person has an increased risk for a particular disease; appropriate intervention can then be administered.
4. The genetic profile can be used to determine if a particular drug therapy is appropriate in a specific clinical condition.
E. Proteomics
1. Proteomics is the study of the structure, function, and interaction of cellular proteins.
2. The information obtained from proteomic studies can be used in designing better drugs, and to correlate drug treatment to the particular genome of the individual.
F. Bioinformatics
1. Bioinformatics is the application of computer technologics to the study of the genome.
2. Information obtained from computer analysis of the genome can show relationships between genetic profiles and genetic disorders.
16.4 Gene Therapy
1. Gene therapy involves procedures to give patients healthy genes to make up for a faulty gene.
2. Gene therapy also includes the use of genes to treat genetic disorders and various human illnesses.
3. There are ex vivo (outside body) and in vivo (inside body) methods of gene therapy.
B. Ex Vivo Gene Therapy
1. Children with severe combined immunodeficiency (SCID) underwent ex vivo gene therapy.
a. Lacking the enzyme ADA involved in maturation of T and B cells, they faced life-threatening infections.
b. Bone marrow stem cells are removed, infected with a retrovirus that carries a normal gene for the enzyme ADA, and returned.
c. Use of bone marrow stem cells allows them to divide and produce more cells with the same genes.
d. Patients who undergo this procedure show significant improvement.
2. Gene therapy trials include treatment of familial hypercholesterolemia where liver cells lack a receptor for removing cholesterol from blood.
a. High levels of blood cholesterol make the patient subject to fatal heart attacks when young.
b. A small portion of the liver is surgically removed and infected with retrovirus with normal gene for receptor.
c. This has lowered cholesterol levels following the procedure.
C. In Vivo Gene Therapy
1. Cystic fibrosis patients lack a gene for trans-membrane chloride ion carriers; patients die from respiratory tract infections.
a. Liposomes, microscopic vesicles that form when lipoproteins are in solution, are coated with healthy cystic fibrosis genes and sprayed into a patient's nostrils.
b. Various methods of delivery are being tested for effectiveness.
2. A gene for vascular endothelial growth factor (VEGF) can be injected alone or within a virus into the heart to stimulate branching of coronary blood vessels.
3. Another strategy is to make cancer cells more vulnerable, and normal cells more resistant, to chemotherapy.
4. Injecting a retrovirus containing a normal p53 gene–that promotes apoptosis–into tumors may stop the growth of tumors.
17.1 History of Evolutionary Thought
1. In 1831, Charles Darwin, a 22-year-old naturalist, accepted a position aboard the ship HMS Beagle that began a voyage around the world; it provided Darwin with many observations.
2. The pre-Darwinian world-view was different from the post-Darwinian.
a. Pre-Darwinian world-view was determined by intractable theological beliefs.
i. The earth is young.
ii. Each species was specially created and did not change over time.
iii. Variations are imperfections varying from a perfectly-adapted creation.
iv. Observations are to substantiate the prevailing worldview.
b. Darwin, however, lived during a time of great change in scientific and social realms.
c. Darwin's ideas were part of a larger change in thought already underway among biologists; this concept would eventually be known as evolution.
B. Mid-Eighteenth-Century Contributions
1. Carolus Linnaeus and Taxonomy
a. Taxonomy is the science of classifying organisms; taxonomy had been a main concern of biology.
b. Carolus Linnaeus (1707–1778) was a Swedish taxonomist.
i. Linnaeus developed a binomial system of nomenclature (two-part names for each species [e.g., Homo sapiens]).
ii. He developed a system of classification for all known plants.
iii. Like other taxonomists of his time, Linnaeus believed in the ideas of
1. special creation—each species had an "ideal" structure and function; and
2. fixity of species—each species had a place in the scala naturae, a sequential ladder of life.
c. Linnaeus thought that classification should describe the fixed features of species and reveal God's divine plan.
d. His ideas reflected the ideas of Plato and Aristotle: the ideal form can be deduced, and organisms can be arranged in order of increasing complexity.
e. His later work with hybridization suggested species might change with time.
2. Georges Louis Leclerc
a. Georges Louis Leclerc, known by his title, Count Buffon (1707–1788), was a French naturalist.
b. He wrote a 44-volume natural history of all known plants and animals.
c. He also provided evidence of descent with modification.
d. His writings speculated on influences of the environment, migration, geographical isolation, and the struggle for existence.
e. Buffon vacillated on whether he believed in evolutionary descent and he professed to believe in special creation and the fixity of species.
3. Erasmus Darwin
a. Erasmus Darwin (1731–1802) was Charles Darwin's grandfather.
b. He was a physician and a naturalist whose writings on both botany and zoology contained many comments that suggested the possibility of common descent.
c. He based his conclusions on
i. changes undergone by animals during development,
ii. artificial selection by humans, and
iii. the presence of vestigial organs (organs that are believed to have been functional in an ancestor but are reduced and nonfunctional in a descendant).
d. Erasmus Darwin offered no mechanism by which evolutionary descent might occur.
C. Late Eighteenth-/Early-Nineteenth Century Contributions
1. Cuvier and Catastrophism
a. George Cuvier (1769–1832), a French vertebrate zoologist, was the first to use comparative anatomy to develop a system of classifying animals.
b. He founded the science of paleontology—the study of fossils—and suggested that a single fossil bone was all he needed to deduce the entire anatomy of an animal.
c. To explain the fossil record, Cuvier proposed that a whole series of catastrophes (extinctions) and re-populations from other regions had occurred.
d. Cuvier was also a staunch advocate of special creation and fixity of species; this presented him with a problem when geological evidence of a particular region showed a succession of life forms in the earth's strata.
e. Catastrophism is the term applied to Cuvier's explanation of fossil history: the belief that catastrophic extinctions occurred, after which repopulation of surviving species occurred, giving an appearance of change through time.
2. Lamarck's Acquired Characteristics
a. Lamarck (1744–1829) was the first to state that descent with modification occurs and that organisms become adapted to their environments.
b. Lamarck, an invertebrate zoologist, held ideas at odds with Cuvier's.
c. Lamarck mistakenly saw "a desire for perfection" as inherent in all living things.
d. Inheritance of acquired characteristics was Lamarck's belief that organisms become adapted to their environment during their lifetime and pass these adaptations to their offspring.
e. Experiments fail to uphold Lamarck's inheritance of acquired characteristics; the molecular mechanism of inheritance shows phenotypic changes do not result in genetic changes that can be passed on to the next generation.
17.2 Darwin's Theory of Evolution
A. Darwin's Background
1. His nature was too sensitive to pursue medicine; he attended divinity school at Cambridge.
2. He attended biology and geology lectures and was tutored by the Reverend John Henslow.
3. Henslow arranged his five-year trip on the HMS Beagle; Darwin was an observant student of nature.
B. Geology and Fossils
1. His study of geology and fossils caused him to concur with Lyell that the observed massive geological changes were caused by slow, continuous processes.
a. Darwin took Lyell's book on the voyage of the HMS Beagle.
b. In his book Principles of Geology, Charles Lyell presented arguments to support a theory of geological change proposed by James Hutton.
c. In contrast to catastrophists, Hutton proposed that the earth was subject to slow but continuous geological processes (e.g., erosion and uplifting) that occur at a uniform rate, a theory called uniformitarianism.
d. The Argentina coast had raised beaches; he witnessed earthquakes raising the earth several feet.
e. Marine shells occurred far inland and at great heights in the Andes.
f. Fossils of huge sloths and armadillo-like animals suggested modern forms were descended from extinct forms with change over time; therefore species were not fixed.
C. Biogeography
1. Biogeography is the study of the geographic distribution of life forms on earth.
2. Patagonian hares replaced rabbits in the South American grasslands.
3. The greater rhea found in the north was replaced by the lesser rhea in the south.
4. Comparison of the animals of South America and the Galápagos Islands caused Darwin to conclude that adaptation to the environment can cause diversification, including origin of new species.
5. The Galápagos Islands
a. These volcanic islands off the South American coast had fewer types of organisms.
b. Island species varied from the mainland species, and from island-to-island.
c. Each island had a variation of tortoise; long and short necked tortoises correlated with different vegetation.
d. Darwin's Finches
1. Finches on the Galápagos Islands resembled a mainland finch but there were more types.
2. Galápagos finch species varied by nesting site, beak size, and eating habits.
3. One unusual finch used a twig or thorn to pry out insects, a job normally done by (missing) woodpeckers (Darwin never witnessed this finch behavior).
4. The variation in finches posed questions to Darwin: did they descend from one mainland ancestor or did islands allow isolated populations to evolve independently, and could present-day species have resulted from changes occurring in each isolated population?
D. Natural Selection and Adaptation
1. Darwin decided that adaptations develop over time; he sought a mechanism by which adaptations might arise.
2. Natural selection was proposed by both Alfred Russel Wallace and Darwin as a driving mechanism of evolution caused by environmental selection of organisms most fit to reproduce, resulting in adaptation.
3. Because the environment is always changing, there is no perfectly-adapted organism.
4. There are three preconditions for natural selection.
a. The members of a population have random but heritable variations.
b. In a population, many more individuals are produced each generation than the environment can support.
c. Some individuals have adaptive characteristics that enable them to survive and reproduce better.
5. There are two consequences of natural selection.
a. An increasing proportion of individuals in succeeding generations will have the adaptive characteristics.
b. The result of natural selection is a population adapted to its local environment.
6. Natural selection can only utilize variations that are randomly provided; therefore there is no directedness or anticipation of future needs.
7. Extinction occurs when previous adaptations are no longer suitable to a changed environment.
E. Organisms Have Variations
1. In contrast to the previous worldview where imperfections were to be ignored, variations were essential in natural selection.
2. Darwin suspected, but did not have today's evidence, that the occurrence of variation is completely random.
3. New variations are as likely to be harmful as helpful.
4. Variations that make adaptation possible are those that are passed on from generation to generation.
5. Darwin could not state the cause of variations because genetics was not yet established.
F. Organisms Struggle to Exist
1. Darwin and Wallace both read an essay by Thomas Malthus, a socioeconomist.
2. Malthus proposed that human populations outgrow food supply and death and famine were inevitable.
3. Darwin applied this to all organisms; resources were not sufficient for all members to survive.
4. Therefore, there is a constant struggle for existence; only certain members survive and reproduce.
G. Organisms Differ in Fitness
1. Organisms whose traits enable them to reproduce to a greater degree have a greater fitness.
a. Fitness is a measure of an organism's reproductive success.
b. Black western diamondback rattlesnakes are more likely to survive on lava flows; lighter-colored rattlesnakes are more likely to survive on desert soil.
2. Darwin noted that humans carry out artificial selection.
a. Early humans likely selected wolf variants; consequently, desirable traits increase in frequency in subsequent generations and produced the varieties of domestic dogs.
b. Many crop plant varieties can be traced to a single ancestor.
c. In nature, interactions with the environment determine which members reproduce more.
d. Evolution by artificial or natural selection occurs when more fit organisms reproduce and leave more offspring than the less fit.
H. Organisms Become Adapted
1. An adaptation is a trait that helps an organism be more suited to its environment.
2. Unrelated organisms living in the same environment often display similar characteristics.
3. Because of differential reproduction, adaptive traits increase in each succeeding generation.
I. On the Origin of Species by Darwin
1. After the HMS Beagle returned to England in 1836, Darwin waited over 20 years to publish.
2. He used the time to test his hypothesis that life forms arose by descent from a common ancestor and that natural selection is a mechanism by which species can change and new species arise.
3. Darwin was forced to publish Origin of Species after reading a similar hypothesis by Alfred Russel Wallace.
17.3 The Evidence of Evolution
A. Common Descent
1. The hypothesis of common descent is supported by many lines of evidence.
2. The more varied the evidence, the more certain it becomes.
3. Darwin synthesized much of the current data but biochemical research was yet to come.
B. Fossils Evidence
1. The fossil record is the history of life recorded by remains from the past.
2. Fossils are at least 10,000 years old and include skeletons, shells, seeds, insects trapped in amber, and imprints of leaves.
3. The fossil record traces history of life and allows us to study history of particular organisms.
4. Fossil evidence supports the common descent hypothesis; fossils can be linked over time because they reveal a similarity in form, despite observed changes.
5. Transitional forms reveal links between groups.
a. Archeopteryx is an intermediate between reptiles and birds.
b. Eustheopteron is an amphibious fish.
c. Seymouria is a reptile-like amphibian.
d. Therapsids were mammal-like reptiles.
6. The fossil record allows us to trace the history of the modern-day horse Equus.
a. Earliest fossils show an ancestral Hyracotherium the size of a dog, with cusped low-crowned molars, four toes on each front foot, three on each hind foot—all adaptations for forest living.
b. When forests were replaced by grasslands, the intermediates were selected for durable grinding teeth, speed, etc. with an increase in size and decrease in toes.
c. Living organisms resemble most recent fossils in the line of descent; underlying similarities allow us to trace a line of descent over time.
C. Biogeographical Evidence
1. Biogeography studies the distribution of plants and animals worldwide.
2. Distribution of organisms is explained by related forms evolving in one locale and spreading to other accessible areas.
a. Darwin observed South America had no rabbits; he concluded rabbits originated elsewhere.
b. Biogeography explains the abundance of finch species on the Galápagos Islands lacking on the mainland.
3. Physical factors, such as the location of continents, determine where a population can spread.
a. Cacti are restricted to North American deserts and euphorbia grow in African deserts.
b. Marsupials arose when South America, Antarctica, and Australia were joined; Australia separated before placental mammals arose, so only marsupials diversified in Australia.
D. Anatomical Evidence
1. Organisms have anatomical similarities when they are closely related because of common descent.
a. Homologous structures in different organisms are inherited from a common ancestor.
b. Analogous structures are inherited from unique ancestors and have come to resemble each other because they serve a similar function.
c. Vertebrate forelimbs contain the same sets of bones organized in similar ways, despite their dissimilar functions.
2. Vestigial structures are remains of a structure that was functional in some ancestors but is no longer functional in the organism in question.
a. Most birds have well-developed wings; some bird species have reduced wings and do not fly.
b. Humans have a tailbone but no tail.
c. Presence of vestigial structures is explained by the common descent hypothesis; these are traces of an organism's evolutionary history.
3. Embryological development reveals a unity of plan.
a. During development, all vertebrates have a post-anal tail and paired pharyngeal pouches.
i. In fishes and amphibian larvae, the pouches become gills.
ii. In humans, first pair of pouches becomes a cavity of middle ear and auditory tube; second pair becomes tonsils, while third and fourth pairs become thymus and parathyroid glands.
iii. The above features are explained if fishes are ancestral to other vertebrate groups.
E. Biochemical Evidence
1. Almost all living organisms use the same basic biochemical molecules, e.g., DNA, ATP, and many identical or nearly identical enzymes.
2. Organisms utilize the same DNA triplet code and the same 20 amino acids in their proteins.
3. Many organisms share the same introns and types of repeats, which is remarkable since there is no obvious functional reason why these components need to be so similar.
4. This is substantiated by the analysis of the degree of similarity in amino acids for cytochrome c among organisms.
5. These similarities can be explained by descent from a common ancestor.
6. Life's vast diversity has come about by only a slight difference in the same genes.
F. Because it is supported by so many lines of evidence, evolution is no longer considered a hypothesis.
1. Evolution is one of the great unifying theories of biology, similar in status to the germ theory of disease in medicine.
2. In science, theory is reserved for those conceptual schemes that are supported by a large number of observations or a large amount of experimental evidence and have not been found lacking.
19.1 Origin of Life
o Chemical evolution is the increase in complexity of chemicals that led to the first cells.
1. Today, we say that "life only comes from life."
2. However, the first cells had to arise from an increased complexity of chemicals.
B. The Early Earth
1. The Earth came into being about 4.6 BYA (BYA).
2. Heat from gravitation and radioactivity formed the Earth in several layers with iron and nickel in a liquid core, silicate minerals in a semi-liquid mantle, and upwellings of volcanic lava forming the first crust.
3. The Earth's mass provides a gravitational field strong enough to hold an atmosphere.
4. Early Earth's atmosphere differed from the current atmosphere, consisting of:
a. water vapor,
b. nitrogen,
c. carbon dioxide,
d. small amounts of hydrogen, methane, ammonia, hydrogen sulfide, and carbon monoxide.
5. The early atmosphere was formed by volcanic out-gassing characteristic of the young Earth.
6. The early atmosphere contained little free oxygen (O2) and was probably a reducing atmosphere with little free oxygen; a reducing atmosphere lacks free O2 and allows formation of complex organic molecules.
7. The early Earth was so hot that H2O only existed as a vapor in dense, thick clouds.
8. As the Earth cooled, H2O vapor condensed to form liquid H2O, and rain collected in oceans.
9. The Earth's distance from the sun allows H2O to exist in all phases: solid, liquid, and gas.
10. NASA photos seem to confirm that Earth is bombarded by comets adding substantial water vapor.
C. Monomers Evolve
0. Comets and meteorites, perhaps carrying organic chemicals, have pelted the Earth throughout history.
1. A meteorite from Mars (ALH84001) that landed on Earth 13,000 years ago, may have fossilized bacteria.
2. Oparin/Haldane Hypothesis (1920s)
. Oparin/Haldane independently suggested organic molecules could be formed in the presence of outside energy sources using atmospheric gases.
a. Experiments performed by Miller and Urey (1953) showed experimentally that these gases (methane, ammonia, hydrogen, water) reacted with one another to produce small organic molecules (amino acids, organic acids).
3. Lack of oxidation and decay allowed organic molecules to form a thick, warm organic soup.
4. Ammonia may have been scarce in the early atmosphere; undersea thermal vents, which line ocean ridges, might have been responsible for converting nitrogen to ammonia.
D. Polymers Evolve
0. Newly formed organic molecules polymerized to produce larger molecules.
. Wachtershauser and Huber formed peptides using iron-nickel sulfides under ventlike conditions.
a. Such minerals have a charged surface that attracts amino acids and provides electrons so they bond together.
1. Protein-first Hypothesis
. Sidney Fox demonstrated amino acids polymerize abiotically if exposed to dry heat.
a. Amino acids collected in shallow puddles along the rocky shore; heat of the sun caused them to form proteinoids (i.e., small polypeptides that have some catalytic properties).
b. When proteinoids are returned to water, they form cell-like microspheres composed of protein.
c. This assumes DNA genes came after protein enzymes; DNA replication needs protein enzymes.
2. The Clay Hypothesis
. Graham Cairns-Smith suggests that amino acids polymerize in clay, with radioactivity providing energy.
a. Clay attracts small organic molecules and contains iron and zinc atoms serving as inorganic catalysts for polypeptide formation.
b. Clay collects energy from radioactive decay and discharges it if temperature or humidity changes.
c. If RNA nucleotides and amino acids became associated so polypeptides were ordered by and helped synthesize RNA, then polypeptides and RNA arose at the same time.
3. RNA-first Hypothesis
. Only the macromolecule RNA was needed at the beginning to lead to the first cell.
a. Thomas Cech and Sidney Altman discovered that RNA can be both a substrate and an enzyme.
b. RNA would carry out processes of life associated with DNA (in genes) and protein enzymes.
c. Supporters of this hypothesis label this an "RNA world" 4 BYA.
E. A Protocell Evolves
0. Before the first true cell arose, there would have been a protocell or protobiont.
1. A protocell would have a lipid-protein membrane and carry on energy metabolism.
2. Fox showed that if lipids are made available to microspheres, lipids become associated with microspheres producing a lipid-protein membrane.
3. Oparin demonstrated a protocell could have developed from coacervate droplets.
. Coacervate droplets are complex spherical units that spontaneously form when concentrated mixtures of macromolecules are held in the right temperature, ionic composition, and pH.
a. Coacervate droplets absorb and incorporate various substances from the surrounding solution.
b. In a liquid environment, phospholipid molecules spontaneously form liposomes, spheres surrounded by a layer of phospholipids; this is called the "membrane-first" hypothesis.
c. A protocell could have contained only RNA to function as both genetic material and enzymes.
4. If a protocell was a heterotrophic fermenter living on the organic molecules in the organic soup that was its environment, this would indicate heterotrophs preceded autotrophs.
. A heterotroph is an organism that cannot synthesize organic compounds from inorganic substances and therefore must take in preformed organic compounds.
a. An autotroph is an organism that makes organic molecules from inorganic nutrients.
5. If the protocell evolved at hydrothermal vents, it would be chemosynthetic and autotrophs would have preceded heterotrophs.
6. The first protocells may have used preformed ATP, but as supplies dwindled, natural selection would favor cells that could extract energy from carbohydrates to transform ADP to ATP.
7. Since glycolysis is a common metabolic pathway in living things, it evolved early in the history of life.
8. As there was no free O2, it is assumed that protocells carried on a form of fermentation.
9. The first protocells had a limited ability to break down organic molecules; it took millions of years for glycolysis to evolve completely.
10. Fox has shown that a microsphere has some catalytic ability; Oparin found that coacervates incorporate enzymes if they are available in the medium.
F. A Self-Replication System Evolves
0. In living systems, information flows from DNA → RNA → protein; it is possible that this sequence developed in stages.
1. The RNA-first hypothesis suggests that the first genes and enzymes were RNA molecules.
. These genes would have directed and carried out protein synthesis.
a. Ribozymes are RNA that acts as enzymes.
b. Some viruses contain RNA genes with a protein enzyme called reverse transcriptase that uses RNA as a template to form DNA; this could have given rise to the first DNA.
2. The protein-first hypothesis contends that proteins or at least polypeptides were the first to arise.
. Only after the protocell develops complex enzymes could it form nucleic acids from small molecules.
a. Because a nucleic acid is complicated, the chance that it arose on its own is minimal.
b. Therefore, enzymes are needed to guide the synthesis of nucleotides and then nucleic acids.
3. Cairns-Smith suggests that polypeptides and RNA evolved simultaneously.
. The first true cell would contain RNA genes that replicated because of the presence of proteins; they become associated in clay in such a way that the polypeptides catalyzed RNA formation.
a. This eliminates the chicken-and-egg paradox; both events happen at the same time.
4. Once the protocell was capable of reproduction, it became a true cell and biological evolution began.
. After DNA formed, the genetic code still had to evolve to store information.
a. Because the current code is subject to fewer errors than other possible codes, and because it minimizes mutations, it likely underwent a natural selection process.
5. Most biologists suspect life evolved in basic steps.
. Abiotic synthesis of organic molecules such as amino acids occurred in the atmosphere or at hydrothermal vents.
a. Monomers joined together to form polymers at seaside rocks or clay, or at vents; the first polymers could have been proteins or RNA or both.
b. Polymers aggregated inside a plasma membrane to make a protocell that had limited ability to grow; if it developed in the ocean it was a heterotroph, if at a hydrothermal vent, a chemoautotroph.
c. Once the protocell contained DNA genes or RNA molecules, it was a true cell.
19.2 History of Life
A. Fossils Tell a Story
1. A fossil is the remains or traces of past life, usually preserved in sedimentary rock.
2. Most dead organisms are consumed by scavengers or decompose.
3. Paleontology is the study of fossils and the history of life, ancient climates, and environments.
4. Sedimentation has been going on since the Earth was formed; it is an accumulation of particles forming a stratum, a recognizable layer in a stratigraphic sequence laid down on land or in water.
5. The sequence indicates the age of fossils; a stratum is older than the one above it and younger than the one below it.
B. Relative Dating of Fossils
1. Strata of the same age in England and Russia may have different sediments.
2. However, geologists discovered that strata of the same age contain the same fossils, termed index fossils.
3. Therefore, fossils can be used for the relative dating of strata.
4. A particular species of fossil ammonite is found over a wide range and for a limited time period; therefore, all strata in the world that contain this ammonite are of the same age.
5. However, relative dating does not establish the absolute age of fossils in years.
C. Absolute Dating of Fossils
1. Absolute dating relies on radioactive dating to determine the actual age of fossils.
2. Radioactive isotopes have a half-life, the time it takes for half of a radioactive isotope to change into a stable element.
3. Carbon 14 (14C) is a radioactive isotope contained within organic matter.
a. Half of the carbon 14 (14C) will change to nitrogen 14 (14N) every 5,730 years.
b. Comparing 14C radioactivity of a fossil to modern organic matter calculates the age of the fossil.
c. After 50,000 years, the 14C radioactivity is so low it cannot be used to measure age accurately.
4. It is possible to determine the ratio of potassium 40 (40K) and argon 40 to date rocks and infer the age of a fossil.
D. The Precambrian
Geologists have devised the geological timescale, which divides the history of Earth into eras, and then periods and epochs.
1. Life arose in the Precambrian Era.
a. The Precambrian encompasses 87% of the geologic time scale.
b. Early bacteria probably resembled the archaea that live in hot springs today.
c. 3.8 BYA, the first chemical fingerprints of complex cells occur; at 3.46 BYA, photosynthetic prokaryotic cells appear.
d. Boulders called stromatolites from this early time resemble living stromatolites with cyanobacteria in the outer surface.
e. Oxygen-releasing photosynthesis by cyanobacteria in stromatolites caused the atmosphere to become oxidizing rather than reducing.
f. By 2 BYA, oxygen levels were high enough that anaerobic prokaryotes were declining.
g. Accumulation of O2 caused extinction of anaerobic organisms and the rise of aerobic organisms.
h. O2 forms ozone or O3 in the upper atmosphere, contributing to the ozone shield and blocking ultraviolet radiation from reaching the Earth's surface; this allowed organisms to live on land.
2. Eukaryotic Cells Arise
a. The eukaryotic cell, which arose 2.1 BYA, is always aerobic and contains a nucleus and organelles.
b. The Endosymbiotic Hypothesis
i. Mitochondria were probably once free-living aerobic prokaryotes.
ii. Chloroplasts were probably once free-living photosynthetic prokaryotes.
iii. A nucleated cell probably engulfed these prokaryotes that became various organelles.
iv. Cilia and flagella may have originated from slender undulating prokaryotes that attached to the host cell.
3. Multicellularity Arises
a. It is not known exactly when multicellular organisms appeared; they would have been microscopic.
b. Separating germ cells from somatic cells may have contributed to the diversity of organisms.
c. Fossils of the Ediacara Hills of South Australia, from about 600-545 MYA, were soft-bodied early invertebrates.
i. These bizarre animals lived on mudflats in shallow marine waters.
ii. They lacked internal organs and could have absorbed nutrients from the sea.
E. The Paleozoic Era
1. The Paleozoic Era lasted over 300 million years and was a very active period with three major mass extinctions.
a. An extinction is the total disappearance of a species or higher taxonomic group.
b. Mass extinction is the disappearance of a large numbers of species or higher groups in a short geological time, just a few million years.
2. Cambrian Animals
a. The Cambrian Period saw invertebrates flourish; invertebrates lack a vertebral column.
b. Today's invertebrates all trace their ancestry to the Cambrian Period, and possibly earlier.
c. A molecular clock, based on a fixed rate of changes in base pair sequences, allows us to trace backward how long current species have evolved separately.
d. Why fossils are easy to find in the Cambrian but not before is a complex question; most likely the animals evolved earlier but without outer skeletons.
e. Cambrian seafloors were dominated by trilobites, now extinct, that had armored exoskeletons.
f. Perhaps the evolution of exoskeletons was due to the presence of plentiful O2 in the atmosphere.
g. A skeleton may have been due to the increased pressures of predation.
3. Invasion of Land
a. Early in the Ordovician Period, marine algae expanded to freshwater.
b. In the Silurian Period, vascular plants invaded land and later flourished in warm swamps in the Carboniferous Period.
c. Spiders, centipedes, mites and millipedes all preceded the appearance of insects on land.
d. The appearance of wings on insects in the Carboniferous Period allowed insects to radiate into a diverse group.
e. The vertebrate line of descent began in the early Ordovician Period.
f. The Devonian Period is called the Age of Fishes and saw jawless and then jawed fishes, including both cartilaginous and ray-finned fishes.
g. The Carboniferous Period was an age of coal-forming forests with an abundance of club mosses, horsetails, and ferns.
i. It is called the "Age of the Amphibians" because amphibians diversified at this time.
ii. Early vascular plants and amphibians were larger and more abundant during the Carboniferous Period; a climate change to colder and drier began the process that produced coal.
F. The Mesozoic Era
1. Although there was a mass extinction at the end of the Paleozoic, evolution of some plants and animals continued into the Triassic, the first period of the Mesozoic Era.
2. The Triassic period
a. Gymnosperms flourished, especially cycads; the Triassic and Jurassic are called the "Age of Cycads."
b. One group of reptiles, the therapsids, had the first mammal features.
c. Reptiles, originating in the Permian, underwent adaptive radiation.
3. The Jurassic Period
a. Many dinosaurs flourished in the sea, on land and in air.
b. Controversy surrounds dinosaurs being ectothermic or endothermic.
4. The Cretaceous Period
a. A new Chinese fossil, Jeholodens, reveals an early mammal with a long snout but sprawling reptile-like hind limbs.
b. The era of dinosaurs ended in a mass extinction in which dinosaurs, most reptiles, and many marine organisms perished.
G. The Cenozoic Era
1. The Cenozoic Era is divided into the Tertiary and the Quaternary Periods.
2. During the Cenozoic Era, mammals with hair and mammary glands diversified and human evolution began.
3. Mammalian Diversification
a. During the Paleocene Epoch, mammals were small and resembled rats.
b. In the Eocene Epoch, all of the modern orders of mammals had developed.
c. Many of the types of herbivores and carnivores of the Oligocene Epoch are extinct today.
4. Evolution of Primates
a. Flowering plants were diverse and plentiful by the Cenozoic Era; primates were adapted to living in flowering trees.
b. The first primates were small squirrel-like animals; from them evolved the first monkeys and apes.
c. Apes diversified during the Miocene and Pliocene Epochs; this includes the first hominids, the group that includes humans.
d. During the Tertiary Period, the world's climate cooled with the last two epochs known as the Ice Age.
e. The Pleistocene Epoch saw many large sloths, beavers, wolves, bison, woolly rhinoceroses, mastodons, and mammoths; modern humans arose and may have contributed to extinction.
19.3 Factors That Influence Evolution
A. Continental Drift
1. Earth's crust is dynamic, not immobile as was once thought.
2. In 1920, German meteorologist Alfred Wegener presented data from across disciplines supporting continental drift.
3. Continental drift was confirmed in the 1960s; the continents moved with respect to one another.
4. During the Permean Period, the continents were joined to form one supercontinent called Pangaea which later divided into Gondwana and Laurasia and then split to form today's configuration.
5. Continental drift explains why the coastlines of several continents (e.g., the outline of the west coast of Africa and that of the east coast of South America) are mirror images of each other.
6. The same geological structures (e.g., mountain ranges) are found in many areas where continents once touched.
7. Continental drift explains unique distribution patterns of several fossils (e.g., species of the seed fern Glossopteris).
8. Continental drift also explains why some fossils (e.g., reptiles Cynognathus and Lystrosaurus) are found on different continents.
9. Continental drift explains why Australia, South America, and Africa have distinctive mammals; current mammalian biological diversity is the result of isolated evolution on separate continents.
B. Plate Tectonics
1. Plate tectonics is the study of the behavior of the Earth's crust in terms of moving plates that are formed at ocean ridges and destroyed at subduction zones.
2. Ocean ridges are ridges on ocean floors where oceanic crust forms; regions in oceanic crust where molten rock rises and material is added to the ocean floor result in seafloor spreading.
3. Seafloor spreading is the lateral movement of oceanic crust away from ocean ridges due to material added to the ocean floor.
4. Subduction zones are regions where oceanic crust collides with the continental crust, causing the oceanic crust to descend into the mantle where it is melted.
5. Where the ocean floor is at the leading edge of a plate, a deep trench forms bordered by volcanoes or volcanic island chains.
6. Two continents colliding form a mountain range (e.g., the Himalayas are the result of the collision of India and Eurasia).
7. Transform boundaries are regions where two crustal plates meet and scrape past one another resulting in relatively frequent earthquakes.
C. Mass Extinctions
1. Five mass extinctions occurred at the ends of the Ordovician, Devonian, Permian, Triassic, and Cretaceous periods.
2. Mass extinctions have been attributed to tectonic, oceanic, and climatic changes.
3. Walter and Louis Alvarez proposed that the Cretaceous extinction was due to a bolide (an asteroid that explodes producing meteorites) striking the Earth.
a. A layer of iridium soot has been identified in the Cretaceous clay, the correct strata.
b. A huge crater near the Yucatan is the impact site.
c. The effect would have resembled a worldwide atomic explosion.
4. David Raup and John Sepkoski proposed that marine fossils show mass extinctions every 26 million years, in periodicity with astronomical movement through the galaxy.
5. Continental drift contributed to Ordovician extinction; Gondwanaland arrived at the south pole and glaciers chilled oceans and land until Gondwanaland drifted away from the pole.
6. The Devonian extinction may have been a bolide event; this saw an end to 70% of the marine invertebrates; other possibilities include drifting back toward the south pole.
7. The Permian extinction was very severe; 90% of ocean species and 70% of land species disappeared perhaps due to an excess of carbon dioxide due to a change in ocean circulation due to a lack of polar ice caps.
8. The Triassic extinction has been attributed to meteorite collision with Earth; a crater in Central Quebec may have been the impact site.
20.1 Taxonomy
o Taxonomy is the branch of biology concerned with identifying, naming, and classifying organisms.
1. A natural system of classification reflects the evolutionary history of organisms.
2. Naming and identifying organisms began with the Greeks and Romans.
3. In the Middle Ages, organisms were described using long Latin descriptions.
4. John Ray (1627-1705), a British naturalist, argued that each organism should have a set name.
B. The Binomial System
1. The number of known organisms expanded greatly in mid-eighteenth century due to European travel.
2. Carolus Linnaeus (1707-1778) developed the binomial system to name species.
3. The binomial system of nomenclature names organisms using a two-part Latin name.
a. First part is the genus; closely related species are assigned to the same genus.
b. Second part is the specific epithet; it usually provides something descriptive about an organism.
c. A scientific name consists of both genus and specific epithet (e.g., Lilium buibiferum and Lilium canadense).
d. Both names are italicized or underlined; the first letter of the genus name is capitalized.
e. The genus can be abbreviated when used with a specific epithet if the full name was given before.
4. Common names vary with different languages, lump many species under one name or have various names for the same species, and the same name may refer to different organisms in different regions.
5. The job of naming all species is far from finished.
. There are estimated to be between 3 and 30 million species living on earth.
a. Currently, one million species of animals and a half million plant and microorganismic species have been named.
b. Some groups, such as birds, are nearly all known; some insect groups are mostly unknown.
C. Identification of a Species
0. Linnaeus considered each species to have a unique structure that made it distinct.
. Distinguishing species on structure can be a problem because variations occur among members of the same species.
a. Males and females may have a different form, as can juveniles and adults.
1. The biological definition of a species states a species can interbreed and share the same gene pool.
. Distinguishing species on the basis of reproductive isolation can also be a problem.
a. Some species do not reproduce sexually.
b. Some species hybridize where their geographic ranges overlap.
c. Reproductive isolation can be difficult to observe.
2. When a species has a wide geographic range, variant types may tend to interbreed where they overlap; these populations are called subspecies, and are designated by a three-part name.
. The rat snakes Elaphe obsoleta obsoleta and Elaphe obsoleta bairdi are subspecies of Elaphe obsoleta.
a. The term "species" is used for a taxonomic category below the rank of genus.
b. A taxon (pl. taxa)is a group of organisms in a classification category; Rosa or Felis are taxa at the genus level.
D. Classification Categories
0. Aristotle classified life into 14 groups (e.g., mammals, birds, etc.), and subdivided them by size.
1. Ray grouped animals and plants according to how he thought they were related.
2. Linnaeus grouped plants by flower parts; his categories were published in Systema Naturae in 1735
3. Today, taxonomists use seven categories of classification: species, genus, family, order, class, phylum, and kingdom.
. A higher category, the domain, has recently been added to these seven categories.
a. The higher the category, the more inclusive it is.
b. Members of a kingdom share general characters; members of a species share quite specific characters.
c. A character is any structural, chromosomal, or molecular feature that distinguishes one group from another.
d. Additional levels of classification can be added by adding super-, sub-, or infra- (e.g., suborder); thus, there are more than 30 categories of classification.
20.2 Phylogenetic Trees
Systematics is the study of the diversity of organisms using information from cellular to population levels.
1. Classification reflects phylogeny; one goal of systematics is to create phylogenetic trees.
2. Phylogeny is the evolutionary history of a group of organisms.
3. A phylogenetic tree indicates common ancestors and lines of descent or lineages.
4. A primitive character is a trait that is present in a common ancestor and all members of a group.
5. A derived character is present only in a specific line of descent.
6. Different lineages diverging from a common ancestor have ancestral characteristics—traits shared by the ancestor and the species in its lines of descent.
B. Classification and Phylogeny
1. Classification is a part of systematics because classification categories list the unique characters of each taxon, which reflect phylogeny.
2. When we say that two species are related, we mean that they share a common ancestor.
C. Tracing Phylogeny
1. Fossil Record
a. Because fossils can be dated, fossils can establish the age of a species.
b. It can be difficult to associate fossils with currently living groups; e.g., a new view of turtle fossils could place them closer to crocodiles.
c. The fossil record is often incomplete because soft-bodied organisms do not fossilize well.
d. Most organisms decay and the chances of becoming a fossil are low.
2. Homology
a. Homology is character similarity that stems from having a common ancestor; homology helps indicate when species belong to a related group.
b. Homologous structures are related to each other through common descent but may differ in structure and function (e.g., the forelimbs of a horse and the wings of a bat).
c. Convergent evolution is acquisition of similar traits in distantly related lines of descent as a result of adaptation to similar environmental conditions; convergent evolution may make it difficult to distinguish homologous from analogous structures.
d. Analogous structures have the same function but are not derived from the same organ in a common ancestor (e.g., the wings of an insect and the wings of a bat).
e. Parallel evolution produces similar characters in related lineages without occurring in a common ancestor.
D. Molecular Data
1. Speciation occurs when mutations bring about changes in base pair sequences of DNA.
2. Each distinct lineage accumulates changes in DNA base pair sequences and amino acid sequences in proteins over time.
3. Advances in analyzing nucleotide and amino acid sequences make abundant data available to researchers.
4. Protein Comparisons
a. Earlier studies used immunological reactions to antibodies, made by injecting a rabbit with cells of one species, to determine the relatedness of two species.
b. Amino acid sequences are now used to determine the differences in proteins between two species.
i. Cytochrome c is a protein found in all aerobic organisms; the amino acid differences in cytochrome c between chickens and humans is 13 but between chickens and ducks is only 3.
c. Since the number of universal proteins is limited, most new studies use RNA and DNA.
5. RNA and DNA Comparisons
a. All cells have ribosomes for protein synthesis; comparing rRNA sequences provides a reliable indicator of similarity.
b. DNA–DNA hybridization separates the DNA strands of two species and combines the strands; the more closely related the two species, the more the DNA strands stick together.
c. Panda evolution
i. The Chinese giant panda resembles a bear, has a false thumb, but bones and teeth resemble a raccoon.
ii. The red panda has similar features but lacks the false thumb.
iii. Results of DNA hybridization suggest the giant panda diverged from the bear lineage and the red panda diverged from the raccoon lineage.
d. Chimpanzees and Humans
i. DNA hybridization shows chimpanzees closer to humans than to other apes.
ii. Yet humans are kept in a separate family and chimpanzees are with the ape family because humans are markedly different in adaptation to the environment.
e. Mitochondria DNA (mtDNA) mutates ten times faster than nuclear DNA; mtDNA is often used for closely related species; North American songbirds were found to have diverged well before retreating glaciation 250,000–100,000 years ago.
6. Molecular Clocks
a. Nucleic acid changes are not tied to adaptation; the fairly constant changes provide a molecular clock.
b. Comparison of mtDNA sequences equated a 5.1% nucleic acid difference among songbird species to 2.5 million years.
c. The fossil record can then be used to calibrate the clock and confirm the hypothesis drawn from molecular data.
20.3 Systematics Today
A. Cladistic Systematics
1. Cladistic systematics is based on the work of Willi Hennig.
2. Cladistics analyze primitive and derived characters and constructs cladograms on the basis of shared derived characters.
3. A cladogram is a diagram showing relationships among species based on shared, derived characters; a cladogram thus traces the evolutionary history of the group being studied.
4. Constructing a Cladogram
a. First step: construct a table of characters of the taxa being compared.
b. Any character found in the outgroup as well is a shared primitive character.
c. Homologies shared by certain lineages are shared derived characters.
d. A clade is an evolutionary branch that includes a common ancestor and all its descendent species.
5. Parsimony
a. Cladists are guided by the principle of parsimony—the minimum number of assumptions is most logical.
b. The best cladogram is one in which the fewest number of shared derived characters are left unexplained or that minimizes the number of assumed evolutionary changes..
c. This approach is vulnerable if convergent evolution produces what appears to be common ancestry.
d. Reliability of cladograms is dependent on the knowledge and skill of a particular investigator gathering data.
B. Phenetic Systematics
1. In phenetic systematics, species are classified according to the number of their similarities, regardless of whether they might be convergent, parallel, or dependent on one another.
2. Systematists of this school do not believe that a classification that actually reflects phylogeny can be constructed; it is better to rely strictly on a method that does away with personal prejudices.
3. Results of their analysis are depicted in a phenogram.
4. Phenograms vary for the same group of organisms, depending on how the data are collected and handled.
C. Traditional Systematics
1. Traditional systematics stresses common ancestry and the degree of structural difference among divergent groups in order to construct phylogenetic trees.
a. The traditional school accepts the tenet that mammals and birds evolved from reptilian ancestors, even though the reptile group is monophyletic—it does not include all groups from all ancestors.
b. But the reptile group is no longer monophyletic—it does not include all groups from all ancestors.
c. To cladists, the traditional method of determining phylogeny is arbitrary.
d. In the bird example, birds are more closely related to dinosaurs and crocodiles than they are different.
2. Strict cladists doubt there should be a class Reptilia because it does not include all organisms derived from reptiles.
20.4 Classification Systems
1. Early biologists recognized two kingdoms: animals (kingdom Animalia) and plants (kingdom Plantae).
2. The microscope revealed unicellular organisms; in the 1880s, Ernst Haeckel proposed the kingdom Protista.
3. Haeckel in the 1880s originally placed bacteria and cyanobacteria in Monera since they lacked a nucleus.
4. H. Whittaker in 1969 suggested a five kingdom system based on cell type, organization, and nutrition:
a. Members of Monera are prokaryotic bacteria that obtain organic molecules by absorption or photosynthesis.
b. Members of Protista are mainly unicellular eukaryotes that obtain organic molecules by absorption, ingestion, or photosynthesis; the classification of protists is debated.
c. The Plantae are multicellular eukaryotes, autotrophic by photosynthesis.
d. Members of Animalia are multicellular eukaryotes, heterotrophic by ingestion, and are generally motile.
e. Members of the Fungi are multicellular eukaryotes, heterotrophic saprotrophs that form spores, lack flagella and have cell walls containing chitin.
5. Generally, protists are considered to have evolved from monerans, and the fungi, plants, and animals evolved from protists via three separate lineages.
B. Three-Domain System
1. Recent research suggests one group of prokaryotes is so distantly related it should be in a separate domain.
2. Sequencing of rRNA suggests all organisms evolved along three distinct lineages: domains Bacteria, Archaea, and Eukarya.
3. Domain Bacteria
a. The bacteria are prokaryotic unicellular organisms that reproduce asexually.
b. Cyanobacteria are large photosynthetic prokaryotes.
c. Most bacteria are heterotrophic.
d. Bacteria are important in ecosystems because they break down organic remains, thereby keeping chemical cycling going.
e. Some bacteria are parasitic and cause disease.
4. Domain Archaea
a. Like bacteria, archaea are prokaryotic unicellular organisms that reproduce asexually.
b. The archaea live in extreme environments: methanogens in anaerobic swamps, halophiles in salt lakes, and thermoacidophiles in hot acidic environments.
c. The archaea cell wall is diverse but not the same as the bacterial cell wall.
5. Domain Eukarya
a. Eukarytes are unicellular to multicellular organisms, always with a membrane-bound nucleus.
b. Sexual reproduction is common; various types of life cycles are seen.
c. Protists and fungi are eukarytes, as are plants and animals (these kingdoms will be studied in detail in later chapters in the text and in this instructor's manual).
21.1 Viruses, Viroids, and Prions
• Viruses
1. are associated with a number of plant, animal, and human diseases;
2. can only reproduce by using the metabolic machinery of the host cell;
3. are noncellular;
4. may have a DNA or RNA genome.
5. In 1884, Pasteur suspected something smaller than bacteria caused rabies; he chose a Latin term for "poison."
6. In 1892, Russian biologist Dimitri Ivanowsky, working with the tobacco mosaic virus, confirmed Pasteur's hypothesis that an infectious agent smaller than a bacterium existed.
7. With the invention of the electron microscope, these infectious agents could be seen for the first time.
A. Viral Structure
1. A virus is similar in size to a large protein, generally smaller than 200 nm in diameter.
2. Many viruses can be purified and crystallized, and the crystals stored for long periods of time.
3. Viral crystals become infectious when the viral particles they contain invade host cells.
4. All viruses have at least two parts:
a. An outer capsid is composed of protein subunits.
b. An inner core contains either DNA (deoxyribonucleic acid) or RNA (ribonucleic acid), but not both.
i. The viral genome at most has several hundred genes; a human cell, in comparison, contains thousands of genes.
ii. The viral envelope is usually partly host plasma membrane with viral glycoprotein spikes.
iii. Viral particles have proteins, especially enzymes (e.g., polymerases), to produce viral DNA or RNA.
iv. Not all viruses have an envelope; such viruses are called naked viruses.
5. The classification of viruses is based on
a. their type of nucleic acid, including whether they are single-stranded or double-stranded;
b. their size and shape; and
c. the presence or absence of an outer envelope.
B. Parasitic Nature
1. Viruses are obligateintracellularparasites that cannot multiply outside a living cell.
a. Animal viruses in laboratories are raised in live chick embryos or in cell tissue culture.
b. Viruses infect all sorts of cells, from bacteria to human cells, but they are host specific.
i. The tobacco mosaic virus only infects certain plants.
ii. The rabies virus infects only mammals.
iii. The AIDS virus, HIV, infects only certain human blood cells.
iv. The Hepatitis virus invades only liver tissues.
v. The Polio virus only reproduces in spinal nerve cells.
2. Virus Evolution
a. Some believe that viruses originated from the very cells that they infect.
b. For example, viral nucleic acids originated from the host cell genome.
c. Therefore, viruses evolved after cells came into existence; new viruses are probably evolving now.
d. Others suggest that viruses arose before the three domains.
3. Viruses often mutate; therefore, it is correct to say that they evolve.
a. Those that mutate are troublesome; a vaccine effective today may not be effective tomorrow.
b. Influenza (flu) viruses mutate regularly.
C. Viral Reproduction
1. Viruses gain entry into and are specific to a particular host cell because portions of the capsid (or spikes of the envelope) adhere to specific receptor sites on the host cell surface.
2. Viral nucleic acid then enters a cell, where viral genome codes for production of protein units in the capsid.
3. A virus may have genes for a few special enzymes needed for the virus to reproduce and exit from a host cell.
4. A virus relies on host cell enzymes, ribosomes, transfer RNA (tRNA), and ATP for its own replication.
D. Reproduction of Bacteriophages
1. Bacteriophages (phages) are viruses that parasitize bacteria.
2. The lytic cycle is a bacteriophage's "life" cycle consisting of five stages:
a. During attachment, portions of the capsid bind with receptors on the bacterial cell wall.
b. During penetration, a viral enzyme digests part of cell wall; the viral DNA is injected into a bacterial cell.
c. Biosynthesis involves synthesis of viral components and begins after the virus brings about inactivation of host genes not necessary to viral replication.
d. During maturation, viral DNA and capsids are assembled to produce several hundred viral particles and lysozyme, coded by the virus, is produced.
e. When lysozyme disrupts the cell wall, release of the viral particles occurs and the bacterial cell dies.
3. With the lysogenic cycle, the virus incorporates its DNA into the bacterium but only later is phage produced.
a. Following attachment and penetration, viral DNA becomes integrated into bacterial DNA with no destruction of the host DNA.
b. At this point, the phage is latent and the viral DNA is called a prophage.
c. The prophage is replicated along with host DNA; all subsequent cells (lysogenic cells) carry a copy.
d. Certain environmental factors (e.g., ultraviolet radiation) induce prophage to enter the biosynthesis stage of the lytic cycle, followed by maturation and release.
E. Reproduction of Animal Viruses
1. Animal viruses replicate similarly to bacteriophages, but there are modifications.
a. If the virus has an envelope, glycoprotein spikes allow it to adhere to plasma membrane receptors.
b. The virus genome covered by the capsid penetrates the host cell.
c. Once inside, the virus is uncoated as the envelope and capsid are removed.
d. Free of its covering, the viral genome (DNA or RNA) proceeds with biosynthesis.
e. Newly assembled viral particles are released by budding.
f. Components of viral envelopes (i.e., lipids, proteins, and carbohydrates) are obtained from the plasma or nuclear membrane of the host cell as the viruses leave.
2. Retroviruses are RNA animal viruses that have a DNA stage.
a. Retroviruses contain the enzyme reverse transcriptase that uses RNA as a template to produce cDNA; cDNA is is a copy of the viral genome.
b. Viral cDNA is integrated into host DNA and is replicated as host DNA replicates.
c. Viral DNA is transcribed; new viruses are produced by biosynthesis and maturation; release is by budding.
F. Viral Infections of Special Concern
1. Viruses cause infectious diseases in plants and animals, including humans.
2. Some animal viruses are specific to human cells: papillomavirus, herpes virus, hepatitis virus, and adenoviruses, which can cause specific cancers.
3. Retroviruses include the AIDS viruses (e.g., HIV) and also cause certain forms of cancer.
4. Emerging Viruses
5. HIV is an example of an emerging virus: the causative agent of a disease that has only recently arisen and infected people.
6. In some cases of emerging diseases, the virus is simply transported from one location to another; e.g., West Nile virus and severe acute respiratory syndrome (SARS).
7. The high mutation rate of viruses also cause infectious viruses to emerge; e.g., AIDS and Ebola fever.
8. A change in the mode of transmission is yet another way infectious viruses could emerge.
G. Viroids and Prions
1. Viroids are naked strands of RNA, a dozen of which cause crop diseases.
2. Like viruses, viroids direct the cell to produce more viroids.
3. Prions (proteinaceous infectious particles) are newly discovered disease agents that differ from viruses and bacteria.
a. Prions are rogue proteins with a wrongly-shaped tertiary structure that cause other proteins to distort.
b. Creutzfeldt-Jakob disease in humans and scrapie and mad cow disease (BSE) in cattle are due to prions.
21.2 The Prokaryotes
• Prokaryotes include the bacteria and archaea.
1. Bacteria were discovered in the seventeenth century when Antonie van Leeuwenhoek examined scrapings from his teeth.
2. The "little animals" Leeuwenhoek observed were thought by him and others to arise spontaneously from inanimate matter.
3. Around 1850, Pasteur devised an experiment showing that the bacteria present in air contaminated the media.
4. A single spoonful of soil contains 1010 prokaryotes; these are the most numerous life forms.
A. Structure of Prokaryotes
1. Prokaryotes range in size from 1–10 μm in length and from 0.7–1.5 μm in width.
2. "Prokaryote" means "before a nucleus"—their cells lack a eukaryotic nucleus.
3. Prokaryotic fossils date back as far as 3.5—3.8 billion years ago.
4. Fossils indicate prokaryotes were alone on earth for 2 billion years; they evolved very diverse metabolic capabilities.
5. Prokaryotes adapted to most environments because they differ in the many ways they acquire and utilize energy.
6. Outside the plasma membrane of most cells is a rigid cell wall that keeps the cell from bursting or collapsing due to osmotic changes by peptidoglycan, a complex molecule containing a unique amino disaccharide and peptide fragments.
a. The cell wall may be surrounded by an organized capsule called a glycocalyx and/or by a loose gelatinous sheath called a slime layer.
b. In parasitic forms, these outer coverings protect the cell from host defenses.
7. Some prokaryotes move by means of flagella.
a. The flagellum has a filament composed of three strands of the protein flagellin wound in a helix and inserted into a hook that is anchored by a basal body.
b. The flagellum is capable of 360° rotation which causes the cell to spin and move forward.
8. Many prokaryotes adhere to surfaces by means of fimbriae.
a. Fimbriae are short hairlike filaments extending from the surface.
b. The fimbriae of Neisseria gonorrhoeae allow it to attach to host cells and cause gonorrhea.
9. Prokaryotic cells lack the membranous organelles of eukaryotic cells.
10. Various metabolic pathways are located on the plasma membrane.
11. A nucleoid is a dense area in prokaryotes where the chromosome is located; it is a single circular strand of DNA.
12. Plasmids are accessory rings of DNA found in some prokaryotes; they can be extracted and used as vectors to carry foreign DNA into bacteria during genetic engineering procedures.
13. Protein synthesis in prokaryotic cells is carried out by thousands of ribosomes, which are smaller than eukaryotic ribosomes.
B. Reproduction in Prokaryotes
1. Binary fission is the splitting of a parent cell into two daughter cells; it is asexual reproduction in prokaryotes.
a. A single circular chromosome replicates; the two copies separate as the cell enlarges.
b. Newly formed plasma membrane and the cell wall separate the cell into two cells.
c. Mitosis, which involves formation of a spindle apparatus, does not occur in prokaryotes.
d. Because prokaryotes have a short generation time, mutations are generated and distributed through a population more rapidly.
e. Prokaryotes are haploid; mutations are therefore immediately subjected to natural selection.
2. In bacteria, genetic recombination can occur in three ways.
a. Conjugation occurs when a bacterium passes DNA to a second bacterium through a tube (sex pilus) that temporarily joins two cells; this occurs only between bacteria in the same or closely related species.
b. Transformation involves bacteria taking up free pieces of DNA secreted by live bacteria or released by dead bacteria.
c. In transduction, bacteriophages transfer portions of bacterial DNA from one cell to another.
d. Plasmids can carry genes for resistance to antibiotics and transfer them between bacteria by any of these processes.
3. Some bacteria form resistant endospores in response to unfavorable environmental conditions.
a. Some cytoplasm and the chromosome dehydrate and are encased by three heavy, protective spore coats.
b. The rest of the bacterial cell deteriorates and the endospore is released.
c. Endospores survive in the harshest of environments: desert heat and dehydration, boiling temperatures, polar ice, and extreme ultraviolet radiation.
d. Endospores also survive very long periods of time; anthrax spores 1,300 years old can cause disease.
e. When environmental conditions are again suitable, the endospore absorbs water and grows out of its spore coat.
f. In a few hours, newly emerged cells become typical bacteria capable of reproducing by binary fission.
g. Endospore formation is not reproduction--it is a means of survival and dispersal to new locations.
C. Prokaryotic Nutrition
1. Bacteria differ in their need for, and tolerance of, oxygen (O2).
a. Obligate anaerobes are unable to grow in the presence of O2; this includes anaerobic bacteria that cause botulism, gas gangrene, and tetanus.
b. Facultative anaerobes are able to grow in either the presence or absence of gaseous O2.
c. Aerobic organisms (including animals and most prokaryotes) require a constant supply of O2 to carry out cellular respiration.
2. Autotrophic Prokaryotes
a. Photoautotrophs are photosynthetic and use light energy to assemble the organic molecules they require.
i. Primitive photosynthesizing bacteria (e.g., green sulfur bacteria and purple sulfur bacteria) use only photosystem I that contains bacteriochlorophyll; they do not give off O2 because hydrogen sulfide (H2S) is used as an electron and H+ donor instead of H2O.
ii. Advanced photosynthesizing bacteria (e.g., cyanobacteria) use both photosystem I and II that contain the same types of chlorophylls found in plants; they do give off O2 because H2O is used as an electron and H+ donor.
b. Chemoautotrophs make organic molecules by using energy derived from the oxidation of inorganic compounds in the environment.
i. Deep ocean hydrothermal vents provide H2S to form chemosynthetic bacteria.
ii. The methanogens are chemosynthetic bacteria that produce methane (CH4) from hydrogen gas and CO2; ATP synthesis and CO2 reduction are linked to this reaction and methanogens can decompose animal wastes to produce electricity as an ecological friendly energy source.
iii. Nitrifying bacteria oxidize ammonia (NH3) to nitrites (NO2) and nitrites to nitrates (NO3).
3. Heterotrophic Prokaryotes
a. Most free-living bacteria are chemoheterotrophs that take in pre-formed organic nutrients.
b. As aerobic saprotrophs, there is probably no natural organic molecule that cannot be broken down by some prokaryotic species.
c. Detritivores (saprophytic bacteria) are critical in recycling materials in the ecosystem; they decompose dead organic matter and make it available to photosynthesizers.
d. Prokaryotes produce chemicals including ethyl alcohol, acetic acid, butyl alcohol, and acetones.
e. Prokaryotic action produces butter, cheese, sauerkraut, rubber, cotton, silk, coffee and cocoa.
f. Antibiotics are produced by some bacteria.
4. Some chemoheterotrophs are symbiotic, forming relationships with members of other species; forms of symbiosis include mutualistic, commensalistic, and parasitic relationships.
a. Mutualistic nitrogen-fixing Rhizobium bacteria live in nodules on roots of soybean, clover, and alfalfa where they reduce N2 to ammonia for their host; bacteria use some of a plant's photosynthetically produced organic molecules.
b. Mutualistic bacteria that live in the intestines of humans benefit from undigested material and release vitamins K and B12, which we use to produce blood components.
c. In the stomachs of cows and goats, mutualistic prokaryotes digest cellulose.
d. Commensalistic bacteria live in or on organisms of other species and cause them no harm.
e. Parasitic bacteria are responsible for a wide variety of infectious plant, animal and human diseases.
5. Bacterial Diseases in Humans
a. Microbes that cause disease are called pathogens.
b. Pathogens may be able to produce a toxin, and or adhere to surfaces and sometimes invade organs or cells.
i. Toxins are small organic molecules, or small pieces of protein or parts of the bacterial cell wall, that are released when bacteria die.
ii. In almost all cases, the growth of the bacteria does not cause disease but instead the toxins they release cause the disease. Example: Clostridiumtetani, the causative agent of tetanus.
c. Adhesion factors allow a pathogen to bind to certain cells, which determines which tissue in the body will be the host. Example: Shigelladysentariae releases a toxin and also sticks to the intestinal wall, making it a life-threatening form of dysentary.
d. Antibacterial compounds either inhibit cell wall synthesis or protein biosynthesis; increasingly, many pathogenic bacteria are becoming resistant to bacteria.
21.3 The Bacteria
1. The Gram stain procedure (developed in the late 1880s by Hans Christian Gram) differentiates bacteria.
a. Gram-positive bacteria stain purple, whereas Gram-negative bacteria stain pink.
b. This difference is dependent on the thick or thin (respectively) peptidoglycan cell wall.
2. Bacteria and archaea have three basic shapes.
a. A spirillum is spiral-shaped.
b. A bacillus is an elongated or rod-shaped bacteria.
c. Coccus bacteria are spherical.
d. Cocci and bacilli tend to form clusters and chains of a length typical of the particular species.
3. Other criteria for classification of bacteria are the presence of endospores, metabolism, growth and nutritional characteristics, and other physiological characteristics.
4. Recent work by Carl Woese has revised bacterial taxonomy based on similarity of 16S rRNA; twelve groups are now recognized based on bacterial 16S ribosomal RNA sequences.
B. Cyanobacteria
1. Cyanobacteria are Gram-negative bacteria with a number of unusual traits.
2. They photosynthesize in the same manner as plants, and thus are responsible for introducing O2 into the primitive atmosphere.
3. They were formerly mistaken for eukaryotes and classified with algae.
4. Cyanobacteria have pigments that mask chlorophyll; they are not only blue-green but also red, yellow, brown, or black.
5. They are relatively large (1–50 μm in width).
6. They can be unicellular, colonial, or filamentous.
7. Some move by gliding or oscillating.
8. Some possess heterocysts, thick-walled cells without a nucleoid, where nitrogen fixation occurs.
9. Cyanobacteria are common in fresh water, soil, on moist surfaces, and in harsh habitats (e.g., hot springs).
10. Some species are symbiotic with other organisms (e.g., liverworts, ferns, and corals).
11. Lichens are a symbiotic relationship where the cyanobacteria provide organic nutrients to the fungus and the fungus protects and supplies inorganic nutrients.
12. Cyanobacteria were probably the first colonizers of land during evolution.
13. Cyanobacteria "bloom" when nitrates and phosphates are released as wastes into water; when they die off, decomposing bacteria use up the oxygen and cause fish kills.
21.4 The Archaea
A. Relationship to Domain Bacteria and Domain Eukarya
1. Archaea are prokaryotes with molecular characteristics that distinguish them from bacteria and eukaryotes; their rRNA base sequence is different from that in bacteria.
2. Because archaea and some bacteria are both found in extreme environments (hot springs, thermal vents, salt basins), they may have diverged from a common ancestor.
3. Later, the eukarya split from the archaea; archaea and eukarya share some ribosomal proteins not found in bacteria; initiate transcription in the same manner, and have similar types of tRNAs.
B. Structure and Function
1. Archaea have unusual lipids in their plasma membranes that allow them to function at high temperatures: glycerol linked to hydrocarbons rather than fatty acids.
2. Cell walls of archaea do not contain the peptidoglycan found in bacterial cell walls.
3. Only some methanogens have the ability to form methane.
4. Most are chemoautotrophs; none are photosynthetic; this suggests chemoautotrophy evolved first.
5. Some are mutualistic or commensalistic but none are parasitic—none are known to cause disease.
C. Types of Archaea
1. Methanogens live under anaerobic environments (e.g., marshes) where they produce methane.
a. Methane is produced from hydrogen gas and carbon dioxide and is coupled to formation of ATP.
b. Methane released to the atmosphere contributes to the greenhouse effect.
c. About 65% of the methane found in our atmosphere is produced by methanogenic archaea.
2. Halophiles require high salt concentrations (e.g., Great Salt Lake).
a. Their proteins have unique chloride pumps that use halorhodopsin to synthesize ATP in the presence of light.
b. They usually require 12–15% salt concentrations; the ocean is only 3.5% salt.
3. Thermoacidophiles live under hot, acidic environments (e.g., geysers).
a. They survive best at temperatures above 80°C; some survive above boiling temperatures.
b. Metabolism of sulfides forms acidic sulfates; these bacteria grow best at pH of 1 to 2.
22.1 General Biology of Protists
• Protists are classified in the domain Eukarya (they have eukaryotic cells) and the kingdom Protista.
1. The endosymbiotic hypothesis suggests how the eukaryotic cells arose.
a. It proposes that aerobic bacteria became mitochondria.
b. Cyanobacteria became chloroplasts after being taken up by eukaryotic cells.
c. Giardia lamblia has two nuclei but no mitochondria, suggesting that a nucleated cell preceded the acquisition of mitochondria.
2. Although many protists are unicellular, they are highly complex.
a. Amoeboids and ciliates possess unique organelles, such as contractile vacuoles.
3. Most protists are free-living; some are parasitic and some (e.g., slime molds) are saprophytic (feed on decaying plant material).
4. Some protists are photoautotrophic; some are heterotrophic.
5. Most protists use asexual reproduction, but sexual reproduction occurs in some species.
a. Formation of spores allows free-living and parasitic protists to survive hostile environments.
b. A cyst is a dormant cell with a resistant outer covering; the cyst allows a free-living species to overwinter and helps certain parasitic species survive the host's digestive juices.
A. Ecological Importance
1. Some protists are of great medical importance because they cause disease; others are ecologically important.
2. Aquatic plankton serve as food for heterotrophic protists and animals.
3. Photosynthetic plankton produce much of the oxygen in the atmosphere.
4. Many protists enter symbiotic relationships; coral reefs rely on symbiotic photosynthetic protists.
B. Classification of Protists
1. Multicellular algae are not plants; they do not protect their gametes and zygote from drying out.
2. None are fungi; those that resemble fungi lack flagella and do not have chitin in their cell wall.
3. None are animals; the heterotrophic protists do not undergo embryonic development.
4. Due to their complexity, protists may deserve more than a dozen kingdoms.
22.2 Diversity of the Protists
1. Algae refers to many phyla of protists that carry out photosynthesis.
2. At one time, algae were grouped with plants because they have chlorophyll a and photosynthesize.
B. The Green Algae
1. Phylum Chlorophyta contains the green algae.
2. They live in the ocean but are more likely found in fresh water and can even be found on moist land.
3. Green algae are not always green; some have pigments that give them an orange, red, or rust color.
4. Organizational forms include single cells, colonies, filaments and multicellular forms.
5. Plants are considered to be most closely related to the green algae.
C. Chlamydomonas, a Unicellular Green Algae
1. Chlamydomonas is a minute, unicellular green alga less than 25 mm long.
2. It has a cell wall and a single, large, cup-shaped chloroplast with a pyrenoid for starch synthesis.
3. The chloroplast contains a light-sensitive eyespot (stigma) that directs the cell to light for photosynthesis.
4. Two long whiplike flagella project from the anterior end to propel the cell toward light.
5. When growth conditions are favorable, Chlamydomonas reproduces asexually with flagellated spores called zoospores.
6. When growth conditions are unfavorable, Chlamydomonas reproduces sexually.
a. Gametes from two different mating types join to form a zygote.
b. A heavy wall forms around the zygote; a resistant zygospore survives until conditions are favorable.
D. Spirogyra, a Filamentous Green Algae
1. Cell division in one plane produces end-to-end chains of cells or filaments.
2. Spirogyra is a filamentous algae found on surfaces of ponds and streams.
a. It has ribbonlike spiral chloroplasts.
b. Two strands may unite in conjugation and exchange genetic material, forming a diploid zygote.
c. The zygotes withstand winter; in spring they undergo meiosis to produce haploid filaments.
E. Multicellular Green Algae
1. Plants are probably related to green algae because both have a cell wall with cellulose, have chlorophyll a and b, and store starch.
2. The multicellular Ulva is called sea lettuce because of its leafy appearance.
3. The thallus (body) is two cells thick but can be a meter long.
4. Ulva has an alternation of generations life cycle, as do plants, but the generations look alike.
5. The gametes look alike (isogametes) and the spores are flagellated.
6. Stoneworts are green algae that live in freshwater lakes and ponds.
7. The stonewort Chara forms a cell plate during cell division and has multicellular sex organs making plants most closely related to this group.
8. Chara also has a stemlike body with nodes and internodes; the cells of the body originate from apical meristem features that are homologous with plants.
F. Volvox, a Colonial Green Algae
1. Volvox is a hollow sphere with thousands of cells arranged in a single layer.
2. Volvox cells resemble Chlamydomonas cells; a colony arises if the daughter cells fail to separate.
3. Volvox cells cooperate when flagella beat in a coordinated fashion.
4. Some cells are specialized forming a new daughter colony within the parental colony.
5. Daughter colonies are inside a parent colony until an enzyme dissolves part of a wall so they can escape.
G. The Red Algae
1. Red algae (phylum Rhodophyta) are chiefly marine multicellular algae that live in warmer seawater.
2. They are generally much smaller and more delicate than brown algae.
3. Some are filamentous, but most are branched, having a feathery, flat, or ribbonlike appearance.
4. Coralline algae are red algae with cell walls with calcium carbonate; they contribute to coral reefs.
5. Red algae are economically important.
a. Mucilaginous material in cell walls of Gelidium and Gracilaria is the source of agar used in drug capsules, dental impressions, and cosmetics.
b. In the laboratory, agar is a major microbiological media, and when purified, is a gel for electrophoresis.
c. Agar is used in food preparation to keep baked goods from drying and to set jellies and desserts.
d. Carrageen, an emulsifying agent extracted from Chondrus crispus, is used in production of chocolate and cosmetics.
H. The Brown Algae
1. The phylum Phaeophyta includes the brown algae; over 1,500 species are known.
2. They range from small forms with simple filaments to large multicellular seaweeds; no unicellular or colonial forms exist.
3. Brown algae have chlorophylls a and c and fucoxanthin that give them their color.
4. Their reserve food is a carbohydrate called laminarin.
5. Seaweed refers to any large, complex alga.
6. Their cell walls contain a mucilaginous water-retaining material that inhibits desiccation.
7. Laminaria is an intertidal kelp that is unique among protists; this genus shows tissue differentiation.
8. Nereocystis and Macrocystis are giant kelps found in deeper water anchored to the bottom by their holdfasts.
9. Individuals of the genus Sargassum sometimes break off from their holdfasts and form floating masses.
10. Brown algae provide food and habitat for marine organisms, and they are also important to humans.
a. Brown algae are harvested for human food and for fertilizer in several parts of the world.
b. Macrocystis is a source of algin, a pectinlike substance added to give foods a stable, smooth consistency.
11. Most have an alternation of generations life cycle.
12. Fucus is an intertidal rockweed; meiotic cell division produces gametes and the adult is always diploid.
I. The Diatoms and Golden Brown Algae
1. Phylum Chrysophyta (Bacillariophyta) contains both diatoms (about 11,000 species), golden-brown algae (500 species), and yellow-green algae (600 species).
2. Diatoms are the most numerous unicellular algae in the oceans; they are an important part of the phytoplankton, photosynthetic organisms that are a source of food and oxygen for heterotrophs.
3. Diatom cell walls consist of two silica-impregnated halves or valves.
a. When diatoms reproduce asexually, each received one old valve.
b. The new valve fits inside the old one; therefore, the new diatom is smaller than the original one.
c. This continues until they are about 30% of their original size.
d. Then they reproduce sexually; a zygote grows and divides mitotically to form diatoms of normal size.
4. The cell wall has an outer layer of silica (glass) with a variety of markings formed by pores.
5. Diatom remains accumulate on the ocean floor and are mined as diatomaceous earth for use as filters, abrasives, etc.
J. The Dinoflagellates
1. Phylum Pyrrophyta contains the 4,000 species of the unicellular dinoflagellates.
2. These algae are bounded by protective cellulose plates.
3. Most have two flagella.
a. One lies in a longitudinal groove and acts as a rudder.
b. The other is located within a transverse groove; beating causes the cell to spin as it moves forward.
4. Some species of dinoflagellates are heterotrophic; some are parasitic.
5. They are extremely numerous (30,000 per ml) and an important source of food in the ecosystem.
6. Under certain conditions, Gymnodinium and Gonyaulax increase in number enormously and cause a "red tide"; they produce a powerful neurotoxin killing fish and causing paralytic shellfish poisoning.
K. The Euglenoids
1. Phylum Euglenophyta includes the euglenoids; about 750 species are known.
2. Euglenoids are small (10–500 μm) freshwater unicellular organisms.
3. One-third of all genera have chloroplasts; those that lack chloroplasts ingest or absorb their food.
4. Their chloroplasts are surrounded by three, rather than two, membranes.
a. Their chloroplasts resemble those of green algae.
b. They are probably derived from a green algae through endosymbiosis.
5. The pyrenoid outside the chloroplast produces an unusual type of carbohydrate polymer (paramylon) not seen in green algae.
6. They possess two flagella, one of which typically is much longer than the other and projects out of a vase-shaped invagination; it is called a tinselflagellum because it has hairs on it.
7. Near the base of the longer flagellum is a red eyespot that shades a photoreceptor for detecting light.
8. They lack cell walls, but instead are bounded by a flexible pellicle composed of protein bands side-by-side.
9. A contractile vacuole, similar to certain protozoa, eliminates excess water.
10. Euglenoids reproduce by longitudinal cell division; sexual reproduction is not known to occur.
L. The Zooflagellates
1. Phylum Zoomastigophora includes the zooflagellates.
2. These protozoa are covered by a pellicle that is often reinforced by underlying microtubules.
3. Many are symbiotic and some are parasitic.
a. Trypanosoma brucei, a trypanosome transmitted by the bite of a tsetse fly, is the cause of African sleeping sickness.
b. Giardia lamblia cysts are transmitted through contaminated water; they cause severe diarrhea.
c. Trichomonas vaginalis is a sexually transmitted organism that infects the vagina and urethra of women and prostate, seminal vesicles and urethra of men.
4. Some scientists place all flagellates among the protozoans; this would then include both photosynthetic and heterotrophic organisms.
M. Protists with Pseudopods
1. Protists that move with pseudopods usually live in aquatic environments.
2. They are part of the zooplankton, microscopic floating organisms.
3. They engulf prey with pseudopods, cytoplasmic extensions formed as cytoplasm streams in one direction.
4. Amoeba proteus is a commonly studied member.
5. Amoeboids phagocytize their food; pseudopods surround and engulf prey.
6. Food is digested inside food vacuoles.
7. Freshwater amoeboids have contractile vacuoles to eliminate excess water.
8. Entamoeba histolytica is an amoebic parasite that invades the human intestinal lining.
9. Foraminiferans (phylum Foraminifera) and radiolarians (phylum Actinopoda)
a. Both are sarcodines with a skeleton called a test.
b. Foraminiferans have a multi-chambered CaCO3 shell; thin pseudopods extend through holes.
10. Radiolaria have a test composed of silica or strontium sulfate.
a. Most have a radial arrangement of spines.
b. Pseudopods (actinopods) project from an external layer of cytoplasm and are supported by rows of microtubules.
11. Tests of dead foraminiferans and radiolarians form deep layers of ocean floor sediment.
12. Dating back to the Precambrian, each layer has distinctive foraminiferans which helps date rocks.
13. Over hundreds of millions of years, the CaCO3 shells have contributed to the formation of chalk deposits (i.e., White Cliffs of Dover, limestone of the great Egyptian pyramids).
N. The Ciliates
1. Phylum Ciliophora contains the 8,000 species of ciliates.
2. Ciliates move by coordinated strokes of hundreds of cilia projecting through holes in a semirigid pellicle.
3. They discharge long, barbed trichocysts for defense and for capturing prey; toxicysts release a poison.
4. Most are holozoic and ingest food through a gullet and eliminate wastes through an anal pore.
5. During asexual reproduction, ciliates divide by transverse binary fission.
6. Ciliates possess two types of nuclei—a large macronucleus and one or more small micronuclei.
a. The macronucleus controls the normal metabolism of the cell.
b. The micronucleus is involved in sexual reproduction.
i. The macronucleus disintegrates and the micronucleus undergoes meiosis.
ii. Two ciliates then exchange a haploid micronucleus.
iii. The micronuclei give rise to a new macronucleus containing only housekeeping genes.
7. Ciliates are diverse with over 8,000 known species.
a. Members of the genus Paramecium are complex.
b. The barrel-shaped didiniums expand to consume paramecia much larger than themselves.
c. Suctoria rest on a stalk and paralyze victims, sucking them dry.
d. Stentor resembles a giant blue vase with stripes.
O. The Sporozoans
1. Phylum Apicomplexa contains the nonmotile parasitic sporozoans (about 3,900 species).
2. The phylum name describes the unique apical complex of organelles.
3. Their common name recognizes that they form spores at some point in their life cycle.
4. Pneumocystis carinii causes the pneumonia seen primarily in AIDS patients.
a. During sexual reproduction, thick-walled cysts form in the lining of pulmonary air sacs.
b. Cysts contain spores that successively divide until the cyst bursts and the spores are released.
c. Each spore becomes a new organism, reproduces asexually and can enter an encysted sexual stage.
5. Plasmodium vivax causes one type of malaria; it is the most widespread human parasite.
a. After the bite of an infected female Anopheles mosquito, the parasite eventually invades red blood cells.
b. Chills and fever appear as red blood cells burst and release toxin into the blood.
c. Malaria remains a major world disease due to insecticide-resistant strains of mosquitoes and drug-resistant strains of Plasmodium.
6. Toxoplasma gondii causes toxoplasmosis, particularly in cats but also in humans.
a. In pregnant women, the parasite can infect the fetus and cause birth defects.
b. In AIDS patients, it can infect the brain and cause neurological symptoms.
P. The Slime Molds and Water Molds
1. These organisms resemble fungi but all have flagellated cells that fungi never have.
2. Water molds possess a cell wall but it is made of cellulose, not chitin as in fungi.
3. Water molds produce diploid (2n) zoospores and meiosis produces the gametes.
4. The Plasmodial Slime Molds
a. Plasmodial slime molds (phylum Myxomycota) exist as a plasmodium.
b. This diploid multinucleated cytoplasmic mass creeps along, phagocytizing decaying plant material.
c. Fan-shaped plasmodium contains tubules of concentrated cytoplasm in which liquefied cytoplasm streams.
d. Under unfavorable environmental conditions (e.g., drought), the plasmodium develops many sporangia, called a fruiting body, that produce spores by meiosis.
e. Mature spores are released and survive until more favorable environmental conditions return; then each releases a haploid flagellated cell or an amoeboid cell.
f. Two flagellated or amoeboid cells fuse to form a diploid zygote that produces a multinucleated plasmodium again.
5. The Cellular Slime Molds
a. Cellular slime molds (phylum Acrasiomycota) exist as individual amoeboid cells.
b. They live in soil and feed on bacteria and yeast.
c. As food runs out, amoeboid cells release a chemical that causes them to aggregate into a pseudoplasmodium.
d. The pseudoplasmodium stage is temporary; it gives rise to sporangia that produce spores.
e. Spores survive until more favorable environmental conditions return, at which time they germinate.
f. Spores germinate to release haploid amoeboid cells, which is again the beginning of the asexual cycle.
g. A sexual cycle occurs under very moist conditions.
Q. The Water Molds
1. Phylum Oomycota includes the water molds; 500 species have been described.
2. Aquatic water molds parasitize fishes, forming furry growths on their gills, and decompose the fish remains.
3. Terrestrial water molds parasitize insects and plants; a water mold caused the 1840s Irish potato famine.
4. Most water molds are saprotrophic, living off dead organic matter.
5. Water molds have a filamentous body but cell walls are composed largely of cellulose.
6. During asexual reproduction, they produce diploid motile spores (2n zoospores) with flagella.
7. Unlike fungi, the adult is diploid; gametes are produced by meiosis.
8. Eggs are produced in enlarged tips called oogonia.
23.1 Characteristics of Fungi
1. The 80,000 species of the Kingdom Fungi are mostly multicellular eukaryotes that share a common mode of nutrition.
2. Like animals, fungi are heterotrophic and consume preformed organic matter.
3. Animals, however, are heterotrophic by ingestion while fungi are heterotrophic by absorption.
4. Fungal cells secrete digestive enzymes; following breakdown of molecules, the nutrients are absorbed.
5. Most fungi are saprotrophic decomposers, breaking down wastes or remains of plants and animals.
6. Some are parasitic, living off the tissues of living plants and animals.
a. Plants are especially subject to fungal diseases.
b. Fungal diseases account for millions of dollars in crop losses each year; fungal diseases also have reduced the numbers of certain species of trees.
c. Fungi also cause human diseases including ringworm, athlete's foot, and yeast infections.
7. Several types of fungi are adapted to mutualistic relationships with other organisms.
a. As symbionts of roots, they acquire inorganic nutrients for plants and receive organic nutrients.
b. Others form an association with a green alga or cyanobacterium to form a lichen.
B. Structure of Fungi
1. Fungi can be unicellular (e.g., yeasts).
2. Most fungi are multicellular in structure.
a. The thallus (body) of most fungi is called a mycelium.
b. A mycelium is a network of hyphae comprising the vegetative body of a fungus.
c. Hyphae are filaments that provide a large surface area and aid absorption of nutrients.
d. When a fungus reproduces, a portion of the mycelium becomes a reproductive structure.
3. Fungal cells lack chloroplasts and have a cell wall made of chitin, not cellulose.
a. Chitin, like cellulose, is a polymer of glucose molecules organized into microfibrils.
b. In chitin, unlike cellulose, each glucose has an attached nitrogen containing amino group.
4. The energy reserve of fungi is not starch, but glycogen, as in animals.
5. Fungi are nonmotile; their cells lack basal bodies and do not have flagella at any stage in their life.
6. Fungi move to a food source by growing toward it; hyphae can grow up to a kilometer a day.
7. Nonseptate fungi lack septa, or cross walls, in their hyphae; nonseptate hyphae are multinucleated.
8. Septate fungi have cross walls in their hyphae; pores allow cytoplasm and organelles to pass freely.
9. The septa that separate reproductive cells, however, are complete in all fungal groups.
C. Reproduction of Fungi
1. In general, fungal sexual reproduction involves the following:
haploid hyphae → dikaryotic stage → diploid zygote
↑-------------←-------- meiosis----←-----------↓
2. During sexual reproduction, haploid hyphae from two different mating types fuse.
3. If nuclei do not fuse immediately, the resulting hypha is dikaryotic (contains paired haploid nuclei, n + n).
a. In some species, nuclei pair but do not fuse for days, months, or even years.
b. The nuclei continue to divide in such a way that every cell has at least one of each type of nucleus.
4. When the nuclei fuse, the resulting zygote undergoes meiotic cell division leading to spore formation.
5. Fungal spores germinate directly into haploid hyphae without embryological development.
6. Fungal Spore Formation
a. Spores are an adaptation to life on land and ensure that the species will be dispersed to new locations.
b. A spore is a reproductive cell that can grow directly into a new organism.
c. Fungi produce spores both during sexual and asexual reproduction.
d. Although nonmotile, the spores are readily dispersed by wind.
7. Asexual reproduction can occur by three mechanisms:
a. Production of spores by a single mycelium is the most common mechanism.
b. Fragmentation is when a portion of a mycelium becomes separated and begins a life of its own.
c. Budding is typical of yeasts; a small cell forms and gets pinched off as it grows to full size.
23.2 Evolution of Fungi
1. Fungi evolved about 570 million years ago.
2. Fungi may not share a common ancestor but may have evolved separately from protist ancestors.
3. Some biologists propose that fungi evolved from red algae; both lack flagella in all stages of the life cycle.
4. In 1969, R. H. Whittaker argued for their own kingdom based on their multicellular nature and mode of nutrition.
5. Fungi share similarities and have differences with other groups of protists.
6. Not knowing phylogeny, fungal groups are classified according to differences in life cycles and the types of structure that produces spores.
B. Zygospore Fungi
1. Phylum Zygomycota contains about 1,050 species of zygospore fungi.
2. Most are saprotrophs living off plant and animal remains in the soil or bakery goods in a pantry.
3. Some are parasites of small soil protists, worms, or insects.
4. The black bread mold, Rhizopus stolonifer, is a common example.
a. With little cellular differentiation among fungi, hyphae specialize for various functions.
b. Stolons are horizontal hyphae that exist on the surface of the bread.
c. Rhizoids are hyphae that grow into the bread, anchor the mycelium, and carry out digestion.
d. Sporangiophores are stalks that bear sporangia.
e. A sporangium is a capsule that produces spores called sporangiospores.
f. During asexual reproduction, all structures are haploid.
5. The phylum name refers to the zygospore seen during sexual reproduction.
a. Hyphae of different mating types (+ and –) are chemically attracted and grow toward each other.
b. Ends of hyphae swell as nuclei enter; cross walls develop behind each end, forming gametangia.
c. Gametangia merge into a large multinucleate cell in which nuclei of two mating types pair and fuse.
d. A thick wall develops around the cell, forming a zygospore.
e. The zygospore undergoes a period of dormancy before meiosis and germination takes place.
f. Germination involves the development of one or more sporangiophores, with sporangia at their tips.
g. The spores are dispersed by air currents and give rise to new haploid mycelia.
C. Sac Fungi
1. Phylum Ascomycota contains about 60,000 species of sac fungi.
2. Two main groups are recognized: the sexual ascomycetes (sexual reproduction) and asexual ascomycetes (sexual reproduction not yet observed).
3. Sexual ascomycetes include the yeasts, the unicellular fungi, red bread molds, morels, and truffles.
a. Many sexual ascomycetes are plant parasites and include the powdery mildews that grow on leaves, leaf curl fungi, chestnut blight, and Dutch elm disease.
b. Ergot is a parasitic fungus on rye; it produces a toxin that can cause hysteria and death.
4. Asexual ascomycetes used to be in the phylum Deuteromycota (imperfect fungi), but are now recognized as sac fungi.
a. Aspergillus and Candida are in this group.
5. Biology of the Sac Fungi
a. The body of the ascomycetes can be a single cell (e.g., yeasts), but more often it is a mycelium composed of septate hyphae.
b. Ascomycetes digest many substances not easily decomposed, such as cellulose, lignin, collagen, jet fuel, and wall paint.
c. Some are symbiotic with algae, forming lichens, and plant roots, forming mycorrhizae.
d. Ascomycetes account for most of the known fungal pathogens.
6. Reproduction
a. Asexual reproduction is the norm among ascomycetes.
i. There are no sporangia in ascomycetes.
ii. Conidiospores (conidia) develop directly on tips of conidiophores, modified aerial hyphae, and are windblown when released.
iii. Yeasts reproduce asexually by budding.
b. The ascus, contained within saclike ascocarp,is a fingerlike sac that develops during sexual reproduction; the asci are protected by sterile hyphae within a fruiting body called the ascocarp.
i. Each ascus contains eight haploid nuclei and produces eight ascospores.
ii. In most ascomycetes, the asci become swollen and burst, expelling the ascospores.
iii. If released into the air, the spores are windblown.
7. Relationship of Ascomycetes to Humans
a. Many ascomycetes produce useful substances, e.g., penicillin, cyclosporin, steroids; some are used in the production of various foods.
b. Some cause diseases (mycoses), e.g., ringworm.
8. Yeasts
a. Saccharomyces cerevisiae is brewer's yeast; because it produces CO2, yeast fermentation is important in the production of bread.
b. Yeasts are also used in genetic engineering experiments requiring a eukaryote.
c. Candidaalbicans is the yeast which causes the widest variety of fungal infections, e.g., vaginal infections, oral thrush, or, in immunocompromised individuals, systemic infections.
9. Molds
a. Aspergillus is a group of green molds that is sometimes pathogenic to humans.
b. It is used to produce soy sauce by fermentation of soybeans; it is used in the production of many other food and cosmetic additives.
c. Aspergillusflavus secretes a potent carcinogen, and it also causes disease of the respiratory tract.
d. Other molds grow in building materials, causing the "sick-building" syndrome.
e. Penicillium produces the important antibiotic penicillin.
f. Moldlike fungi in the genus Tinea cause diseases of the skin called tineas; examples are athlete's foot and ringworm.
g. Histoplasmacapsulatum, which grows in both mold form and yeast form, causes the mild "fungal flu"; the fungus grows within cells of the immune system and causes systemic disease.
10. Control of Fungal Infections
a. Because fungal cells are similar to human cells, antifungal medications are difficult to design.
b. Some fungicides are directed against steroid biosynthesis in the fungus; some are based on heavy metals.
D. Club Fungi
1. Club fungi are in the phylum Basidiomycota and include over 22,000 species.
2. They have septate hyphae and include mushrooms, bracket fungi, puffballs, bird's nest fungi, and stinkhorns.
3. Biology of Club Fungi
a. Sexual reproduction involves production of basidiospores within fruiting bodies in the basidium.
i. Sexual reproduction begins when monokaryotic hyphae of two different mating types meet and fuse to form a dikaryotic (n + n) mycelium.
ii. The dikaryotic mycelium continues its existence for years (perhaps even hundreds of years).
iii. A basidium contains four projections; cytoplasm and a haploid nucleus enters to form four basidiospores.
iv. Released basidiospores are windblown; when they germinate, a new haploid mycelium forms.
v. In a puffball, spores inside parchmentlike membranes are released through a pore or when the parchment breaks down.
vi. In bird's nest fungi, raindrops splatter basidiospore-containing "eggs" through the air.
vii. Stinkhorns have a slimy cap and attract flies by their bad odor to pick up and distribute spores.
4. Smuts and Rusts
a. Smuts and rusts are club fungi that parasitize cereal crops (e.g., corn, wheat, oats, and rye) and cause great economic crop losses every year.
b. Rusts and smuts do not form basidiocarps; their spores are small and numerous, resembling soot.
c. Some smuts enter seeds and exist inside the plant, becoming visible only at maturity.
d. Corn smut mycelia grow between the corn kernels and secrete substances to cause tumors on ears.
e. Rusts have a complex life cycle that may involve more than one host; thus, control measures may center on eradicating the alternate hosts.
23.3 Symbiotic Relationships of Fungi
A. Lichens
1. Lichens are a symbiotic association between a fungus and a cyanobacterium or a green alga.
2. The body of a lichen is composed of three layers:
3. a thin, tough upper layer and a loosely packed lower layer that shield the photosynthetic cells in the middle layer.
4. Special fungal hyphae penetrate or envelope the photosynthetic cells and transfer nutrients directly to the rest of the fungus.
5. Lichens can reproduce asexually by releasing fragments that contain hyphae and an algal cell.
6. This association was considered mutualistic, but experimentation suggests a controlled parasitism by the fungus of the alga.
a. The algae grow faster when they are alone rather than when they are part of a lichen.
b. On the other hand, it is difficult to cultivate the fungus, which does not grow naturally alone.
c. Different lichen species are identified based on the fungal partner.
7. Three types of lichens are recognized.
a. Compact crustose lichens are often seen on bare rocks or tree bark.
b. Foliose lichens are leaflike.
c. Fruticose lichens are shrublike.
8. Lichens are efficient at acquiring nutrients; they survive with low moisture, temperature, or poor soil.
9. Lichens may live in extreme environments and on bare rocks; they help form soil.
10. Lichens also take up pollutants and cannot survive where the air is polluted.
B. Mycorrhizae
1. Mycorrhizae are mutualistic relationships between soil fungi and roots of most plants.
2. The fungus enters the cortex of roots but does not enter the cytoplasm of plant cells.
3. Ectomycorrhizae form a mantle that is exterior to the root, growing between cell walls.
4. It helps the roots absorb more minerals; in turn, the plant passes on carbohydrates to the fungus.
5. The truffle lives in association with oak and beech tree roots; it can be inoculated with the fungus.
6. The fossil record indicates that the earliest plants had mycorrhizae associated with them; mycorrhizae helped plants adapt to and flourish on land.
24.1 Evolutionary History of Plants
1. Plants are multicellular photosynthetic eukaryotes placed in kingdom Plantae; 280,000 species are known.
2. Plants are believed to have evolved from a freshwater green algal ancestor over 500 million years ago.
a. Both utilize chlorophylls a and b and various accessory pigments.
b. In both, the food reserve is starch.
c. The cell walls of both contains cellulose.
d. DNA base codes for rRNA suggest plants are most closely related to green algae known as stoneworts.
3. The common ancestor would have existed sometime in the Paleozoic era.
4. Plants, from nonvascular to vascular, nourish a multicellular embryo within the body of the female plant; this distinguishes them from green algae.
5. Vascular plants have vascular tissue, specialized elongated cells that conduct water and solutes through the plant.
6. Vascular plants evolved about 430 million years ago during the Silurian period.
7. The cone-bearing gymnosperms and flowering angiosperms both produce seeds.
a. A seed is an embryo and organic nutrient within a protective coat.
b. Seeds are resistant to drought and somewhat resistant to predators.
c. Gymnosperms appeared about 400 million years ago, during the Devonian Period.
8. Flowers evolved as reproductive structures to attract pollinators; they first appeared about 135 million years ago.
9. All of the above features are adaptations for life on land.
B. Alternation of Generations
1. Plants have a two-generation life cycle called alternation of generations.
a. The sporophyte is a diploid (2n) generation producing haploid spores by meiotic cell division.
b. The gametophyte is a haploid (n) generation producing haploid gametes by mitotic division.
c. In the plant life cycle, a spore undergoes mitosis and becomes a gametophyte.
d. Note that meiosis produces haploid spores.
e. Mitosis occurs as a spore becomes a gametophyte, and also as a zygote becomes a sporophyte.
f. It is the occurrence of mitosis twice in the life cycle that results in two generations.
2. Plants differ in which generation–gametophyte or sporophyte–is dominant.
a. In nonvascular plants, the gametophyte is dominant.
b. In the vascular groups, the sporophyte is dominant.
c. The shift to sporophyte dominance is an adaptation to life on land.
d. As the sporophyte gains dominance, the gametophyte becomes microscopic and dependent on the sporophyte.
3. Appearance of the generations among plants varies widely.
a. In ferns, the gametophyte is a small heart-shaped structure.
b. Eggs are fertilized by flagellated sperm that swim to the archegonia (the female portion of the gametophyte that produces and protects the egg) in a film of water.
c. The female gametophyte in flowering plants (the embryo) is retained within the body of the plant as a few cells inside an ovule.
d. In seed plants, pollen grains are mature sperm-bearing male gametophytes; these are produced by antheridia.
e. In seed plants, pollen grains are mature sperm-bearing male gametophytes.
f. Pollen grains are transported by wind, insects, or birds and do not need water to reach the egg.
g. In the life cycle of seed plants, reproductive cells are protected from desiccation.
C. Other Adaptations to a Terrestrial Environments
1. Sporophyte dominance is accompanied by adaptation for water transport and conservation.
2. Vascular tissues transports water and nutrients in the body of the plant.
3. Leaves and stems are covered by a waxy cuticle that holds in water but limits gas exchange; the thickness of the cuticle varies among different species of plants.
4. Leaves and some other tissues have openings (stomata) that regulate gas and water exchange.
24.2 Nonvascular Plants
1. Nonvascular plants lack true roots, stems, and leaves, although they have rootlike, stemlike, or leaflike structures.
2. The term "bryophyte" is a general term for nonvascular plants.
3. The gametophyte is the dominant generation we recognize in bryophytes.
a. Flagellated sperm swim to the vicinity of the egg in a continuous film of water.
b. The gametophyte produces eggs in the archegonia, flagellated sperm in the antheridia.
c. The sperm swim to the egg in a continuous film of water.
d. The sporophyte is attached to and derives nourishment from the photosynthetic gametophyte.
4. Nonvascular plants are quite small because of lack of vascular tissue and the need for sperm to swim to the archegonia in water.
a. Because sexual reproduction involves flagellated sperm, they are usually found in moist habitats.
b. Bryophytes compete well in harsh environments because the gametophyte can reproduce asexually.
5. Approximately 24,000 species of nonvascular plants have been described and classified into three phyla.
B. Hornworts
1. The phylum Anthocerophyta contains the hornworts; about 100 species are known.
2. They are photosynthetic, but also have a symbiotic relationship with cyanobacteria, which can fix atmospheric nitrogen.
3. The small sporophytes of a hornwort look like tiny green broom handles and are attached to a filmy gametophyte that is less than two cm in diameter.
C. Liverworts
1. The phylum Hepatophyta contains the 8,000 species of liverworts.
2. There are two groups: the thallose liverworts with flattened bodies known as a thallus, and the leafy liverworts, which superficially resemble mosses.
3. Marchantia is a example of a liverwort.
a. It has a flat, lobed thallus about a centimeter in length.
b. The upper surface of thallus is smooth; the lower surface bears numerous rhizoids projecting into the soil.
c. It reproduces asexually and sexually.
4. Rhizoids are the hairlike extensions that anchor it and absorb water and minerals from the soil.
5. Asexual reproduction involves gemmae in gemmae cups on the upper surface of the thallus; gemmae can start a new plant.
6. Sexual reproduction depends on antheridia and archegonia.
a. Antheridia are on disk-headed stalks and produce flagellated sperm.
b. Archegonia are on umbrella-headed stalks and produce eggs.
c. The zygote develops into a tiny sporophyte composed of a foot, short stalk, and capsule.
d. Spores produced within the capsule of the gametophyte are disseminated by wind.
D. Mosses
1. Mosses are in the phylum Bryophyta; over 15,000 species are known.
a. Three distinct classes exist: peat mosses, true mosses, and rock mosses.
2. Mosses are found from the Arctic through the tropics to parts of the Antarctic.
3. They prefer damp, shaded localities but some survive in deserts; others in bogs and streams.
4. Mosses store much water; when they dry out, they become dormant; when it rains, they become green.
5. Copper mosses live only in the vicinity of copper and serve as an indicator of ore deposits.
6. Luminous moss lives in caves and glow with a golden-green light.
7. Some "mosses" are not true mosses:
a. Irish moss is an edible red alga of northern seacoasts.
b. Reindeer moss is a lichen that is a mainstay of caribou.
c. Club mosses are vascular plants.
d. Spanish moss, which hangs from trees in the southern U.S., is a flowering plant related to pineapple.
8. Most mosses can reproduce asexually by fragmentation.
9. The moss life cycle begins with algalike protonema developing from the germination of a haploid spore.
a. Three days of favorable growing conditions produce upright shoots covered with leafy structures.
i. Rhizoids (rootlike filaments) anchor the protonema, to which the shoots are attached.
ii. The shoots bear antheridia and archegonia at their tips.
iii. The antheridia produce flagellated sperm which need external water to reach eggs in the archegonia.
iv. The archegonium looks like a vase with a long neck; it has an outer layer of sterile cells with a single egg at the base.
v. Fertilization results in a diploid zygote that undergoes mitotic division to develop a sporophyte.
b. The sporophyte consists of a foot (which grows down into the gametophyte tissue starting at the former archegonium), a stalk, and an upper capsule (sporangium) where spores are produced.
i. At first the sporophyte is green and photosynthetic.
ii. At maturity it is brown and nonphotosynthetic.
10. Uses of Mosses
a. Sphagnum (bog or peat moss) has tremendous ability to absorb water and is important in gardening.
b. Sphagnum does not decay in some acidic bogs; the accumulated dried peat can be used as fuel.
24.3 Vascular Plants
• Evolutionary History
1. Rhyniophytes (phylum Rhyniophyta) were dominant from mid-Silurian Period of the Paleozoic Era to the mid-Devonian.
2. Cooksonia may have been the first vascular plant and colonizer of land.
3. The photosynthetic stems, not true leaves or roots, have sporangia at their tips; they are attached to a rhizome.
4. Similar to bryophytes, these plants were homosporous, producing one type of spore.
• Vascular Tissue
1. Xylem is vascular tissue that conducts water and minerals upward from the roots.
2. Phloem is vascular tissue that transports sucrose and hormones throughout the plant.
3. Lignin strengthens the walls of conducting cells in xylem.
4. The cuticle and stomata are also characteristics of a dominant sporophyte.
5. Seedless plants are mostly homosporous, using spores for dispersal.
6. All seed plants are heterosporous, using pollen grain and seeds.
24.4 Seedless Vascular Plants
1. Seedless vascular plants were dominant from the late Devonian Period through the Carboniferous Period.
2. Club mosses (35 m), horsetails (18 m), and ferns (8 m) were larger than today's specimens and formed great swamps.
B. Club Mosses
1. Club mosses are in the phylum Lycophyta.
2. They are common in temperate woodlands where they are called ground pines and spike mosses.
3. A branching rhizome sends up aerial stems less than 30 cm tall.
4. Tightly packed, scalelike microphylls cover stems and branches; each contains one strand of vascular tissue.
5. Sporangia are borne on the surface of leaves called sporophylls which are grouped in club-shaped stroboli.
6. Spores germinate into inconspicuous and independent gametophytes.
7. Most club mosses live in tropics or subtropics as epiphytes, plants that live on trees without harming them.
8. Closely related spike mosses (Selaginella) and quillworts (Isoetes) produce heterospores, suggesting that heterospory arose independently at least twice.
C. Ferns and Allies
1. Phylum Sphenophyta today contains one genus, Equisetum; these are known as horsetails.
2. A rhizome produces aerial stems that stand about 1.3 meters tall.
3. Whorls of slender side branches encircle nodes of a stem, resembling a horse's tail.
4. Small scalelike leaves also form whorls at each node.
5. Many horsetails have strobili at the tip of all stems; others send up special buff-colored stems that bear stroboli.
6. The spores germinate into inconspicuous and independent gametophytes.
7. The tough, rigid stems have silica in the cell walls; early Americans used them as "scouring rushes."
8. Whisk Ferns
a. Whisk ferns are in the phylum Psilotophyta; there are two genera of whisk ferns.
b. Whisk ferns occur in the southern United States and in the tropics.
c. Whisk ferns have no leaves or roots.
d. A branched rhizome with rhizoids and a mycorrhizal fungus helps gather nutrients.
e. Aerial stems with tiny scales fork repeatedly and carry on photosynthesis.
f. Sporangia are located at the ends of short branches.
g. Other genera including Tmesipteris have true leaves that are microphylls.
D. Ferns
1. Ferns (about 11,000 species) belong to the phylum Pterophyta.
2. Ferns are widespread, and especially abundant in warm, moist tropical regions.
3. Ferns range in size from low-growing mosslike forms to tall trees.
4. Fronds are leaves that are variable in size and shape.
5. Nearly all fronds first appear as a fiddlehead which unrolls as it grows.
6. Ferns are the only group of seedless plants to have well-developed megaphylls; megaphylls may have evolved by fusion or branching of stems.
7. Fern Reproduction
a. A tiny green gametophyte is independent from the sporophyte for nutrition.
b. Flagellated sperm are released by antheridia and swim to the archegonia in a film of water.
8. Uses of Ferns
a. Ferns are used heavily as ornamental plants by florists and as home decorations.
b. Fern wood is very decay and termite resistant.
c. Fern medicines are used by natives to stop bleeding after childbirth; also as an expectorant.
24.5 Seed Plants
1. Seeds are mature ovules containing embryonic sporophyte and stored food enclosed in a protective seed coat.
2. Seeds are resistant to adverse conditions such as dryness and temperature extremes.
3. A food reserve supports the emerging seedling until it can exist on its own.
4. The survival value of seeds contributes greatly to the success of seed plants and to their present dominance.
5. There are separate male and female gametophytes.
6. Pollen grains are drought resistant and become a multicellular male gametophyte.
7. Pollination is the transfer of pollen to the vicinity of the female gametophyte.
8. Sperm is delivered to an egg through a pollen tube; no external water is required for fertilization.
9. The whole male gametophyte, rather than just the sperm, moves to the female gametophyte.
10. A female gametophyte develops within an ovule which, after fertilization, becomes an embryonic plant or "seed."
11. In gymnosperms, the ovules are not completely enclosed by sporophyte tissue at pollination.
12. In angiosperms, the ovules are completely enclosed within diploid sporophyte tissues which become a fruit.
24.6 Gymnosperms
• The Gymnosperms include the conifers, cycads, ginkgo, and gnetophytes; they are classified into 780 species.
1. All have ovules exposed on the surface of sporophylls or similar structures.
2. Ancient gymnosperms were present in swamp forests of the Carboniferous Period.
A. Conifers
1. About 575 species of conifers are in phylum Coniferophyta.
2. Conifers are cone-bearing trees and shrubs such as pines, hemlocks, and spruces.
3. Conifers usually have evergreen needlelike leaves well adapted to withstand extremes in climate.
4. The oldest and largest trees in existence are conifers:
a. The coastal redwood (Sequoia semperivirens) is the tallest living vascular plant and grows to nearly 100 meters high.
b. Bristlecone pines grow in the White Mountains of California and Nevada mountains; one is 4,900 years old.
5. Conifer forests cover vast areas of northern temperate regions.
6. Pine needles have thick cuticle and recessed stomata.
7. Uses of Pines
a. Pine is a major wood used in construction.
b. With xylem tissue that lacks some of the rigid cell types, it is a "soft" rather than "hard" wood.
c. Pine resin is an insect and fungal deterrent harvested for turpentine.
B. Cycads
1. About 140 species of cycads belong to the phylum Cycadophyta.
2. The trunk is stout and unbranched; the large leaves are compound giving a palmlike appearance.
3. Cycads have pollen and seed cones on separate plants.
4. The cycad life cycle is similar to that of pine trees except they are pollinated by insects.
5. The pollen tube bursts in the vicinity of the archegonium and multiflagellated sperm swim to reach an egg.
6. Cycads flourished during the Mesozoic Era and probably were food for herbivorous dinosaurs.
7. Today, cycads are endangered because of their very slow growth.
C. Ginkgoes
1. Only one species of ginkgo (maidenhair tree) survives in the phylum Ginkgophyta.
2. It is called the maidenhair trees because its forked-veined, fan-shaped leaves resemble the maidenhair fern.
3. Ginkgoes are dioecious—some trees produce seeds.
4. Ginkgo ovules are at the end of short, paired stalks; female trees produce seeds with a fleshy covering and foul odor.
5. Similar to cycads, the pollen tube of Gingkoes bursts to release multiflagellated sperm that swim to the egg produced by the female gametophyte in an ovule.
D. Gnetophytes
1. Three living genera with about 70 species are in the phylum Gnetophyta.
2. Gnetum consists of trees and climbing vines with broad leaves; they live mainly in the tropics.
3. Ephedra is found in U.S. desert regions, and is a many-branched shrub with small, scalelike leaves.
4. Welwitschia is found in deserts in southwest Africa; most of it exists underground and it has two enormous leaves.
5. The xylem and stroboli are uniform across all three genera, and all lack archegonia.
6. Angiosperms also lack archegonia, suggesting that gnetophytes are the gymnosperms most closely related to angiosperms.
7. Some gnetophytes produce nectar in their reproductive structures, recruiting insects in pollination.
24.7 Angiosperms
1. 240,000 known species of angiosperms (flowering plants) belong to the phylum Anthophyta.
2. This group contains six times the number of species of all other plant groups combined.
3. Angiosperms live in all habitats from freshwater to desert and from tropics to subpolar regions.
4. Flowering plant size ranges from microscopic duckweed to Eucalyptus exceeding 100 m tall.
5. They are important in everyday human life: clothing, food, medicine, and commercial products.
6. Unlike gymnosperms, angiosperms enclose their ovules within diploid tissues.
B. Origin and Radiation of Angiosperms
1. Flowering plants became the dominant plants in the late Cretaceous and early Tertiary Periods..
2. Although the first fossils are no older than 135 million years, angiosperms probably arose much earlier, perhaps 200 million years ago.
3. Gene sequencing data indicates Amborella trichopoda, a small shrub from New Caledonia in the South Pacific may be the most primitive survivor.
4. Possibly the rise to dominance of angiosperms is tied to the increasing diversity of flying insects which serve as pollinators.
C. Monocots and Eudicots
1. Most flowering plants belong to one of two classes: Monocotyledones (65,000 species), called monocots, or the Eudicotyledones (175,000 species), called eudicots.
2. The term eudicots is preferred to the earlier dicots; some former dicots are now known to have split off before the rise of these two major classes.
3. Monocot produce one cotyledon (seed leaf) at germination and have flower parts mostly in threes or multiples of threes.
4. Dicots produce two cotyledons (seed leaves) at germination and have flower parts mostly in fours or fives, or multiples of these numbers.
D. The Flower
1. The peduncle, a flower stalk, expands slightly at the tip into a receptacle.
2. The receptacle is a modified stem tip to which flower parts are attached.
3. Sepals are outer ring of modified leaves of flowers; usually green, they enclose the flower before it opens.
4. Petals (collectively a corolla) are a ring of modified leaves inside of sepals; large and colorful, they help attract pollinators.
5. Stamens consist of two parts: each slender filament has an anther at its tip.
6. The anther produces pollen.
7. At the center of the flower is the carpel; it consists of a stigma, style, and ovary.
a. Carpels are modified sporophylls that contain ovules in which megasporangia are located.
b. A stigma is a landing platform for pollen and the site where the pollen tube enters the style.
c. The style is a slender column that holds up the stigma to receive pollen.
d. Pollen grains develop a pollen tube that takes sperm to the female gametophyte in the ovule.
e. Glands located in the region of the ovary produce nectar, a nutrient gathered by pollinators as they go flower to flower.
E. Flowering Plant Life Cycle
1. A megaspore located in an ovule within an ovary of a carpal develops into an egg-bearing female gametophyte called the embryo sac.
2. Usually, the embryo sac has seven cells; one is an egg and one contains two polar nuclei.
3. Microspores produced in anthers become pollen grains which mature into sperm-bearing male gametophytes.
4. The mature male gametophyte consists of three cells; the tube cell and two sperm cells.
5. Pollination brings the male gametophyte to the stigma where it germinates.
6. During germination, the tube cell produces a pollen tube that carries the two sperm to the micropyle opening of an ovule.
7. In double fertilization, one sperm fertilizes an egg and one sperm unites with polar nuclei to form the triploid endosperm.
8. The ovule becomes the seed and contains the embryo (the sporophyte of the next generation) and stored food enclosed within a seed coat.
9. A fruit is derived from an ovary and possibly accessory parts of the flower; some fruits are fleshy and some are dry.
F. Flowers and Diversification
1. Flower variety is related to the numerous means by which flowers are pollinated and fruits are dispersed.
2. Inconspicuous flowers disperse pollen by wind; colorful flowers attract specific pollinators (e.g., bees, wasps, flies, butterflies, moths, and even bats) which carry only a particular pollen.
3. Flowers promote efficient cross pollination; they also aid in dispersal through production of fruits.
4. There are fruits that utilize wind, gravity, water, and animals for dispersal.
5. Since animals live in certain habitats or have particular migration patterns, they can deliver a fruit-enclosed seed to a suitable location for germination and development.
25.1 Plant Organs
1. Structures of flowering plants are well-adapted to varied environments, including water.
2. Flowering plants usually have a root system (the roots) and a shoot system (the stems and leaves).
3. Roots, stems, and leaves are the vegetative organs of plants; flowers, seeds, and fruits are reproductive structures.
B. Roots
1. A plant's root system is underground.
2. The root system is the primary root plus the branch roots.
3. It is generally equal in size to the shoot system, the part above ground.
4. Root systems have the following functions:
a. Roots anchor a plant in soil and give support.
b. Roots absorb water and minerals from soil; root hairs are central to this process.
i. Root hair cells are in a zone near the root tip.
ii. Root hairs are numerous to increase the absorptive surface of a root.
iii. Transplanting plants damages a plant when the root hairs are torn off.
iv. Water and nutrients absorbed are distributed to the rest of the plant.
v. Roots produce hormones that must be distributed to the plant
c. Perennials "die back" to regrow the next season; roots of herbaceous perennials store food (e.g., carrots, sweet potatoes).
C. Stems
1. The shoot system of a plant consists of the stem, the branches, and the leaves.
2. The stem forms the main axis of the plant, along with lateral branches.
3. Upright stems produce leaves and array them to be exposed to as much sun as possible.
4. A node occurs where a leaf attaches to the stem and an internode is the region between nodes; nodes and internodes identify a stem even if it is underground.
5. The stem has vascular tissue to transport water and minerals from roots and sugar from leaves.
6. Nonliving cells form a continuous pipeline through vascular tissue.
7. A cylindrical stem expands in girth and length; trees use woody tissue to strengthen stems.
8. Stems may function in storage: cactus stems store water and tubers are horizontal stems that store nutrients.
D. Leaves
1. A leaf is the major organ of photosynthesis in most plants.
2. Leaves receive water from roots by way of the stem.
3. Broad, thin leaves have a maximum surface area to absorb CO2 and collect solar energy.
4. A blade is the wide portion of a leaf with most photosynthetic tissue.
5. The petiole is a stalk that attaches a leaf blade to the stem.
6. The leaf axil is the upper acute angle between petiole and stem where an axillary (lateral) bud originates.
7. Some leaves protect buds, attach to objects (tendrils), store food (bulbs), or capture insects.
25.2 Monocot Versus Eudicot Plants
• Criteria for Monocots and Eudicots
1. Cotyledons are embryonic seed leaves providing nutrition from the endosperm before the mature leaves begin photosynthesis.
2. Flowering plants are divided into monocots and eudicots based on these traits.
Monocots Eudicots
a. Number of cotyledons in seed one two
b. Distribution of root xylem and root xylem and phloem in a ring root phloem between phloem of xylem
c. Distribution of vascular bundles scattered in stem arranged in a distinct ring
d. Pattern of leaf veins form a parallel pattern form a net pattern
e. Number of flower parts in threes and multiples of three in fours and fives and multiples of four or five
f. Number of apertures in pollen grains usually one usually three
3. Representative members: grasses, lilies, orchids, rice, wheat, corn dandelions to oak trees and palm trees
4. The distinction between monocots and eudicots represents an important evolutionary division that relates to many structures.
25.3 Plant Tissues
1. Plants continually grow due to meristematic (embryonic) tissue in the stem and root tips (apexes).
2. Apical meristems are located near the tips of stems and roots, where they increase the size of these structures; this is called primary growth.
3. Monocots also have intercalary meristem, which allows them to regrow lost parts.
4. Apical meristem produces three types of meristem, which develop into the three types of specialized primary tissues in the body of the plant.
a. Protoderm is the outermost primary meristem giving rise to epidermis.
b. Ground meristem is the inner meristem producing ground tissue.
c. Procambium produces vascular tissue.
5. Three specialized tissues are include:
a. Epidermal tissue forms the outer protective covering.
b. Ground tissue fills the interior of the plant.
c. Vascular tissue transports water and nutrients and provides support.
B. Epidermal Tissue
1. Epidermis is an outer protective covering tissue of plant roots, leaves, and stems of nonwoody plants.
2. It contains closely packed epidermal cells.
3. Waxy cuticle covers the walls of epidermal cells, minimizing water loss and protecting against bacteria.
4. In roots, certain epidermal cells are modified into root hairs that increase the surface area of the root for absorption of water and minerals and help to anchor plants in the soil.
5. Protective hairs called trichomes are produced by epidermal cells of stems, leaves, and reproductive organs.
6. Trichomes may help protect a plant from herbivores by producing a toxic substance.
7. On the lower epidermis of eudicot leaves, and both surfaces of monocot leaves, special guard cells form microscopic pores (stomata) and regulate gas exchange and water loss.
8. In older woody plants, the epidermis of the stem is replaced by periderm, the majority component of which is cork cells.
a. At maturity, dead cork cells may be sloughed off.
b. Cork cambium is meristem that produces new cork cells.
c. As cork cells mature, they encrust with the lipid suberin that renders them waterproof and inert.
d. Cork protects a plant and makes it resistant to attack by fungi, bacteria, and animals.
e. When the cork cambium overproduces cork in certain areas of the stem surface, ridges and cracks, called lenticels, appear; lenticels are important in gas exchange between the interior of the stem and the air.
C. Ground Tissue
1. Ground tissue forms the bulk of the plant; it contains parenchyma, collenchyma and sclerenchyma cells.
2. Parenchyma are the least specialized of all plant cell types.
a. Cells of this type contain plastids (e.g., chloroplasts or colorless storage plastids).
b. They are found in all organs of a plant.
c. They divide to form more specialized cells (e.g., roots develop from stem cuttings in water).
3. Collenchyma resemble parenchyma but has thicker primary cell walls.
a. Collenchyma cells are uneven in the corners.
b. They usually occur as bundles of cells just beneath the epidermis.
c. They give flexible support to immature regions of plants (e.g., a celery stalk is mostly collenchyma).
4. Sclerenchyma cells have thick secondary cell walls.
a. They are impregnated with lignin that makes the walls tough and hard.
b. They provide strong support to mature regions of plants.
c. Most cells of this type are nonliving.
d. Sclerenchyma cells form fibers (used in linen and rope) and shorter sclereids (found in seed coats, nut shells, and gritty pears).
D. Vascular Tissue
1. Xylem conducts water and mineral solutes upward through a plant from roots to leaves.
a. Xylem contains tracheids and vessel elements.
b. Tracheids
i. Tracheids are smaller, hollow, thin, long nonliving cells with tapered overlapping ends.
ii. Water moves across end and sidewalls because of pits or depressions in the secondary cell wall.
c. Vessel Elements
i. Vessel elements are hollow non-living cells lacking tapered ends.
ii. They are larger than tracheids.
iii. They lack transverse end walls.
iv. They form a continuous pipeline for water and mineral transport.
d. Xylem also contains sclerenchyma cells to add support.
e. Vascular rays are flat ribbons of parenchyma cells between rows of tracheids; they conduct water and minerals across the width of the plant.
2. Phloem is vascular tissue that conducts the organic solutes in plants, from the leaves to the roots; it contains sieve-tube members and companion cells.
a. Sieve-tube Members
i. Sieve-tube cells contain cytoplasm but no nucleus.
ii. They are arranged end to end.
iii. They have channels in their end walls (thus, the name "sieve-tube"), through which plasmodesmata extend from one cell to another.
b. Companion Cells
i. Companion cells are closely connected to sieve-tube cells by numerous plasmodesmata.
ii. They are smaller and more generalized than sieve-tube cells.
iii. They have a nucleus which may control and maintain the function of both cells.
iv. They are also thought to be involved in the transport function of phloem.
3. Vascular tissue extends from root to leaves as vascular cylinder (roots), vascular bundles (stem) and leaf veins.
25.4 Organization of Roots
1. The eudicot root has various zones where cells are in various stages of differentiation and where primary growth occurs.
2. The root apical meristem is the region protected by the root cap, a protective cover; its cells are replaced constantly because they are soon ground off.
3. The primary meristems are in the zone of cell division, which continuously provides cells to the zone of elongation by mitosis.
4. The zone of elongation is above the zone of cell division where cells become longer and more specialized.
5. The zone of cell division contains meristematic tissue and adds cells to the root tip and the zone of elongation.
6. The zone of maturation is above the zone of elongation; cells are mature and differentiated and it has root hairs.
B. Tissues of a Eudicot Root
1. Epidermis is a single layer of thin-walled, rectangular cells.
a. The epidermis forms the protective outer layer of the root.
b. In the region of maturation, there are many root hairs.
c. Root hairs project as far as 5–8 mm into the soil.
2. Cortex is a layer of large, thin-walled, irregularly shaped parenchyma cells.
a. These cells contain starch granules; the cortex functions in food storage.
b. The cells are loosely packed; water and minerals can diffuse through the cortex without entering cells.
3. Endodermis is single layer of rectangular cells that forms the boundary between the cortex and inner vascular cylinder.
a. Its cells fit closely together and are bordered on four sides by the Casparian strip.
b. It regulates the entrance of minerals into the vascular cylinder.
c. The Casparian strip is an impermeable lignin and suberin layer that excludes water and mineral ions.
d. The only access to the vascular bundle is through endodermal cells.
4. Vascular tissue
a. The pericycle is the first layer of cells within the vascular cylinder
i. Its cells have retained the capacity to divide.
ii. It can start the development of branch or secondary roots.
b. The main portion of the vascular cylinder is composed of
i. xylem, whose cells are arranged in a star-shaped pattern; and
ii. phloem, whose cells are located in regions between arms of xylem.
C. Organization of Monocot Roots
1. Monocot roots have the same zones as a eudicot root but do not undergo secondary growth.
2. The monocot root has a ring of vascular tissue where alternating bundles of xylem and phloem surround pith.
3. Monocot roots also have pericycle, endodermis, cortex, and epidermis.
D. Root Diversity
1. Roots have adaptations to help anchor plants, absorb water and minerals, and store carbohydrates.
2. There are three general root types.
a. A taproot is common in eudicots; this first or primary root grows straight down and remains the dominant root of a plant; it is often fleshy and adapted to store food (e.g., carrots, beets).
b. The fibrous root system of monocots is a mass of slender roots and lateral branches that hold the plant secure in the soil.
3. Adventitious roots develop from underground stems or from the base of above-ground stems.
4. A prop root's main function is to anchor a plant (e.g., corn and mangrove plants).
5. Pneumatophores of mangrove plants project above the water from roots to acquire oxygen.
6. Ivy has holdfast roots to anchor aerial shoots.
7. Haustoria are rootlike projections from stems on parasitic plants (e.g., dodders and broomrapes).
a. Haustoria grow into the host plant.
b. They contact vascular tissue from which they extract water and nutrients.
8. Mycorrhizae are an association between fungus and roots.
a. In this mutualism, the fungus receives sugars and amino acids from the plant.
b. The plant receives water and minerals from the fungus.
9. Legumes (e.g., peas and beans) have root nodules containing nitrogen-fixing bacteria.
a. Bacteria extract nitrogen from air and reduce it to a form that can be used by plant tissues.
b. Legumes are often planted to bolster the nitrogen supply in the soil.
25.5 Organization of Stems
1. The terminal bud contains the shoot tip protected by bud scales, which are modified leaves.
2. Dormant auxillary buds that can give rise to branches or flowers are here also.
3. Bud scales are scalelike coverings protecting terminal buds during winters when bud growth stops.
4. The stem tip is the site of primary growth where cell division extends the length of stems or roots.
5. The apical meristem produces new cells that elongate and increase the height of the stem.
6. The shoot apical meristem is protected within a terminal bud of leaf primordia (immature leaves).
7. Three specialized types of primary meristem develop from shoot apical meristem.
a. Protoderm is the outermost primary meristem that gives rise to epidermis.
b. Ground meristem produces two tissues composed of parenchyma cells: the pith and the cortex.
c. Procambium is the inner meristem that produces primary xylem and primary phloem.
8. Differentiation continues; cells become the first tracheids or vessel elements within the vascular bundle.
9. First sieve-tube cells are short-lived and do not have companion cells.
10. Mature phloem develops later after all surrounding cells have stopped expanding and a lateral meristem, called vascular cambium, has developed.
B. Herbaceous Stems
1. Herbaceous stems are mature nonwoody stems that exhibit only primary growth.
2. The outermost tissue of herbaceous stems is epidermis covered by a waxy cuticle to prevent water loss.
3. Xylem and phloem are in distinctive vascular bundles.
a. In each bundle, xylem is found to the inside of the stem; phloem is found to the outside.
b. In the eudicot herbaceous stem, vascular bundles are arranged in a ring towards the outside of the stem and separating the cortex from the central pith.
c. In a monocot stem, vascular bundles are scattered throughout the stem; there is no well-defined cortex or pith.
4. The cortex sometimes carries on photosynthesis; pith may function as a storage site.
C. Woody Stems
1. Woody plants have both primary and secondary tissues.
2. Primary tissues are new and form each year from primary meristem right behind the apical meristem.
3. Secondary tissues develop from second year onward from growth of lateral meristem.
4. Primary growth increases the length of a plant; secondary growth increases its girth.
5. As secondary growth continues, it is not possible to distinguish individual vascular bundles.
6. The woody eudicot stem has a different organization with three distinct areas: bark, wood, and pith.
7. Bark
a. The bark of a tree contains cork, cork cambium, and phloem.
b. Secondary phloem is produced each year by vascular cambium but does not build up.
c. This phloem tissue is soft; therefore it is easy to remove the bark of a tree.
d. Cork cambium is meristem beneath the epidermis that produces new cork cells when needed.
e. Cork cambium begins to divide, producing cork that disrupts epidermis replacing it with cork cells.
f. Cork cells become impregnated with suberin, causing them to die but making them waterproof.
g. Consequently, cork forms an impervious barrier, even to gas exchange, except at lenticels.
8. Wood
a. Wood is a secondary xylem which builds up each year; the vascular cambium is dormant during the winter.
b. Spring wood is composed of wide xylem vessel elements with thin walls, necessary to conduct sufficient water and nutrients to supply abundant growth that occurs during spring.
c. Summer wood forms when moisture is scarce; composed of a lower proportion of vessels, it contains thick-walled tracheids and numerous fibers.
d. An annual ring is one ring of spring wood followed by a ring of summer wood; this equals one year's growth.
e. Sapwood is the outer annual rings where transport occurs.
f. Heartwood is the inner annual rings of older trees.
i. Vessels no longer function in transport; they become plugged with resins and gums that inhibit growth of bacteria and fungi.
ii. Heartwood may help to support a tree.
9. Woody Plants
a. The first flowering plants were probably woody shrubs; herbaceous plants evolved later.
b. It is advantageous to be woody when there is adequate rainfall; woody plants can grow taller and have adequate tissue to support and service leaves.
c. It takes energy to support secondary growth and prepare the plant for winter in temperate zones.
d. Long-lasting plants need more defense mechanisms against attack by herbivores and parasites.
e. Trees need years to mature before reproducing; they are more vulnerable to accident or disease.
D. Stem Diversity
1. Stolons are stems that grow along the ground; new plants grow where the nodes contact the soil.
2. The succulent stems of cacti are modified for water storage.
3. Tendrils of grapes and morning glories are stems adapted for wrapping around support structures.
4. Rhizomes are underground horizontal stems.
a. Rhizomes are long and thin in grasses and thick and fleshy in irises.
b. Rhizomes survive winter and contribute to asexual reproduction because each node bears a bud.
c. Some rhizomes have tubers that function in food storage (e.g., potatoes).
5. Corms are bulbous underground stems that lie dormant during winter, like rhizomes.
6. Humans use stems: sugarcane is primary source of table sugar, cinnamon and quinine are from bark, wood is from paper, etc.
25.6 Organization of Leaves
1. Leaves are organs of photosynthesis in plants; they are made of a flattened blade and a petiole.
2. The leaf veins reveal the presence of vascular tissue within the leaves.
3. The vascular tissues of leaves transport water and nutrients.
4. Leaf veins have a net pattern in eudicot leaves and a parallel pattern in monocot leaves.
5. A petiole is a stalk that attaches a leaf blade to the plant stem.
6. Epidermis is the layer of cells that covers the top and bottom sides of a leaf.
a. The epidermis often bears protective hairs or glands; epidermal glands produce irritating substances.
b. The epidermis is covered by a waxy cuticle that keeps the leaf from drying out.
c. The epidermis, particularly lower epidermis, contains stomata that allow gases to move into and out of the leaf.
7. Mesophyll is the body of a leaf and the site of most photosynthesis.
a. Palisade mesophyll is the layer of mesophyll containing elongated parenchyma cells with many chloroplasts.
b. Spongy mesophyll contains loosely packed parenchyma cells that increase the surface area for gas exchange.
B. Leaf Diversity
1. Simple leaves have margins not deeply lobed or divided into smaller leaflets.
2. 2 Compound leaves are divided into smaller leaflets, and each leaflet may have its own stalk.
3. Leaves are variously modified.
a. Pinnately compound leaves have the leaflets occurring in pairs.
b. Palmately compound leaves have all of the leaflets attached to a single point.
c. Some plants have bipinnately compound leaves.
d. Leaves can be arranged in one of three ways: alternate, opposite, or whorled.
e. Cactus spines are modified leaves; succulents have fleshy leaves to hold moisture.
f. Onion bulbs have leaves surrounding a short stem.
g. The tendrils of peas and cucumbers are leaves.
h. The Venus's-flytrap has leaves to trap and digest insects.
26.1 Plant Nutrition and Soil
1. The ancient Greeks considered plants "soil-eaters" that converted soil into plant tissue.
2. The 17th Century Dutchman Jean-Baptiste Van Helmont conducted an experiment.
a. He planted a five pound young willow tree in a pot with 200 pounds of soil.
b. After five years of watering, the tree weighed 170 pounds but only a few ounces of soil was missing.
c. He concluded the increase in tree weight came from water; he was unaware of substances in air.
B. Essential Inorganic Nutrients
1. Approximately 95% of a plant's dry weight is carbon, hydrogen, and oxygen.
a. Carbon dioxide is the source of carbon for a plant.
b. Water is the source of hydrogen.
c. Oxygen can come from either atmospheric oxygen, carbon dioxide, or water.
2. A mineral is an organic substance usually containing two or more elements.
3. To be classified as an essential nutrient, the following criteria must be fulfilled.
a. It must have an identifiable nutritional role.
b. No other element can substitute and fulfill the same role.
c. A deficiency of the element causes the plant to die.
4. These elements are divided into macronutrients and micronutrients, according to their relative concentrations in plant tissue.
5. Beneficial nutrients are elements required for or to improve growth of a particular plant.
a. Horsetails require silicon as a mineral nutrient.
b. Sugar beets show better growth in the presence of sodium.
c. Soybeans use nickel when root nodules are present.
C. Determination of Essential Nutrients
1. When a plant is burned, most mineral elements (except for nitrogen) remain in the ash.
2. Hydroponics (water culture) is the preferred method for determining plant mineral requirements.
a. Hydroponics is cultivation of plants in water.
b. Nutrient requirements of plants are determined by omitting a mineral and observing the effects.
c. If plant growth suffers, it can be concluded that the omitted mineral is a required nutrient.
d. This works for macronutrients but impurities make micronutrient measurement difficult.
D. Soil
1. Soil Formation
a. Soil formation begins with weathering of rock by freezing, glacier flow, stream flow, and chemicals.
b. Lichens and mosses grow on barren rock and trap particles and leave decaying tissues.
c. Decayed organic matter (humus) takes time to accumulate; its acidity leaches minerals from rocks.
d. Depending on parent material and weathering, a centimeter of soil may develop within 15 years.
2. The Nutritional Function of Soil
a. Soil consists of mineral particles, decaying organic matter, living organisms, air and water.
b. The best soil includes particles of different sizes; this provides critical air spaces.
c. Mineral Particles
i. Mineral particles vary in size.
1. Sand particles are largest: 0.05–2.0 mm in diameter.
2. Sand particles are medium sized: 0.002–0.05 mm in diameter.
3. Clay particles are smallest: below 0.002 mm in diameter.
ii. Sandy soils lose water too readily; clay packs tightly to hold water.
iii. Clay particles are negatively charged and attract positively charged ions (e.g., calcium [Ca2+] and potassium [K+]).
iv. In acidic soils, hydrogen ions replace positively charged nutrients and the nutrient ions float free and are leached; this is why acid rain kills trees.
v. Clay cannot retain negatively charged NO3-, and the nitrogen content of clay soil is low.
vi. Loam (a mixture of the three mineral particles) retains water and nutrients; roots take up oxygen in the air spaces.
d. Humus
i. A mixture of 10–20% humus mixed with a top layer of soil particles is best for plants.
ii. Humus keeps soil loose and crumbly, decreases runoff and aerates soil.
iii. Humus is acidic and retains positively charged minerals for plants to use later.
iv. Bacteria and fungi break down organic matter in humus and return inorganic nutrients to plants.
e. Living Organisms
i. Small plants play a major role in the formation of soil from rock and in succession.
ii. Roots of larger plants penetrate the soil and weather rocks.
iii. Larger moles and badgers and smaller earthworms help turn over the soil.
iv. Soil animals, from mites to millipedes, help break down leaves and other plant remains.
v. Fungi, protozoa, algae and bacteria complete decomposition.
vi. Soil bacteria make nitrate available to plants.
vii. Some soil organisms (roundworms and insects) are crop pests that feed on roots.
3. Soil Profiles
a. A soil profile is a vertical section from the ground surface to the unaltered rock below; usually, a soil profile has parallel layers called horizons.
i. The A horizon is the uppermost topsoil layer that contains litter and humus.
ii. The B horizon lacks organic matter but contains inorganic nutrients leached from the A horizon.
iii. The C horizon is weathered and shattered bedrock.
iv. Soil profiles vary by parent material, climate and ecosystem.
v. Grassland soils have deep A horizons from turnover of decaying grasses and lack of leaching.
vi. Forest soils have thinner A horizons but enough inorganic nutrients for tree root growth.
vii. Tropical rain forest A horizons are shallow due to rapid decomposition; the B horizon is deeper due to extensive leaching.
4. Soil Erosion
a. Soil erosion is caused by water or wind carrying away soil.
b. Erosion removes 25 billion tons of topsoil worldwide annually.
c. Deforestation and desertification contribute to erosion.
d. U.S. farmlands lose soil faster than it is formed on one-third of cropland.
e. The coastal wetlands are also losing soil at a high rate.
26.2 Uptake of Water and Minerals
• Pathways
1. Minerals follow the path of water uptake.
a. Some mineral ions diffuse in between the cells; this is called apoplastic transport.
b. Because of the impermeable Casparian strip (a band of suberin and lignin bordering four sides of root endodermal cells), water must eventually enter the cytoplasm of endodermal cells.
c. Water can enter epidermal cells at their root hairs, and progress across the cortex and endodermis of a root.
d. In contrast to water, minerals are actively taken up by plant cells.
e. Mineral nutrient concentration in roots may be 10,000 times more than in surrounding soil.
f. During transport throughout a plant, minerals can exit xylem and enter cells that require them.
2. Mineral ions cross plasma membranes by a chemiosmotic mechanism.
a. Plants absorb minerals in ionic form: nitrate (NO3-), phosphate (HPO4= ), and potassium ions (K+); all have difficulty crossing a charged plasma membrane.
b. It has long been known plants expend energy to actively take up and concentrate mineral ions.
c. A plasma membrane pump, called a proton pump, hydrolyzes ATP to transport H+ ions out of the cell; this sets up an electrochemical gradient that causes positive ions to flow into cells.
d. Negative ions are carried across the plasma membrane in conjunction with H+ ions as H+ ions diffuse down their concentration gradient.
A. Adaptations of Roots for Mineral Uptake
1. Two symbiotic relationships are known to assist roots in acquiring nutrients.
2. Legumes have roots colonized by the bacterium Rhizobium.
a. Plants cannot use atmospheric nitrogen because they lack enzymes to break the N ° N bond.
b. Rhizobium makes nitrogen compounds available to plants in exchange for carbohydrates.
c. Bacteria live in root nodules—structures on plant roots that contain nitrogen-fixing bacteria.
d. Rhizobial bacteria reduce atmospheric nitrogen (N2) to ammonium (NH4+) (nitrogen-fixation).
e. Other plants have a relationship with free-living, nitrogen-fixing microorganisms in soil.
3. Most plants have mycorrhizae; those lacking mycorrhizae are limited in where they can grow.
a. Mycorrhizae are a mutualistic symbiotic relationship between soil fungi and plant roots.
b. The fungal hyphae may enter the cortex of roots but do not enter plant cells.
c. Ectomycorrhizae form a mantle exterior to the root, and they grow between cell walls.
d. Fungus increases the surface area for mineral and water uptake and breaks down organic matter.
e. In return the root furnishes the fungus with sugars and amino acids.
f. Orchid seeds are small with limited nutrients; they germinate only when invaded by mycorrhizae.
g. Nonphotosynthetic plants (e.g., Indian pipe) use mycorrhizae to extract nutrients from nearby trees.
4. Some plants have poorly developed roots or no roots; other mechanisms supply minerals and water.
a. Epiphytes take nourishment from air; their attachment to other plants gives them support.
b. Parasitic plants (e.g., dodders, broomrapes, pinedrops) send out haustoria (rootlike projections) that grow into the host and tap into the xylem and phloem of the host.
c. Venus's-flytrap and sundew obtain nitrogen and minerals as leaves capture and digest insects.
26.3 Transport Mechanisms in Plants
• Transport Tissues
1. Flowering plants have transport tissues as an adaptation to living on land.
2. Xylem transport tissue conducts water and mineral solutes from roots to leaves; it contains two types of conducting cells: tracheids and vessel elements.
a. Tracheids:
i. are hollow, nonliving cells with tapered overlapping ends;
ii. are thinner and longer than vessel elements; and
iii. water crosses the end and sidewalls because of pits in secondary cell wall.
b. Vessel elements:
i. are hollow, nonliving cells that lack tapered ends;
ii. are wider and shorter than tracheids;
iii. lack transverse end walls; and
iv. form a continuous pipeline for water and mineral transport.
3. Phloem is vascular tissue that conducts organic nutrients to all parts of the plant; it contains sieve-tube members and companion cells.
a. Sieve-tube members lack a nucleus, are arranged end to end and have channels in end walls (thus, the name "sieve-tube") through which plasmodesmata extend from one cell to another.
b. Companion cells connect to sieve-tube cells by numerous plasmodesmata, and are smaller and more generalized than sieve-tube cells; they have a nucleus.
4. These transport systems rely on the mechanical properties of water.
a. Diffusion moves molecules from higher to lower concentrations.
b. Water potential considers both water pressure and osmotic pressure.
5. They also rely on the chemical properties of water: polarity of water and hydrogen bonding.
A. Water Transport
1. Movement of water and minerals in a plant involves entry into roots, xylem, and leaves.
2. Water and minerals enter root cells before they reach xylem by the two routes already described.
3. Water entering root cells creates a positive pressure called root pressure.
a. Root pressure (which occurs primarily at night) tends to push xylem sap upward in a plant.
b. Guttation is the appearance of drops of water along the edges of leaves, as a result of water being forced out of leaf vein endings; it is the result of root pressure.
c. Root pressure is not a sufficient mechanism for water to rise to the tops of trees.
4. Cohesion-Tension Model of Xylem Transport
a. Water and dissolved minerals must be transported upward from roots to xylem, perhaps as high as 90 meters.
b. The cohesion-tension model states that transpiration creates a tension (i.e., a negative pressure) that pulls water upward in xylem.
c. Water molecules are cohesive with one another, adhesive with xylem walls.
d. Transpiration is a plant's loss of water to the atmosphere through evaporation at leaf stomata.
e. Cohesion is the tendency of water molecules to cling together due to their forming hydrogen bonds.
f. Adhesion is the ability of water (a polar molecule) to interact with molecules comprising the walls of xylem vessels; adhesion gives a water column extra strength and prevents it from slipping back down.
g. In daytime, the negative water potential created by transpiration extends from leaves to roots; the water column must be continuous.
h. If a water column within xylem is broken by cutting a stem, the water column will drop back down the xylem vessel away from the site of breakage, making it more difficult for conduction to occur.
i. At least 90% of the water taken up by roots is lost through stomata by transpiration.
j. With plenty of water, stomata will remain open, allowing CO2 to enter the leaf and photosynthesis to occur.
k. Transpiration exerts a driving force or tension that draws the water column up in vessels.
l. Under water stress, more water is lost through a leaf than can be brought up and the stomata close; the leaves are then protected from water loss by the waxy cuticle of the upper and lower epidermis.
m. Photosynthesis requires CO2 to enter the leaf; there must be sufficient water so stomata can remain open and allow CO2 to enter.
B. Opening and Closing of Stomata
1. Each stoma, a small pore in the leaf epidermis, has two guard cells.
2. Stomata open from turgor pressure when guard cells take up water; when they lose water, turgor pressure decreases and stomata close.
3. Guard cells are attached to each other at their ends; the inner walls are thicker than outer walls.
4. Radial expansion is prevented by cellulose microfibrils in the walls but outer walls can expand lengthwise.
5. As they take up water, they buckle out, thereby creating an opening between cells.
6. Since 1968, it has been known that when stomata open, there is accumulation of K+ ions in guard cells.
7. A proton pump run by breakdown of ATP to ADP and P transports H+ outside the cell; this establishes an electrochemical gradient allowing K+ to enter by way of a channel protein.
8. The blue-light component of sunlight is a signal that can cause stomata to open.
a. There is evidence that flavin pigments absorb blue light.
b. This pigment sets in motion a cytoplasmic response activating the proton pump that causes K+ ions to accumulate in guard cells.
9. Evidence suggests a receptor in the plasma membrane of guard cells brings about inactivation of the proton pump when CO2 concentration rises, as happens when photosynthesis ceases.
10. Abscisic acid (ABA) produced by cells in wilting leaves, also causes stomata to close; photosynthesis cannot occur, but water is conserved.
11. In plants kept in the dark, stomata open and close on a 24-hour basis as if responding to sunlight in the daytime and the absence of sunlight at night; some sort of internal biological clock must keep time.
C. Organic Nutrient Transport
1. Marcello Malpighi (1679) suggested bark transferred sugars from leaves to roots.
a. He observed the results of removing a strip of bark from a tree (girdling).
b. Bark swells just above the cut and sugar accumulates in the swollen tissue.
c. Today, we know phloem was removed but xylem remained; therefore, phloem does transport sugars.
2. Radioactive tracer studies using 14C confirmed phloem transports organic nutrients.
a. When 14C-labeled carbon is supplied to mature leaves, radioactively labeled sugar moves to roots.
b. Similar studies confirm phloem transports amino acids, hormones, and mineral ions.
3. Aphids Used in Study
a. It is difficult to take samples of sap from just the phloem cells without injuring the phloem.
b. Aphids (small insects) drive their mouth stylets into a sieve-tube cell; then samples are easily taken.
c. The aphid body is cut off; the stylet becomes a small needle from which phloem is collected.
d. Such research indicates that sap can move through phloem from 60–100 cm per hour or more.
4. Pressure-Flow Model of Phloem Transport
a. The pressure-flow model explains the transport of sap through sieve tubes by a positive pressure potential.
b. The buildup of water creates a positive pressure potential within the sieve tubes that moves water and sucrose to a sink (e.g., at the roots).
c. Pressure exists from the leaves to the roots; at the roots, sucrose is transported out and water also flows through due to the pressure.
d. Consequently, this pressure gradient causes a flow of water from leaves to roots.
e. The conducting cells of phloem are sieve tubes lined end to end.
f. Cytoplasm extends through the sieve plates of adjoining cells to form a continuous tube system.
g. During the growing season, leaves produce sugar.
h. Sucrose is actively transported into phloem by an electrochemical gradient established by a H+ pump.
i. Water flows passively into sieve tubes by osmosis.
j. A sink can be at the roots or any other part of the plant that requires nutrients.
k. Because phloem sap flows from source to sink, sap can move any direction along phloem.
27.1 Plant Responses
1. One defining characteristic of life is an ability to respond to stimuli.
2. Adaptive organisms respond to environmental stimuli because it leads to longevity and survival of the species.
3. Animals have nerves and muscles; plants respond by growth patterns.
B. Tropisms
1. A tropism is plant growth toward or away from a unidirectional stimulus.
a. The stimulus comes from only one direction instead of many.
b. Growth toward a stimulus is a positive tropism; growth away from a stimulus is a negative tropism.
c. By differential growth, one side elongates faster; the result is a curving toward or away from a stimulus.
2. Three well-known tropisms are named for the stimulus that causes the response.
a. Phototropism is growth of plants in response to light; stems show positive phototropism.
b. Gravitropism is response to Earth's gravity; roots demonstrate positive gravitropism
c. and stems demonstrate negative gravitropism.
d. Thigmotropism is unequal growth due to touch (e.g., coiling of tendrils around a pole).
3. Response to a stimulus first involves reception of the stimulus.
4. The next step is transduction of the stimulus into a form meaningful to the organism.
5. Finally, there is a response by the organism.
C. Phototropism
1. Early researchers, including Charles Darwin and his son Francis, observed plants curve toward light.
2. Phototropism occurs because cells on shady side of stems elongate.
3. It is believed that a yellow pigment related to riboflavin acts as a photoreceptor for light.
a. Following reception, the plant hormone auxin migrates from the bright side to the shady side of a stem.
b. How reception of stimulus couples to production of auxin is not yet known.
4. Auxin is also involved in gravitropism, apical dominance, and root and seed development.
D. Gravitropism
1. An upright plant placed on its side displays negative gravitropism; it grows upward opposite gravity.
2. Charles Darwin and his son were the first to show that roots display positive gravitropism.
a. If the root cap is removed, roots no longer respond to gravity.
b. Later researchers showed root cap cells contain statoliths, starch grains within amyloplasts.
c. Due to gravity, amyloplasts settle to the lowest part of the cell.
3. The hormone auxin underlies both positive and negative gravitropisms.
a. The two tissues respond differently to auxin, which moves to the lower side of both stems and roots.
b. Auxin inhibits the growth of root cells; cells of the upper surface elongate and the root curves downward.
c. Auxin stimulates the growth of stem cells; cells of the lower surface elongate and the stem curves upward.
E. Thigmotropism
1. Unequal growth due to contact with solid objects is thigmotropism.
2. The coiling of morning glory or pea tendrils around posts, etc., is a common example.
3. Cells in contact with an object grow less while those on the opposite side elongate.
4. This process is quite rapid; tendrils have been observed to encircle an object in ten minutes.
5. A couple of minutes of touching can bring about a response that lasts for several days.
6. The response can be delayed; tendrils touched in the dark will respond when illuminated.
a. ATP rather than light can cause the response; the need for light is simply a need for ATP.
b. The hormones auxin and ethylene are involved; they induce curvature of tendrils in the absence of touch.
7. Thigmomorphogenesis is a touch response involving the whole plant.
a. An entire plant responds to the presence of wind or rain.
b. A plant growing in a windy location has a shorter, thicker trunk.
c. Simple rubbing of a plant inhibits cellular elongation and produces a shorter, sturdier plant.
F. Nastic Movements
1. In contrast to tropisms, nastic movements are independent of the direction of stimulus.
2. Seismonastic movements result from touch, shaking, or thermal stimulation.
3. When a Mimosa pudica leaf is touched, the leaflets fold because the petiole droops.
4. This response takes only a second or two and is due to a loss of turgor pressure within cells.
5. A pulvinus is a thickening at the base of such leaflets where turgor pressure can rapidly drop.
6. Potassium ions move out of the cell and water follows by osmosis.
7. A single stimulus, such as a hot needle, can cause all of the leaves to respond; this requires a nerve impulse-like stimulus for communication.
8. Venus's-flytrap
a. This plant has three sensitive hairs at the base of the trap.
b. When touched by an insect, an impulse-type stimulus triggers the trap to close.
c. Turgor pressure in leaf cells then propel the trap.
G. Sleep Movements
1. Sleep movements are nastic responses to the daily changes in light level; an example is the prayer plant that folds its leaves each night.
2. Movement is due to changes in turgor pressure of motor cells in a pulvinus.
3. Some plant movements correspond to environmental changes in light, temperature, etc.
4. Circadian rhythms are biological rhythms with a 24-hour cycle.
5. A biological clock is an internal mechanism maintaining biological rhythms in the absence of stimuli.
6. Biological clocks are synchronized by external stimuli to twenty-four-hour rhythms.
7. Photoperiod is more reliable an indicator of seasonal changes than temperature change.
8. Stomata and flowers usually open in the morning, close at night; some plants secrete nectar at the same time of day.
27.2 Plant Hormones
1. For plants to respond to stimuli, activities of plant cells and structures have to be coordinated.
2. Almost all plant communication is done by hormones.
3. Hormones are chemical messengers, produced in very low concentrations, that are active in another part of the organism.
4. A response is influenced by several hormones and may require a specific ratio of two or more hormones.
5. Hormones are synthesized in one part of a plant; they travel in the phloem after a plant receives an appropriate stimulus.
6. Each naturally-occurring hormone has a specific chemical structure.
7. Other chemicals that differ only slightly from natural hormones also affect the growth of plants.
8. Plant growth regulators are hormone imitators plus naturally-occurring plant growth hormones.
B. Auxins
1. Apically produced auxin prevents the growth of axillary buds, a phenomenon called apical dominance.
a. When a terminal bud is removed, the nearest buds begin to grow and the plant branches.
b. Application of a weak solution of auxin causes roots to develop from the ends of cuttings.
c. Auxin production by seeds promotes growth of fruit.
d. As long as auxin is concentrated in leaves and fruits rather than stem, they do not fall off.
2. Auxin-controlled cell elongation is involved in gravitropism and phototropism.
a. When gravity is perceived, auxin moves to the lower surface of roots and stems.
b. The Darwins discovered with oat seedlings, phototropism would not occur if the tip of a seedling is cut off or covered by a cap; they concluded the cause of curvature moved from the coleoptile (the protective sheath for the leaves of a seedling) tip to the rest of the shoot.
3. Frits W. Went (1926) experimented with coleoptiles.
a. He cut off tips and placed them on agar.
b. When an agar block was placed to one side, the coleoptile would curve away from that side regardless of the light.
c. He deduced a chemical caused the curved growth, and he named it auxin after the Greek word for "promoting growth."
C. How Auxins Work
1. In a plant exposed to unidirectional light, auxin moves from the bright side to the shady side of a stem.
2. Auxin binds to receptors and activates the ATP-driven proton (H+) pump.
3. As hydrogen ions are pumped out of the cell, the cell wall becomes acidic, breaking hydrogen bonds.
4. Cellulose fibrils are weakened and activated enzymes further degrade the cell wall.
5. The electrochemical gradient established causes of uptake of solutes and water follows by osmosis.
6. The turgid cell presses against the cell wall, stretching it so that elongation occurs.
7. Auxin-mediated elongation occurs in younger cells; older cells may lack auxin receptors.
D. Gibberellins
1. Gibberellins are a group of some 70 plant hormones that chemically differ only slightly.
2. GA3 is the most common of the natural gibberellins.
3. Gibberellins are growth promoters that elongate cells.
4. Gibberellins were discovered in 1926 by Ewiti Kurosawa, a Japanese scientist investigating a fungal disease of rice plants called "foolish seedling disease."
a. His fungus-infected plants produced an excess chemical gibberellin, named after the fungus.
b. By 1956, gibberellic acid was finally isolated from a flowering plant rather than fungus.
5. Mode of action
a. The hormone GA3 binds to a receptor; a second messenger (Ca2+) inside the cell combines with the protein a DNA-binding protein.
b. This complex activates a gene coding for the enzyme amylase.
c. Amylase acts on starch to release sugars used as a source of energy by the growing embryo.
d. The dormancy of seeds and buds can be broken by applying gibberellins.
E. Cytokinins
1. Cytokinins are a class of plant hormones that promote cell division.
2. Cytokinins are derivatives of the purine base adenine.
3. A natural cytokinin zeatin is found in corn kernels; kinetin is a synthetic cytokinin.
4. Researchers discovered cytokinins in work on growing plant tissues in culture.
5. Oligosaccharins, chemical fragments released from the cell wall, also direct differentiation.
6. Researchers hypothesize that auxin and cytokinins are part of a reception-transduction-response pathway that activates enzymes that release these fragments from the cell wall.
F. Senescence
1. Aging processes are senescence; large molecules break down and are transported elsewhere in the plant.
2. Cytokinins prevent senescence of leaves; they also initiate development of leaf growth.
3. Cytokinins initiate growth of lateral buds despite apical dominance.
G. Abscisic Acid
1. Abscisic acid (ABA) is sometimes called the "stress hormone"; it maintains seed and bud dormancy and causes closure of stomata.
2. Dormancy occurs when a plant organ readies itself for adverse conditions by stopping growth.
a. ABA moves from leaves to vegetative buds in the fall; thereafter these buds are converted to winter buds which are covered by thick, hardened scales.
b. Reduction in ABA and increase in gibberellins break seed and bud dormancy; seeds germinate and buds send forth leaves.
3. Abscisic acid brings about the closing of stomata when a plant is under water stress.
a. By some unknown mechanism, ABA causes K+ ions to leave guard cells.
b. As a result, guard cells lose water and the stomata close.
4. Although external application of ABA promotes abscission (the dropping of leaves, fruits, and flowers), it is not believed to function in this process; the hormone ethylene is considered to have this natural function.
H. Ethylene
1. Ethylene is involved in abscission, the dropping of leaves, fruits, or flowers.
a. Lower levels of auxin in these areas (compared to stem) probably initiate abscission.
b. Once abscission begins, ethylene stimulates production of enzymes such as cellulase that cause leaf, fruit, or flower drop.
2. It was an early practice to prepare citrus fruit for market by storage in a room with a kerosene stove.
3. Later work revealed incomplete combustion of kerosene produced ethylene which ripens fruit.
4. Ethylene is a gaseous plant hormone; it ripens fruit by increasing the activity of enzymes that soften fruit.
5. Ethylene stimulates production of cellulase, an enzyme that hydrolyzes cellulose in plant cell walls.
6. A barrel of ripening apples can induce ripening of a bunch of bananas some distance away.
7. Ethylene releases from the site of a physical wound; therefore one rotten apple spoils the whole bunch.
8. The presence of ethylene in air inhibits the growth of plants in general.
9. Ethylene is present in auto exhaust and in homes heated with natural gas.
10. Inhibition of plant growth occurs in low concentrations (one part ethylene per 10 million parts of air).
27.3 Photoperiodism
A. Many physiological changes in plants (e.g., seed germination, the breaking of bud dormancy, and the onset of senescence) are related to a seasonal change in day length.
1. Photoperiodism is a physiological response to relative lengths of daylight and darkness.
2. Research by the U.S. Department of Agriculture in the 1920s in controlled greenhouses revealed this mechanism.
3. In some plants, photoperiodism also affects flowering.
B. Plants can be divided into three groups, based on photoperiodism.
1. Short-day Plants
a. These plants flower when day length is shorter than a critical length.
b. Examples include cocklebur, poinsettia, and chrysanthemum.
c. In effect, they require a period of darkness that is longer than a critical length to flower.
2. Long-day Plants
a. These plants flower when the day length is longer than a critical length.
b. Examples include wheat, barley, clover, and spinach.
c. In effect, they require a period of darkness that is shorter than a critical length to flower.
3. Day-neutral Plants
a. These are plants for which flowering is not dependent on day length.
b. Examples include tomato and cucumber.
C. A long-day and a short-day plant can have the same critical length.
1. These plants differ in specific sequence of day lengths as a season progresses.
2. Spinach is a long-day plant that flowers in summer when day length increases to 14 hours.
3. Ragweed is a short-day plant that flowers in fall when day length shortens to 14 hours or less.
4. In 1938, K. C. Hammer and J. Bonner experimented with artificial lengths of dark and light periods.
a. Cocklebur, a short-day plant, flowers as long as the dark period lasts over 8.5 hours.
b. If the dark period is interrupted by a flash, it does not flower; but darkness amidst a day cycle has no effect.
c. Long-day plants require a dark period shorter than a critical length regardless of the length of the light period.
d. Therefore, length of the dark period controls flowering, not length of the light period.
D. Phytochrome and Plant Flowering
1. U.S.D.A. scientists discovered phytochrome, a blue-green leaf pigment that exists in two forms.
2. Pr (phytochrome red) absorbs red light (wavelength of 660 nm); it is converted to Pfr.
3. Pfr is phytochrome far-red and absorbs far-red light (wavelength of 730 nm); it is converted to Pr.
4. During a 24-hour period, there is a shift in ratio of these two pigments.
a. Direct sunlight contains more red than far-red light; Pfr is present in plant leaves during the day.
b. Shade and sunsets have more far-red than red light; Pfr is converted to Pr as night approaches.
c. There is a slow metabolic replacement of Pfr by Pr during night.
5. Phytochrome conversion may be a first step in reception-transduction-response pathway resulting in flowering.
E. Other Functions of Phytochrome
1. The Pr → Pfr conversion cycle controls other growth functions in plants.
2. In addition to being involved in flowering, Pfr promotes seed germination and inhibits stem elongation.
3. Following germination, the presence of Pr dominates; the stem elongates and grows toward sunlight while the leaves remain small.
4. Once a plant is exposed to sunlight and Pr is converted to Pfr, the plant begins to grow normally—leaves expand and the stem branches.
5. The Pfr form of phytochrome triggers activation of one or more regulatory proteins in the cytosol.
6. These proteins migrate to the nucleus and bind to "light-stimulated" genes coding for proteins found in chloroplasts.
28.1 Reproductive Strategies
A. Life Cycles
1. In contrast to animals with only one multicellular stage in their life cycle, all plants have two: a diploid and a haploid generation.
2. The sporophyte is a diploid generation in an alternation of generations life cycle.
a. A sporophyte produces haploid spores by meiotic division.
b. Spores develop, by mitosis, into a haploid gametophyte.
3. A flower, the reproductive structure of an angiosperm, produces two types of spores, microspores and megaspores.
a. A microspore is a plant spore that develops into a male gametophyte, which is a pollen grain.
b. A megaspore is a plant spore that develops into a female gametophyte, the embryo sac which remains within a sporophyte plant.
4. At maturity, a pollen grain (which is either windblown or carried by an animal to the vacinity of the female gametophyte) travels via a pollen tube to the embryo sac and fertilizes the egg.
5. The zygote becomes an embryo, which develops into a seed.
6. The sporophyte is the generation that contains vascular tissue and has other adaptations suitable to living on land, including production of flowers.
7. Flowers are unique to angiosperms; aside from producing the spores and protecting gametophytes, flowers attract pollinators and produce fruits to enclose the seeds.
B. Flowers
1. A flower is the reproductive organ of a flowering plant; it develops in response to environmental signals.
2. The shoot apical meristem stops forming leaves to form flowers; axillary buds can become flowers directly.
3. Monocot flower parts are in threes or their multiples; eudicot flower parts are in fours or fives or their multiples.
4. Sepals are leaflike, usually green; this outermost whorl protects the bud as a flower develops within.
5. Petals are interior to sepals; coloration accounts for attractiveness of many flowers.
a. The size, shape, and color of a flower are attractive to a specific pollinator.
b. Wind-pollinated flowers often have no petals at all.
6. Grouped about a pistil are stamens, stalked structures that have two parts.
a. The anther is a saclike container within which pollen grains develop.
b. A filament is a slender stalk that supports the anther.
7. The carpel is the vaselike structure located at the center of a flower; carpels usually have three parts.
a. The stigma is an enlarged sticky knob on the end of a style; stigma serves to receive pollen grains.
b. The style is a slender stalk that connects stigma with the ovary.
c. The ovary is an enlarged base of a carpel that contains a number of ovules.
8. Not all flowers have sepals, petals, stamens, and a pistil.
a. Complete flowers have sepals, petals, stamens, and a pistil; incomplete flowers do not.
b. Perfect (bisexual) flowers have both stamens and a pistil.
c. Imperfect (unisexual) flowers have only stamens or carpels.
d. Carpellate flowers have only carpels.
9. If staminate and carpellate flowers are on the same plant, the plant is monoecious.
10. If staminate and carpellate flowers are on different plants, the plant is dioecious.
C. Life Cycle of Flowering Plants
1. In plants, the sporophyte produces haploid spores by meiosis; in animals, meiosis produces gametes.
2. Flowering plants are heterosporous, producing microspores and megaspores that become sperm-bearing pollen grains and egg-bearing embryo sacs, respectively.
3. Development of Male Gametophyte
a. Microspores are produced in the anthers of flowers.
b. An anther has four pollen sacs; each contains many microsporocytes (microspore mother cells).
c. Microsporocytes undergo meiotic cell division to produce four haploid microspores.
d. The haploid nucleus then divides mitotically forming two cells enclosed in a finely sculptured wall; this is a pollen grain, which is at first an immature male gametophyte, containing a tube cell and a generative cell.
e. The larger tube cell will eventually form the pollen tube.
f. Eventually each generative cell will divide mitotically to form two sperm.
g. Once both events have occurred, the pollen grain is the mature male gametophyte.
4. Development of Female Gametophyte
a. The ovary contains one or more ovules.
b. An ovule is covered by parenchymal cells except for one small opening, the micropyle.
c. One parenchyma cell enlarges to become a megasporocyte that undergoes meiotic cell division to produce four haploid megaspores.
d. Three megaspores are nonfunctional; one megaspore nucleus divides mitotically into eight nuclei in a female gametophyte.
e. When cell walls form around the nuclei later, there are seven cells, one of which is binucleate.
f. The female gametophyte (or embryo sac) consists of seven cells:
i. one egg cell,
ii. two synergid cells,
iii. one central cell with two polar nuclei, and
iv. three antipodal cells.
5. Development of Sporophyte
a. Walls separating the pollen sacs in the anther break down when the pollen grains are to be released.
b. Pollination is the transfer of pollen from an anther to the stigma of a carpel.
c. Self-pollination is transfer of pollen from anther to stigma of the same plant.
d. Cross pollination is transfer of pollen from anther of one plant to stigma of another plant; plants often have mechanisms that promote cross pollination such as the carpel only maturing after anthers have released their pollen.
e. Using a pollinator to carry pollen from flower to flower of only one species increases the efficiency.
f. Secretion of nectar is one way to attract certain pollinators, and they may be adapted to reach only one type of flower.
g. When a pollen grain lands on a stigma, it germinates, forming a pollen tube.
h. A germinated pollen grain, containing a tube cell and two sperm, is the mature male gametophyte.
i. As a pollen tube grows, it passes between the cells of the stigma and the style to reach the micropyle of an ovule.
j. Double fertilization occurs after the release of both sperm cells into the ovule.
k. One sperm nucleus unites with the egg nucleus, forming a 2n zygote.
l. The other sperm nucleus migrates and unites with the polar nuclei of the central cell, forming a 3n endosperm cell.
m. The zygote divides mitotically to become the embryo; the endosperm nucleus divides mitotically to become the endosperm.
n. The embryo, in most plants, is a young sporophyte.
o. The endosperm is tissue that will nourish the embryo and seedling as they undergo development.
28.2 Seed Development
1. Development of the seed is the next event in the life cycle of the angiosperm.
2. Plant growth and development involves cell division, cell elongation, and differentiation of cells into tissues and then organs.
B. Development of the Eudicot Embryo
1. Immediately after double fertilization, the endosperm nucleus divides to produce a mass of endosperm surrounding the embryo.
2. The single-celled zygote also divides, but asymmetrically, forming two parts: embryo and suspensor, which anchors the embryo and transfers nutrients to it from the sporophyte plant.
3. Globular Stage
a. During this stage, the proembryo is a ball of cells.
b. The root-shoot axis is established; cells near the suspensor will become a root, those at the opposite end will become a shoot.
c. The outermost cells become dermal tissue; by dividing with the cell plate perpendicular to the surface, they produce one outer cell layer.
d. Dermal tissue prevents dessication and also has stomata that regulate gas exchange.
4. The Heart Stage and Torpedo Stage Embryos
a. The embryo has a heart-shape when the cotyledons appear; it then grows to a torpedo shape.
b. With elongation, the root and shoot apical meristems are distinguishable.
c. Ground meristem responsible for most of the interior of the embryo is also present now.
5. The Mature Embryo
a. After differentiation into embryo and suspensor, one or two cotyledons develop.
b. The embryo continues to differentiate into three parts.
c. The epicotyl is between the cotyledons and first leaves; it contributes to shoot development.
d. The hypocotyl is below the cotyledon and contributes to stem development.
e. The radicle is below the hypocotyl and contributes to root development.
f. The cotyledons are quite noticeable in a eudicot embryo, and may fold over.
C. Monocots Versus Eudicots
1. Monocots have only one cotyledon.
a. In monocots, the cotyledon rarely stores food.
b. It absorbs food molecules from the endosperm and passes them to the embryo.
2. Eudicots have two cotyledons
a. During development of a eudicot embryo, cotyledons usually store the nutrients the embryo uses.
b. The endosperm seemingly disappears as the nutrients are consumed.
28.3 Fruit Types and Seed Dispersal
1. A fruit is a mature ovary that encloses seeds; sometimes they retain other flower parts.
2. Fruits serve to protect and disperse offspring.
3. The fruit protects the peach seed well but makes germination difficult; the peas escape easily but are lightly protected.
B. Simple Fruits
1. Simple fruit develops from a single carpel or several fused carpels of a compound ovary.
2. A pea pod breaks open on both sides and releases seeds; legumes are fruits that split to two sides when mature.
3. Legumes and cereal grains are examples of dry fruits; such fruits are mistaken for seeds because a dry pericarp adheres to the seed within.
4. For plants to be widely distributed, seeds have to be dispersed away from the parent plant.
a. Hooks and spines of clover, bur, and cocklebur attach to the fur of animals.
b. Woolly hairs, plumes, and wings disperse by wind.
5. A fleshy fruit has a fleshy pericarp (e.g., peach, plum, olive, grape, tomato, apple, and pear).
a. Birds and mammals eat fruits, including seeds, and defecate them at a distance.
b. Squirrels and other animals gather seeds and fruits and bury them some distance away.
6. An apple is an example of an accessory fruit; the bulk of the fruit is not from the ovary but from the receptacle; a cross-section shows it came from a compound ovary with several chambers.
C. Compound Fruits
1. A compound fruit develops from several individual ovaries.
2. An aggregate fruit develops from ovaries from a single flower (e.g., blackberry).
3. An aggregate fruit where each ovary becomes a one-seeded fruit is called an achene (e.g., strawberry).
4. A multiple fruit develops from ovaries from separate flowers fused together (e.g., pineapple).
D. Seed Germination
1. Seed germination occurs when growth and metabolic activity resume.
2. The embryo forms with both shoot and root apical meristem enclosed in a seed.
a. Protoderm gives rise to the epidermis.
b. Ground meristem produces the cells of the cortex and pith.
c. Procambium produces vascular tissue.
3. Seeds retain their viability for varying times: maples seeds only last a week while lotus seeds are viable for hundreds of years.
4. Some seeds do not germinate until they have been dormant for a period of time.
a. Seed dormancy is a time during which no growth occurs even though conditions are favorable.
b. In temperate zones, seeds may have to be exposed to cold weather before dormancy is broken.
c. In deserts, germination requires rain; this ensures that seeds do not germinate until a favorable growing season has arrived.
5. Germination has environmental requirements.
a. Oxygen must be available for increased metabolism.
b. Adequate temperature allows enzymes to act.
c. Adequate moisture hydrates cells.
d. Light may also be required.
6. Respiration and metabolism continue throughout dormancy but at a reduced level.
7. Some seeds have a surface coating that attracts water; imbibing plant cells can swell dramatically.
8. Seeds that must be planted near the surface probably require light (e.g., lettuce).
9. When a seedling grows in the dark, it becomes long and spindly (etiolated); phytochrome induces normal growth in light.
10. Germination in Eudicots and Monocots
a. Prior to germination, a eudicot embryo consists of the following:
i. two cotyledons that supply nutrients to the embryo and seedling, but soon shrivel and disappear;
ii. a plumule—a rudimentary plant that consists of an epicotyl bearing young leaves;
iii. the hypocotyl, which becomes the stem; and
iv. the radicle, which develops into roots.
b. As dicot seedling emerges, the shoot is hook-shaped to protect the delicate plumule.
c. In monocots, the endosperm is the food-storage tissue and the cotyledon does not have a storage role.
d. A monocot "seed" such as a corn kernel is actually the fruit and the outer covering is the pericarp.
e. The plumule and radicle are enclosed in protective sheaths, the coleoptile and the coleorhiza, respectively.
f. The plumule and radicle burst through these coverings when germination occurs.
28.4 Asexual Reproduction in Plants
1. Plants contain nondifferentiated meristem tissue and often reproduce asexually by vegetative propagation.
2. In asexual reproduction, offspring arise from a single parent and inherit genes of that parent only.
3. Vegetative propagation utilizes the meristematic tissue of a parent plant.
a. Violet plants grow from nodes of rhizomes.
b. The nodes of stolons will produce strawberry plants.
c. Each eye of a potato plant tuber is a bud that produces a new plant.
d. Sweet potatoes can be propagated from their modified roots.
e. Many trees can be started from small "suckers."
4. Stem cuttings have long been used to propagate a wide array of plants (e.g., sugarcane, pineapple).
5. The discovery that auxin will cause roots to develop has expanded our ability to use stem cuttings.
B. Tissue Culture of Plants
1. In 1902, German botanist Gottleib Haberlandt suggested producing entire plants from tissues.
2. Tissue culture is the process of growing tissue artificially in a liquid or solid culture medium.
3. Haberlandt stated plant cells were totipotent; each cell has full genetic potential of the organism.
4. In 1958, Cornell botanist F. C. Steward grew a complete carrot plant from a tiny piece of phloem.
5. When cultured cells were provided with sugars, minerals, vitamins, and cytokinin from coconut milk, the undifferentiated cells divided and initially formed a callus, an aggregation of undifferentiated cells.
6. The callus then differentiated into shoot and roots and developed into a complete plant.
7. Micropropagation is a commercial method of producing thousands to millions of identical seedlings, by tissue culture in limited space.
8. Meristem culture micropropagates many new shoots from a single shoot apex culture in a medium with correct proportions of auxin and cytokinin.
a. Since the shoots are genetically identical, the adult plants that develop are clonal plants.
b. Clonal plants have the same genome and display the same traits.
c. Meristem culture generates meristem that is virus-free; the plants produced are also virus-free.
9. Entire plants can be grown from single plant cells.
a. Enzymes can digest cell walls and produce naked plant cells called protoplasts.
b. Protoplasts regenerate a cell wall and begin cell division.
c. Clumps of cells can be manipulated to form somatic embryos.
d. Somatic embryos encapsulated in a hydrated gel ("artificial seeds") can be shipped anywhere.
e. Somatic embryos are cultured by the millions in large tanks (bioreactors).
f. Plants generated from somatic embryos vary because of mutations; these somaclonal variations may produce new traits.
10. Anther cultures mature anthers in a medium of vitamins and growth regulators.
a. The haploid tube cells within a pollen grain divide, producing proembryos made of 20 to 40 cells.
b. Finally the pollen grains rupture, releasing haploid embryos.
i. The researcher can then generate a haploid plant.
ii. Chemical agents are added to encourage chromosomal doubling; the resulting plants are diploid and homozygous for all alleles.
iii. This produces plants that express recessive alleles.
11. Cell suspension culture uses rapidly growing calluses cut into small pieces and shaken in a liquid nutrient medium.
a. Single cells or small clumps form a suspension of cells; all produce the same chemicals as the entire plant.
b. This technique is a more efficient way of producing chemicals used in drugs, cosmetics, and agricultural applications than farming plants simply to acquire chemicals they produce.
C. Genetic Engineering of Plants
1. Traditionally hybridization (crossing different varieties or species) was used to produce new plants.
2. Tissue Culture and Genetic Engineering
a. Genetic engineering alters genes of organisms so they have new and different traits.
b. Protoplasts in particular lend themselves to direct genetic engineering in tissue culture.
c. High voltage electric pulses create pores in plasma membrane so new DNA can be introduced.
d. When genes for production of firefly enzyme luciferinase were inserted into tobacco protoplasts, adult plants glowed when sprayed with the substrate luciferin.
e. Regeneration of cereal grains from protoplasts has been difficult; corn and wheat protoplasts produce infertile plants.
f. Foreign DNA can be inserted into a plasmid of Agrobacterium; this bacterium infects plant cells and can be used to deliver the recombinant DNA to target cells.
g. John C. Sanford and Theodore M. Klein of Cornell University developed a gene gun to bombard a callus with DNA coated metal particles; later, adult plants are generated.
h. Crops have been engineered to resist frost, fungal and viral infections, insect predation, and herbicides.
i. Future crops could have higher protein content and require less water and fertilizer.
j. Sequencing the genomes of a dicot Arabidopsis thaliana and rice will give a blueprint to the genes of other monocots and dicots.
3. Agricultural Plants with Improved Traits
a. Corn, potato, soybean, and cotton plants have been engineered to be resistant to insect predation or herbicides.
b. If the crops are resistant to herbicide and the weeds are not, then less tillage is needed.
c. Salt-tolerant Arabidopsis has been developed by cloning a gene for sequestering sodium ions in a vacuole.
d. Such techniques would allow development of crops that could grow where irrigation causes salinization.
e. Genes from Vernonia and castor bean seeds have been inserted into soybeans to produce vernolic acid and ricinoleic acid used as hardeners in paints and plastics.
f. Other genetic engineering goals could be the increase in productivity by altering water and carbon dioxide intake.
g. Genetic engineering is attempting to improve efficiency of RuBP carboxylase and introduce C4 photosynthesis to rice.
4. Commercial Products
a. Corn has made antibodies to deliver radioisotopes to tumor cells.
b. Soybeans make an antibody to treat genital herpes.
c. Researchers can introduce a human gene into tobacco plants using the tobacco mosaic virus.
d. Tobacco plants produced antigens to treat non-Hodgkin's lymphoma.
29.1 Evolution of Animals
1. Members of the kingdom Animalia are multicellular heterotrophs that ingest their food.
2. Over 1.5 million species of animals are known, many more likely exist.
3. There are over 35 animal phyla—all but one contain only invertebrates (no backbone) hordata, is composed mainly of vertebrates (with a backbone of bone or cartilage).
B. A Criteria for Classification
1. Organization of Tissues
a. Animals may have the cellular, tissue, or organ level of organization.
b. Germlayers refer to the number of layers of tissues.
c. Most animals are made of three tissue layers: endoderm, ectoderm and mesoderm.
d. Animals with only ectoderm and endoderm (diploblastic) have the tissue level of organization.
e. Animals with three tissue layers (triploblastic) have the organ level of organization.
2. Types of Symmetry
a. Asymmetry means there is no particular body shape (e.g., sponge).
b. Radial symmetry describes body parts arranged around an axis, like spokes of a wheel (e.g., starfish).
i. Radially symmetrical animals may be sessile (i.e., attached to a substrate or less motile).
ii. This symmetry enables an animal to reach out in all directions from one center.
c. Bilateral symmetry describes a body having a right and left, or complementary halves.
i. Only one longitudinal cut down the center produces mirror halves.
ii. Bilaterally symmetrical animals tend to be active and to move forward at an anterior end.
iii. The development of a head to localize the brain and sensory organs at the anterior end is called cephalization.
3. Type of Body Plan
a. Animals with a sac plan have an incomplete digestive system with only one opening used for both entrance and exit of food.
b. Animal with the tube-within-a-tube plan have a complete digestive system with separate entrance and exit to the digestive system; this allows specialization zones along the digestive tract.
4. Type of Coelom
a. Acoelomates lack a body cavity or coelom although they have mesoderm.
b. Pseudocoelomates possess a body cavity that is incompletely lined by mesoderm because the cavity develops between the mesoderm and endoderm.
c. Coelomates possess a body cavity completely lined with mesoderm; coelomates are either protostomes or deuterostomes.
d. Protostomes have an embryonic development where the first embryonic opening (blastopore) becomes the mouth.
e. Deuterostomes develop with the blastopore becoming the anus.
5. Segmentation is repetition of body parts along the length of the body.
a. Among coelomates, molluscs and echinoderms are non-segmented.
b. Annelids, arthropods, and chordates are segmented.
c. Segmentation leads to specialization of parts as they differentiate for specific purposes.
6. Molecular Data
a. When classifying animals, nucleotide sequences are taken into account—the assumption being that the more closely related two organisms are, the more nucleotide sequences they will have in common.
29.2 Multicellularity
A. About 8,000 species of sponges are in the phylum Porifera.
1. Sponges have no symmetry and no tissues; they are to the side from the mainstream of animal evolution.
2. Sponges remain at a cellular level of evolution, having evolved separately from protozoan ancestors.
3. Sponges are aquatic, largely marine animals, that vary greatly in size, shape, and color.
4. Their saclike bodies are perforated by many pores; Porifera means "pore-bearing."
5. Their cell organization is demonstrated by fragmenting a sponge; cells survive and reorganize into a sponge.
6. Several types of cells are found in a sponge.
a. The outer layer of their wall contains flattened epidermal cells; some have contractile fibers.
b. The middle layer is a semifluid matrix with wandering amoeboid cells and spicules.
c. The inner layer is composed of collar cells (choanocytes).
d. There are no nerve cells or means of coordination between cells.
7. Beating collar cells produce currents through pores in the wall into a central cavity and out through osculum.
8. Simple sponges 10 cm tall can filter as much as 100 liters of water a day.
9. Sponges are sessile filter feeders; they stay in one place and filter food from the water.
a. Collar cells engulf and digest food particles in food vacuoles, or pass them to amoeboid cells.
b. Amoeboid cells transport nutrients from cell to cell, and they also produce gametes and spicules.
10. Sponges reproduce asexually by budding, which can produce quite large colonies.
11. Fragmentation occurs when sponges are chopped up; each piece can start a complete sponge.
12. Sponges reproduce sexually when eggs and sperm are released into a central cavity; the zygote develops into a ciliated larva.
13. Sponge classification is partly based on the chemistry of its skeleton composed of spicules.
a. Spicules are tiny needle-shaped structures with one to six rays, depending on chemical structure.
b. Chalk sponges have spicules composed of calcium carbonate.
c. Glass sponges have spicules of silica.
14. Spongin is a protein made of modified collagen; it gives a sponge its flexibility.
29.3 True Tissue Layers
A. About 90 species of comb jellies are in the phylum Ctenophora.
1. Comb jellies develop only ectoderm and endoderm (are diploblasts), and are radially symmetrical.
2. Comb jellies are transparent and often luminescent; their eight plates of fused cilia resemble long combs.
3. Most of their body is a jellylike packing material called mesoglea.
4. They are the largest animals to be propelled by beating cilia.
5. Long tentacles covered with sticky filaments—or an entire body covered by sticky mucus—captures prey.
B. Cnidarians
1. About 9,000 species of cnidarians are in the phylum Cnidaria.
2. Cnidaria also develop only ectoderm and endoderm and are tubular or bell-shaped.
3. They mostly live in coastal waters but there are oceanic jellyfish and freshwater hydras.
4. Cnidaria have cnidocytes, a specialized cell that contains a nematocyst.
a. The nematocyst is a fluid-filled capsule, which contains a long, spirally coiled hollow thread.
b. When the trigger of the cnidocyte is touched, the nematocyst is discharged.
c. Some threads merely trap prey or predators; others have spines that penetrate and inject paralyzing toxins.
5. A cnidarian body is a two-layered sac with the epidermis derived from ectoderm.
6. The inner tissue layer derived from the endoderm secretes digestive juices into the gastrovascular cavity.
7. The gastrovascular cavity digests food and circulates nutrients.
8. There are muscle fibers at the base of the epidermal and gastrodermal cells.
9. Nerve cells located below the epidermis near the mesoglea interconnect and form a nerve net.
a. Unlike highly organized nervous systems, this nerve net transmits impulses in several directions at once, which results in multiple firings of nematocysts in parts not directly stimulated.
b. Having both muscle fibers and nerve fibers, these animals are capable of directional movement, the body can contract or extend, and tentacles that ring the mouth can extend to grasp prey.
10. Cnidaria have two basic body plans.
a. A polyp is vase-shaped and the mouth is directed upward.
b. A medusa is bell-shaped and the mouth is directed downward.
c. A medusa has more mesoglea than a polyp; tentacles are concentrated on the margin of the bell.
d. Both body forms may have been a part of life cycle of early cnidaria.
e. When both stages are present, the animal is dimorphic and the polyp stage is sessile and produces the medusae.
f. The medusa stage is motile and produces the egg and sperm, dispersing the species.
C. Cnidarian Diversity
1. Sea anemones and corals are in the class Anthozoa.
a. Sea anemones are solitary polyps 5–100 mm in length and 5–200 mm in diameter or larger.
b. Many are brightly colored and resemble flowers.
c. A thick, heavy body supports a mouth that is surrounded by hollow tentacles.
d. Sea anemones feed on invertebrates and fish.
e. They attach to rocks, timbers, etc., or may be mutualistic and attach to a hermit crab's shell.
f. Corals may be solitary; most are colonial.
g. Most corals live in shallow waters; the accumulation of their calcium-carbonate remains builds reefs.
2. Class Hydrozoa includes hydras and Portuguese man-of-war (Physalia).
a. The polypoid stage is dominant.
b. The Portuguese man-of-war is a colony of polyps; the original polyp becomes a gas-filled float.
c. Other polyps bud to specialize for feeding or reproduction.
d. It can cause serious injury to swimmers with a tentacle having numerous nematocysts; each tentacle arises from the base of each feeding polyp.
3. The class Scyphozoa includes the true jellyfishes (e.g., Aurelia).
a. The medusa stage is dominant in jellyfish; the polyp remains small.
b. Jellyfishes are an important part of the zooplankton, the food for larger marine animals.
D. Hydra and Obelia
1. Hydras are solitary, freshwater hydrozoan polyps.
a. The hydra body is a small tube about one-quarter inch in length.
b. Four to six tentacles containing nematocysts surround the mouth, the only opening at one end.
c. Hydra move by gliding or somersaulting.
d. Hydras have both muscular and nerve fibers, and respond to touch.
2. Hydra Anatomy
a. Epidermal cells are termed epitheliomuscular cells and contain muscle fibers.
b. Cnidocytes and sensory cells are in the epidermis.
c. Interstitial cells can produce an ovary or testis, and may assist regeneration.
d. Gland cells secrete digestive juices into a gastrovascular cavity; the tentacles capture and stuff in prey.
e. Digestion is completed within food vacuoles of nutritive-muscular cells.
f. Nutrients then diffuse to the rest of the cells in the body.
3. Hydra reproduction
a. Hydra reproduces asexually by budding.
b. They can also reproduce sexually: sperm from a testis swim to an egg within an ovary; after early development within an ovary, a protective shell allows the egg to survive until conditions are optimum for it to emerge.
4. Obelia is a colony of polyps enclosed in a hard, chitinous covering.
a. Feeding polyps have nematocysts and extend beyond the covering to capture tiny worms, etc.
b. Polyps are connected and partially digested food is distributed through the colony.
c. A colony increases in size by asexually budding new polyps.
d. Reproductive polyps bud off the medusae stage which is free-living or attached to the colony.
e. Obelia medusae tentacles have nematocysts; the gastrovascular cavity extends into tentacles.
f. A nerve net runs in two rings around the bell margin; it has statocysts for balance and ocelli to detect light.
g. Sperm and egg from medusae unite; the resulting zygote develops into a ciliated planula larva that settles down to develop into a polyp colony.
5. Evolutionary history of Cnidaria
a. Some biologists propose that a planuloid-type organism gave rise to both the cnidaria and the flatworms.
b. Cnidaria have two tissues and are radially symmetrical; flatworms have three germ layers and bilateral symmetry.
29.4 Bilateral Symmetry
1. All animals beyond this point are triploblasts with three germ layers.
2. Flatworms have a sac body plan and thus an incomplete digestive tract; ribbon worms have a tube-within-a-tube body plan and thus a complete digestive tract.
3. The tube-within-a-tube plan allows specialization of parts along the tube.
B. Ribbon Worms
1. About 650 species of marine ribbon worms are in phylum Nemertea.
2. Ribbon worms have a distinctive proboscis apparatus.
3. The proboscis is a long, hollow tube that is shot outward through a pore located just above the mouth.
4. The proboscis is used for prey capture, defense, locomotion, and burrowing.
C. Flatworms
1. Over 15,000 species of flatworms belong to phylum Platyhelminthes.
2. Classification
a. Planaria and relatives are freshwater animals in the class Turbellaria.
b. Flukes are external or internal parasites in the class Trematoda.
c. Tapeworms are internal parasites in the class Cestoda.
3. In addition to endoderm and ectoderm, a mesoderm layer gives rise to muscles and reproductive organs.
4. There is no coelom; they are acoelomates.
5. A branched gastrovascular cavity is the site of extracellular digestion and it distributes nutrients about the body.
6. The flat, thin body allows gas exchange to occur by diffusion.
7. An excretory system functions as an osmotic-regulating system.
8. Flatworms are bilaterally symmetrical; free-living forms exhibit cephalization, the development of a head region.
9. Flatworms have a ladder-type nervous system.
a. Paired ganglia form a brain.
b. Sensory cells are in the body wall.
D. Free-living Flatworms
1. Turbellaria (3,000 species) include freshwater planaria such as the planaria Dugesia.
2. Planaria live in lakes, ponds, and streams and feed on small living or dead organisms.
3. The head is bluntly arrow-shaped; side extensions (auricles) are sensory organs to detect food and enemies.
4. The two light-sensitive eyespots have pigmentation that makes them look cross-eyed.
5. Three muscle layers—an outer circular, an inner longitudinal, and a diagonal—allow for varied movement.
6. In larger forms, locomotion is accomplished by movement of cilia on ventral and lateral surfaces.
7. Numerous gland cells secrete a mucous material upon which the animal moves.
8. Digestion
a. It captures food by wrapping itself around prey, entangling it in slime, and pinning it down.
b. The pharynx is a muscular tube that extends through the mouth and through which food is ingested.
c. In a three-branched gastrovascular cavity, digestion is both extracellular and intracellular.
9. Excretion
a. The flame-cell system consists of a series of interconnecting canals that run the length of the body on either side of the longitudinal axis and side branches of the canals.
b. A flame cell is a bulb-shaped cell containing a tuft of cilia inside the hollow interior of the bulb; cilia move back and forth, bringing water into canals that empty through pores at the surface.
c. It functions in both water excretion and osmotic regulation.
10. Planaria can reproduce both sexually and asexually.
a. They constrict beneath the pharynx; each half will grow into a whole animal—regeneration.
b. Planaria are hermaphroditic, possessing both male and female sex organs.
c. Planaria cross-fertilize each other.
d. Fertilized eggs are enclosed in a cocoon and hatch in two to three weeks into tiny worms.
E. Parasitic Flatworms
1. As parasites, flukes and tapeworms have characteristic modifications.
a. Loss of predation allows a lack of cephalization; the head carries hooks and suckers to attach to a host.
b. There is extensive development of the reproductive system with loss of other systems.
c. Well-developed nervous and gastrovascular systems are not needed; it does not seek out or digest prey.
d. Flukes and tapeworms are covered by a tegument that protects them from host digestive juices.
2. Two hosts
a. Flukes and tapeworms use a secondary (intermediate) host to travel from primary host to primary host.
b. A primary host is infected with sexually mature adults; the secondary host contains the larval stage(s).
3. Flukes
a. The class Trematoda includes the flukes.
b. Blood, liver, and lung flukes inhabit those organs.
c. Fluke bodies are generally oval and elongate.
d. At the "head," an oral sucker is surrounded by sensory papillae; another sucker also helps attach.
e. Flukes have reduced digestive, nervous, and excretory systems.
f. The reproductive system is well developed and they are usually hermaphroditic.
g. The blood fluke causes schistosomiasis.
i. Schistosomiasis disease is found predominantly in tropical Africa and South America where about 800,000 infected persons die each year.
ii. Blood flukes are male or female; the female fluke deposits eggs in blood vessels around the intestines.
iii. The eggs migrate to the intestine and are passed out with feces.
iv. Tiny larvae hatch in water and swim until they detect and enter a particular species of snail.
v. The larvae reproduce asexually inside the snail and eventually leave the snail.
vi. If the larvae penetrate the skin of the human body, they begin to mature in the liver and implant in the small intestine blood vessels.
vii. A weakened person is more likely to die from secondary diseases.
h. The Chinese liver fluke (Clonorchis sinensis) requires two intermediate hosts (a snail and a fish).
i. Humans become infected when they eat uncooked fish.
ii. Adults live in the liver and deposit eggs in the bile duct, which carries eggs to the intestine.
iii. Larval flukes must then pass through two intermediate hosts, a snail and a fish.
4. Tapeworms
a. A tapeworm head or scolex contains hooks and suckers for attachment to the intestinal wall of the host.
b. Behind this head is a short neck and then a long string of proglottids.
c. Each proglottid segment contains a full set of both male and female sex organs and little else.
d. There are excretory canals but no digestive system and only rudiments of nerves.
e. After fertilization, proglottids become a bag of eggs; mature proglottids break off and pass out with feces.
f. If eggs of tapeworms are ingested by pigs or cattle, larvae become encysted in the muscle of hosts.
g. The covering of ingested eggs is digested away and larvae burrow through the intestinal wall and travel by bloodstream to lodge and encyst in muscle.
h. A cyst is a hard-walled structure sheltering a larval worm.
i. If humans eat the meat of infected pigs or cattle and fail to cook the meat properly, they too become infected.
29.5 Body Cavity
A. Roundworms
1. The pseudocoelom is a body cavity that is incompletely lined with mesoderm.
2. Body support is provided by the hydrostatic pressure of fluid in the pseudocoelom pressing against a tough cuticle.
3. Over 90,000 species of roundworms are in the phylum Nematoda.
4. These worms are unsegmented and have a smooth outside wall.
5. Nematode worms are found almost everywhere: sea, fresh water, soil, etc.
6. Many are scavengers or parasites; some are predators.
7. Ascaris
a. In Ascaris lumbricoides, males are smaller than females and their posterior end curves to a point.
b. These worms move by whiplike motions because only longitudinal muscles lie next to the body wall.
c. Mating produces eggs that mature in the soil; therefore, most are limited to warmer climates.
d. When eggs are swallowed, larvae burrow through the intestinal wall to the liver, heart and/or lungs.
e. In the lungs, the larvae molt; after 10 days they migrate up the windpipe to the throat and are swallowed.
f. Back in the intestine, mature worms mate and the female deposits eggs that are lost with feces.
g. Feces must reach the mouth of the next host to complete a life cycle; therefore proper sanitation easily prevents infection.
8. Trichinosis is a serious infection.
a. Humans contract Trichinella spirallis by eating raw pork with encysted larvae.
b. After maturation, the adult female burrows into the wall of the small intestine and produces living offspring that are carried by the bloodstream to skeletal muscles where they encyst.
9. Filarial worms cause various diseases.
a. In the U.S., a filarial worm is a parasite of dogs.
b. It lives in the heart and the arteries that serve the lungs and thus is called heartworm disease.
10. Elephantiasis occurs in tropical Africa.
a. It is caused by a filarial worm (Wuchereria bancroftji) that utilizes the mosquito as a secondary host.
b. Adult worms reside in and block the lymphatic vessels; ultimately this results in the limbs of an infected individual swelling to monstrous size.
c. It is treatable only in the early stages but not after scar tissue has blocked the lymphatic vessels.
B. Rotifers
1. About 2,000 species of rotifers belong to phylum Rotifera.
2. Rotifers are abundant in freshwater.
3. Although microscopic and easily confused with protozoans, rotifers are multicellular with a pseudocoelom and organs.
4. A crown of cilia (corona) causes a rotating motion; this organ of locomotion also directs food to the mouth.
5. They can dessicate for lengthy periods of time and thus are called "resurrection animacules."
30.1 Advantages of Coelom in Protostomes and Deuterostomes
A. Protostomes include molluscs, annelids, arthropods.
1. Protostomes exhibit the following events during embryological development
a. Spiral cleavage, in which the cells divide without an increase in size.
b. The fate of the cells is fixed—each contributes to development in only one particular way.
c. The blastopore is associated with the mouth.
d. A coelom (schizocoelom) forms by splitting of the mesoderm, which has arisen from cells near the blastopore.
B. Deuterostomes include echinoderms and chordates.
1. They exhibit the following events during embryological development.
a. Radial cleavage, where the new daughter cells sit atop the previous cells; the fate of these cells is indeterminate.
b. The blastopore is associated with the anus; the mouth appears later.
c. A coelom (enterocoelom) forms by the fusion of mesodermal pouches from the primitive gut.
C. The presence of a coelom has many advantages.
1. Body movements are freer.
2. Complex organs and organ systems are allowed to develop.
3. It can serve as a storage area for eggs and sperm before release.
4. Coelomic fluid protects internal organs.
5. It can assist in respiration and circulation.
6. It can allow for a site of accumulation of metabolic wastes.
7. It can provide a hydrostatic skeleton for movement.
D. Advanced Characteristics
1. The animals discussed in this chapter will have the following characteristics:
a. Bilateral symmetry
b. Organ level of organization
c. Complete digestive tract
d. They evolved in the sea
30.2 Molluscs
1. Over 110,000 living species of molluscs belong to the phylum Mollusca.
2. Most are marine, but some are freshwater and some are terrestrial.
3. Molluscs have a three-part body plan: a visceral mass, a mantle, and a foot.
4. The visceral mass contains internal organs: digestive tract, paired kidneys, and reproductive organs.
5. A mantle covering partly surrounds the visceral mass; it may secrete a shell and help develop the gills or lungs.
6. The foot is muscular and adapted for locomotion, attachment, food capture, or a combination of functions.
7. The radula in the mouth bears many rows of teeth and is used for feeding; a radula is not present in all molluscs.
8. In molluscs, the coelom is reduced and limited to the region around the heart.
9. Most molluscs have an open circulatory system: a heart pumps hemolymph through vessels into a hemocoel.
10. Blue hemocyanin, not red hemoglobin, is the respiratory pigment found in molluscs.
11. Some are slow moving with no head; others are active predators with a head and sense organs.
12. The nervous system consists of several ganglia connected by nerve cords.
13. Chitons are in the class Polyplacophora.
a. Chiton shells consists of a row of eight overlapping plates.
b. The flat chiton foot is muscular and creeps along or clings to rocks.
c. They scrape algae and other plant food from rocks with a well-developed radula.
B. Bivalves
1. Class Bivalvia contains the bivalves (clams, oysters, mussels, scallops).
2. Bivalves are two-part shells that are hinged and closed by powerful muscles.
3. They have no head, no radula, and little cephalization.
4. Clams burrow with a hatchet-shaped foot; mussels use it to produce threads to attach to objects.
5. Scallops both burrow and swim; rapid clapping of their valves releases water in spurts.
6. The shell is secreted by the mantle.
a. The shell is composed of protein and calcium carbonate with an inner layer of pearl.
b. The shell deposits around a foreign particle inserted between the mantle and the shell to form a pearl.
7. A compressed muscular foot projects down from the shell; by expanding the tip, it pulls in the body.
8. The beating cilia of the gills causes water to enter the mantle cavity by way of an incurrent siphon and exit by way of an excurrent siphon.
9. The cilia of gills move water through the mantle cavity.
10. Gills capture particles and move them toward the mouth; the mouth leads to the stomach, which leads to the intestine, which passes through a heart and ends at the anus.
11. The circulatory system is open; the heart pumps hemolymph into vessels that open into the hemocoel.
12. The nervous system consists of three pairs of ganglia that connect the front, back, and foot.
13. Two excretory kidneys below the heart remove ammonia waste from the pericardial cavity.
14. The sexes are separate; the gonad is located around coils of intestine.
15. Some clams and annelids have the same type of larva, indicating an evolutionary relationship between molluscs and annelids.
C. Cephalopods
1. Class Cephalopoda ("head-footed") includes squids, cuttlefish, octopuses, and nautiluses.
2. Squids and octopuses squeeze water out of the mantle cavity; the water forced out through a funnel propels them by jet propulsion.
3. Around the head are tentacles with suckers or adhesive secretions adapted for grasping prey.
4. A head equipped with a powerful beak can tear prey apart.
5. Well-developed sense organs include focusing camera-type eyes.
6. Cephalopods, particularly octopuses, have well-developed brains with a capacity for learning.
7. Nautiluses are enclosed in shells; squids have a reduced internal shell and octopuses lack shells.
8. Squids and octopuses possess ink sacs; they squirt a cloud of ink to escape predators.
9. Squids possess a vestigial shell under the mantle (the pen) which surrounds the visceral mass.
10. Squids direct the funnel to squeeze water out to move forward or backward.
11. Squids have a closed circulatory system where blood is always within blood vessels or the heart.
12. A squid has three hearts; one pumps blood to internal organs; two pump blood to the gills.
13. Gonads make up a large portion of the visceral mass; the sexes are separate.
a. Spermatophores are packets that contain sperm which a male tentacle passes to the female mantle cavity.
b. After the eggs are fertilized, they are attached to substratum in strings of up to 100 eggs.
D. Gastropods
1. Class Gastropoda includes snails, land slugs, whelks, conchs, periwinkles, sea hares, and sea slugs.
2. Most are marine but some are freshwater or terrestrial.
3. Herbivores use a radula to scrape food from surfaces; carnivorous gastropods use the radula to bore through a surface, such as a bivalve shell, to obtain food.
4. A developed head with eyes and tentacles projects from a coiled shell that protects visceral mass.
5. Nudibranchs (sea slugs) and terrestrial slugs lack shells.
6. In development, gastropods undergo a torsion—the body is twisted to bring the anus and mantle cavity downward, forward and around to a position above the head—to position the visceral mass above the foot.
7. In aquatic gastropods, the gills are in the mantle cavity.
8. In land gastropods, a mantle has blood vessels and functions as a lung when air is moved in and out through respiratory pores.
9. Terrestrial gastropod embryonic development does not go through a swimming larval stage found in aquatic species.
10. For terrestrial snails, the shell not only offers protection, but it also prevents desiccation.
11. The foot contracts in peristaltic waves from anterior to posterior; this movement is aided by a lubricating mucus that is secreted.
12. Land snails are hermaphroditic.
a. In pre-mating behavior, they meet and shoot calcareous darts into each other's body wall.
b. Each inserts a penis into the other's vagina; this provides sperm for future fertilization of eggs.
c. Eggs are deposited in soil and development proceeds without the formation of a larvae.
13. Hermaphroditism assures that any two animals can mate–very useful in slow-moving animals; this is an adaptation to terrestrial life.
30.3 Annelids
1. About 15,000 species of segmented worms are in phylum Annelida.
2. Segmentation is evidenced by the rings that encircle the body; septa are internal walls that partition the coelom.
3. A well-developed, fluid-filled coelom and tough integument act as a hydrostatic skeleton.
4. Segmentation may have evolved in conjunction with a hydrostatic skeleton.
5. Using a hydrostatic skeleton, partitioning the coelom allows for independent movement of the segments so it can not only burrow but crawl on the surface.
6. Once segmentation and the tube-within-a-tube plan appeared, each segment could specialize to perform a particular function.
7. The digestive system is specialized to include a pharynx, stomach, and accessory glands.
8. The extensive closed circulatory system has blood vessels running the length of the body and to every segment; muscular aortic arches ("hearts") propel blood through the vessels.
9. The nervous system has a brain connected to a ventral solid nerve cord with a ganglion in each segment.
10. Paired nephridia in each segment collect waste material from coelom and excrete it through openings in the body wall.
B. Polychaete Worms
1. Most polychaetes (class Polychaeta) are marine.
2. Polychaetes possess parapodia and setae.
a. Parapodia are paddle-like appendages used in swimming and for respiration.
b. Setae are bristles, attached to parapodia, that help anchor polychaetes or help them move.
3. Clam worms such as Nereis are active predators.
4. Many have well-developed cephalization; the head has well-developed jaws, eyes, and other sense organs.
5. Other sedentary filter feeders possess tentacles with cilia to create water currents and sort out food particles.
6. Only during breeding seasons do polychaetes have reproductive organs.
7. In Nereis, many worms coordinate the shedding of a portion of their bodies that contain either eggs or sperm; these segments float to the surface where fertilization takes place.
8. Marine worm zygotes develop larva similar to those of marine clams; again this shows a relatedness between annelids and molluscs.
C. Oligochaetes
1. Class Oligochaeta includes earthworms with only a few setae, protruding in pairs directly from the body; about 3,500 species are known.
2. Oligochaetes lack both a well-developed head and parapodia.
3. Locomotion requires coordinated movement of body muscles and the help of setae.
a. As longitudinal muscles contract, segments bulge and setae protrude to anchor into soil.
b. When circular muscles contract, a worm lengthens, setae are withdrawn and the segment can be pulled forward.
4. Earthworms live in moist soil; a moist body wall allows for gas exchange.
5. Earthworms are scavengers that extract organic remains from soil they eat.
6. A muscular pharynx draws food into the mouth.
7. Food is stored in a crop and ground up in a thick, muscular gizzard.
8. The dorsal surface of the intestine is expanded into a typhlosole for more surface area for digestion.
9. Each external segment corresponds to an internal septum; a wall that separates each body segment.
10. A long ventral nerve cord leads from the brain to ganglionic swellings and lateral nerves in each segment.
11. Segmentation
a. The ventral nerve cord leading from the brain has ganglionic swellings and lateral nerves in each segment.
b. The coiled nephridia tubules in most segments have two openings: one is a ciliated funnel that collects coelomic fluid, and the other is an exit through the body wall.
c. Between the two openings, a coiled nephridia tubule removes waste from blood vessels.
d. The dorsal blood vessel moves red blood anteriorly; five pairs of hearts pump blood to a ventral vessel.
e. Thus, segmentation in the earthworm is evidenced collectively by
i. body rings,
ii. coelom divided by septa,
iii. setae on most segments,
iv. ganglia and lateral nerves in each segment, nephridia in most segments,
v. branch blood vessels in each segment.
12. Reproduction
a. Earthworms are hermaphroditic.
b. The male organs are the testes, sperm ducts, and seminal vesicles.
c. The female organs are the ovaries, oviducts, and seminal receptacles.
d. Mating involves aligning parallel to each other facing opposite directions to exchange sperm.
e. Each possesses a clitellum that secretes mucus that slides off, forming a slime tube that protects the sperm and eggs from drying out.
f. The slime tube forms a cocoon around the fertilized eggs as they develop.
g. Embryonic development lacks a larval stage.
13. Comparison with Clam Worm
a. Earthworms and clam worms show adaptations to marine and terrestrial life through presence or absence of cephalization, parapodia, a slime tube cocoon, and trochophore larvae.
D. Leeches
1. Leeches (about 500 species) belong to the class Hirudinea.
2. Most are freshwater species but a few are marine or terrestrial.
3. They lack setae and each body ring has several transverse grooves.
4. Leeches possess a small anterior sucker around the mouth and a larger posterior sucker.
5. Although some are free-living predators, most feed on body fluids.
6. Leeches keep blood from coagulating by hirudin, an anticoagulant in their saliva.
30.4 Arthropods
1. Over 1,000,000 species are in phylum Arthropoda; they are considered highly successful because they have adapted to so many different habitats.
2. Arthropods have a rigid exoskeleton with freely movable jointed appendages.
a. The exoskeleton is a strong but flexible outer covering composed mainly of chitin.
b. Chitin is a strong, flexible, nitrogenous polysaccharide.
c. The exoskeleton serves for protection, attachment for muscles, locomotion, and prevention of desiccation.
d. Because the exoskeleton is hard and nonexpandable, arthropods must molt (shed) the exoskeleton to grow larger.
i. Before molting, the body secretes a larger soft and wrinkled exoskeleton underneath.
ii. Enzymes partially dissolve and weaken the old exoskeleton.
iii. The arthropod breaks the old exoskeleton open and wriggles out.
iv. The new exoskeleton then quickly expands and hardens.
3. Some segments of arthropods have fused into regions (e.g., a head, a thorax, and an abdomen).
a. Trilobites (Cambrian Period) had a pair of appendages on each body segment.
b. Modern arthropod appendages specialize for walking, swimming, reproduction, etc.
c. These modifications account for the diversity and success of arthropods.
4. Arthropods have a well-developed nervous system.
a. A brain is connected to a ventral solid nerve cord.
b. The head bears various sensory organs.
c. Compound eyes have many complete visual units; each collects light independently.
d. The lens of each visual unit focuses the image on the light-sensitive membranes of a few photoreceptors.
e. In simple eyes, a single lens brings the image to focus into many receptors, each of which receives only a portion of the image.
5. Arthropods use a variety of respiratory organs.
a. Marine forms use gills with vascularized, thin-walled tissue specialized for gas exchange.
b. Terrestrial forms have book lungs (e.g., spiders) or air tubes called tracheae (e.g., insects).
6. Metamorphosis is a drastic change in form and physiology that occurs as a larva becomes an adult.
a. Metamorphosis contributes to the success of arthropods.
b. A larva eats foods and lives in environments different from the adult.
c. Competition between the immatures and adults of a species is reduced.
d. This reduction in competition allows more members of the species to exist at one time.
B. Crustaceans
1. About 40,000 species of crustaceans belong to subphylum Crustacea.
2. Crustaceans are successful and mostly marine arthropods.
3. The head usually bears compound eyes and five pairs of appendages.
a. The first two appendages are antennae and antennules; in front of the mouth, they have sensory functions.
b. The next three pairs (mandibles, first and second maxillae) lie behind the mouth and are used in feeding.
4. Biramous appendages have two branches; one branch is a gill and the other is the leg branch.
5. Copepods and krill feed on algae; numerous, they are an important link in marine food chains.
6. Barnacles have a thick, heavy shell and are sessile.
a. Stalked barnacles attach by their stalk; stalkless barnacles attach directly to the shell.
b. Barnacles begin as free-swimming larvae and become sessile on wharf pilings, rocks, etc.
c. They extend feathery structures (cirri) to filter feed.
7. Decapods include shrimps, lobsters, crabs, etc.
a. Their thorax bears five pairs of walking legs; the first pair may be modified as claws.
b. Usually respiratory gills are above the walking legs.
c. The nonsegmented carapace covers the fused head and thorax (cephalothorax).
d. Abdominal segments have a pair of swimmerets, small paddlelike structures.
e. The first pair of swimmerets in a male are stronger to pass sperm to the female.
f. The last tail segments are the uropod and the telson, which together make a fan-shaped tail.
g. A crayfish awaits prey and uses its claws to carry it to the mouth.
h. The crayfish stomach has two main regions.
i. The anterior gastric mill with chitinous teeth grinds food.
ii. A posterior region filters coarse particles before absorption in the digestive glands.
i. Green glands in the head area excrete metabolic wastes through a duct at the base of the antennae.
j. The coelom is reduced in arthropods and forms the space about the reproductive system.
k. The heart pumps hemolymph containing bluish hemocyanin into a hemocoel where it washes around the organs.
l. A brain is connected to a ventral nerve cord; periodic ganglia give off lateral nerves.
m. The sexes are separate in crayfish.
i. The male has a coiled sperm duct that opens to the outside at the base of its fifth walking leg.
ii. The female's ovaries open at the base of the third walking legs.
iii. The fold between the bases of the fourth and fifth pair of legs serves as a seminal receptacle.
iv. Following fertilization, the eggs are attached to the swimmerets of the female.
C. Uniramians
1. The subphylum Uniramia includes the millipedes, centipedes, and insects.
2. The appendages attached to the thorax and abdomen have only one branch—the leg branch.
3. The head appendages include one pair of antennae, one pair of mandibles, and one or two pairs of maxillae.
4. Uniramia live on land and breathe by air tubes called tracheae.
5. Insects
a. Over 900,000 species of insects are in the superclass Insecta; this number exceeds all other animal species combined.
b. Most insects live on land; some are secondarily aquatic.
c. The insect body is divided into a head, thorax, and abdomen.
i. The head bears sense organs and mouthparts.
ii. The thorax bears three pairs of legs and one or two pairs of wings; the wings provide advantages in escaping enemies, finding food, and mating.
iii. The abdomen contains most of the internal organs.
d. The exoskeleton of an insect is lighter and contains less chitin than that of many other arthropods.
e. Grasshoppers have adaptations as herbivorous insects.
i. The third pair of legs is suited to jumping.
ii. The front wings are protective and leathery; the thin hind pair of wings fold up.
iii. Each side of the first abdominal segment has a tympanum for sound wave reception.
iv. Two paired projections form an ovipositor in females used to dig a hole for laying eggs.
v. The grasshopper digestive system is complex.
1. The mouth mechanically breaks apart food, and salivary secretions begin digestion.
2. The crop temporarily stores food.
3. A gizzard finely grinds the food.
4. Digestion is completed in the stomach; gastric ceca cavities assist absorption of nutrients.
f. The excretory system consists of Malpighian tubules.
i. The tubules extend into the hemocoel.
ii. Nitrogenous wastes are collected and excreted into the digestive system.
iii. Formation of solid nitrogenous wastes (uric acid) conserves water.
g. The respiratory system begins with openings in the exoskeleton called spiracles.
i. Air enters into small tubules called tracheae.
ii. The tracheae branch many times until they reach moist areas of gas absorption.
iii. Air movement through this tracheal system is assisted by air sacs.
iv. Air enters the anterior four spiracles and exits by the posterior six.
v. Breathing by tracheae is a factor that limits the size of insects.
h. The circulatory system contains a slender, tubular heart.
i. This heart lies against the dorsal wall of the abdominal exoskeleton.
ii. The heart pumps hemolymph into a hemocoel where it circulates before returning to the heart.
iii. Insect hemolymph is colorless—it lacks any respiratory pigment since the tracheal system transports gases.
i. Reproduction is adapted to life on land.
i. A male grasshopper has a penis to transfer sperm to the female.
ii. Internal fertilization protects the gametes from drying out.
iii. Female grasshoppers deposit eggs in the ground with the ovipositor.
j. Grasshoppers undergo incomplete metamorphosis; the immature nymph resembles the adult.
k. Other insects undergo complete metamorphosis; the wormlike larvae reorganize into different adults.
l. Some species (e.g., bees and ants) exhibit colonial social behavior.
m. Entomology, the study of insects, is a major field of biology.
6. Comparison with Crayfish
a. Crustacea are primarily marine or freshwater organisms, use gills and an oxygen carrying pigment, and excrete liquid ammonia wastes; insects are primarily terrestrial or freshwater organisms, use tracheae, lack respiratory pigments, and excrete solid wastes.
D. Centipedes and Millipedes (Superclass Myriapoda)
1. There are about 3,000 species of centipedes ("hundred-leggers").
a. The body is composed of a head and trunk with many segments; each segment has one pair of legs.
b. Centipedes are carnivorous; the head bears antennae and mouthparts with poison fangs.
2. The millipedes ("thousand-leggers") have a cylindrical segmented body.
a. Some body segments are fused with two pair of legs on each resulting segment.
b. They possess more legs than centipedes, although not one thousand as the name states.
c. Millipedes dwell in the soil, feeding on dead organic matter.
E. Chelicerates
1. Chelicerates in the subphylum Chelicerata include spiders, scorpions, and horseshoe crabs.
2. The first pair of appendages are chelicerae, the second pair are pedipalps, and the next four pairs are walking legs.
a. Chelicerae are appendages that function as feeding organs.
b. Pedipalps are feeding or sensory structures.
3. All of the appendages attach to a cephalothorax, a fusion of head and thoracic regions.
4. The head lacks antennae, mandibles, or maxillae appendages.
5. The marine horseshoe crab (genus Limulus) is common along the east coast of North America.
a. They scavenge sandy and muddy substrates for annelids, molluscs and other invertebrates.
b. The anterior shield is a horseshoe-shaped carapace with two compound eyes.
c. A long, unsegmented telson tail projects to the rear.
d. They possess book gills that resemble the pages in a book.
6. Scorpions
a. Scorpions are arachnids and are the oldest terrestrial arthropods known from fossils.
b. They are nocturnal and spend the day hidden under a log or rock.
c. Pedipalps are large pincerlike appendages; the abdomen ends in a stinger containing venom.
7. Ticks and mites number over 25,000 species.
a. Ticks are parasites that suck blood and sometimes transmit diseases.
b. Chiggers are larvae of certain mites and feed on the skin of vertebrates.
8. Spiders (over 35,000 species) have a narrow waist separating the cephalothorax from the abdomen.
a. Spiders have numerous simple eyes rather than compound eyes.
b. Spider chelicera are modified as fangs with ducts leading from poison glands.
c. The abdomen has silk glands; they may spin a web to trap prey.
d. Invaginations of the body wall form lamellae (pages) of book lungs.
9. The harvestman is not a true spider.
a. About 5,000 species are known; the most commonly-known is the "daddy long legs."
31.1 Chordates
1. Chordates include 45,000 species in the phylum Chordata.
2. At some time during their life, all chordates have four basic characteristics.
a. Notochord
i. This supporting rod is located dorsally just below the nerve cord.
ii. It provides support and is replaced by the vertebral column in vertebrates.
b. Dorsal Hollow Nerve Cord
i. This cord contains a fluid-filled canal.
ii. In vertebrates, this is the spinal cord and it is protected by vertebrae.
c. Pharyngeal Pouches
i. These openings function in feeding, gas exchange, or both.
ii. They are seen only during embryonic development in most vertebrates.
iii. In invertebrate chordates, fish, and amphibian larvae, they become functioning gills.
iv. In terrestrial vertebrates, the pouches are modified for various purposes.
v. In humans, the first pair of pouches become the auditory tubes, the second become tonsils, and the third and fourth pairs become the thymus and parathyroid glands.
d. A postanal tail extends beyond the anus; in some, this only appear in embryos.
B. Nonvertebrate Chordates
1. The notochord persists and is never replaced by the vertebral column in these species.
2. Lancelets
a. Approximately 25 species of lancelets are in genus Branchiostoma in the subphylum Cephalochordata.
b. An elongated, lance-shaped body resembles the lancelet, a two-edged surgical knife.
c. They inhabit shallow coastal waters; they filter feed partly buried in sandy substrates.
d. They feed on microscopic particles filtered from a constant stream of water that enters the mouth and exits through gill slits into an atrium that opens at the atriopore.
e. Lancelets retain the four chordate characteristics as adults.
f. The notochord extends from head to tail, accounting for the name "Cephalochordata."
g. They possess segmented muscles and the dorsal hollow nerve cord has periodic branches.
3. Sea Squirts
a. The subphylum Urochordata contains about 2,000 species of sea squirts, also called tunicates.
b. Adults have a body composed of an outer tunic; an excurrent siphon squirts out water when it is disturbed.
c. The larvae are bilaterally symmetrical and have the four chordate characteristics.
d. The larvae undergo metamorphosis to develop into sessile adults.
e. Water passes into a pharynx and out numerous gill slits, the only chordate characteristic that remains in adults.
f. Beating cilia lining the inside of the pharynx create a current to move water through.
g. Microscopic particles adhere to a mucous secretion in the pharynx and are eaten.
h. If the larvae became sexually mature without developing tunicate characteristics, the urochordate larva may have been ancestral to vertebrates.
31.2 Vertebrates
1. The 43,700 species of vertebrates are in the subphylum Vertebrata.
2. Vertebrates have all four chordate characteristics at some time during their lives.
3. The embryonic notochord is replaced by a vertebral column.
a. The vertebral column is individual vertebrae that surround a dorsal hollow nerve cord.
b. The vertebral column is part of a flexible, strong endoskeleton that is also evidence of segmentation.
4. The vertebrate skeleton is living tissue (either cartilage or bone) that grows with the animal.
5. The endoskeleton and muscles together permit rapid and efficient movement.
6. The pectoral and pelvic fins of fish evolved into jointed appendages allowing vertebrates to move onto land.
7. A skull is an anterior component of the main axis of vertebrate endoskeleton; it encases the brain.
8. The high degree of cephalization in vertebrates is accompanied by complex sense organs.
a. The eyes developed as outgrowths of the brain.
b. The ears—equilibrium devices in water—function as sound-wave receivers in land vertebrates.
9. Vertebrates possess a complete digestive system and a large coelom.
10. The circulatory system is closed and the blood is contained within blood vessels.
11. Gills or lungs provide efficient gas exchange.
12. The kidneys efficiently excrete nitrogenous waste and regulate water.
13. Reproduction is usually sexual with separate sexes.
14. Evolution of the amnion allowed reproduction to take place on land.
15. The development of the placenta in mammals allowed development in the uterus of the female.
B. Fishes
1. Fishes are aquatic, gill-breathing vertebrates that usually have fins and skin covered with scales.
2. Small, jawless, and finless ostracoderms are the earliest vertebrate fossils.
a. They were filter feeders also able to move water through their gills by muscular action.
b. Ostracoderms are dated from Cambrian to as late as Devonian; then they became extinct.
c. Although living jawless fishes lack protection, early jawless fishes had large defensive head shields.
C. Jawless Fishes
1. Jawless fishes are agnathans; 65 species belong to superclass Agnatha.
2. Lampreys and hagfishes are modern jawless fishes and they lack a bony skeleton.
3. They have smooth nonscaly skin.
4. They have cylindrical bodies and are up to a meter long.
5. Hagfishes are scavengers feeding on soft-bodied invertebrates and dead fishes.
6. Many lampreys are filter feeders similar to their ancestors.
7. Parasitic lampreys have a round muscular mouth equipped with teeth; they attach themselves to fish and suck nutrients from the host's circulatory system.
8. Marine parasitic lampreys entered the Great Lakes and devastated the trout population in the 1950s.
D. Fishes with Jaws
1. Animals beyond this point have jaws, the tooth-bearing bones of the head.
2. Jaws evolved from the first pair of gill arches of agnathans; the second pair of arches became support structures for the jaws.
3. Placoderms are extinct jawed fishes of the Devonian Period.
a. They were armored with heavy plates and had strong jaws.
b. Like extant fishes, they had paired pectoral and pelvic fins.
c. Paired fins allow a fish to balance and maneuver well in water; this helps predation.
E. Cartilaginous Fishes
1. About 850 species of sharks, rays, and skates are in the class Chondrichthyes, the cartilaginous fishes.
2. They have a cartilaginous skeleton rather than bone.
3. Five to seven gill slits are on both sides of the pharynx; they lack the gill covers found on bony fish.
4. Their body is covered by epidermal placoid (toothlike) scales.
5. The teeth of sharks are enlarged scales; there are many rows of replacement teeth growing behind the front teeth.
6. They have three well developed senses to detect electric currents in water, pressure (a lateral line system), and smell.
7. The largest sharks are filter feeders, not predators; the basking and whale sharks eat tons of crustacea.
8. Most sharks are fast, open-sea predators; a great white shark eats dolphins, sea lions and seals.
9. Rays and skates live on the ocean floor; their pectoral fins are enlarged into winglike fins and they swim slowly.
10. Stingrays have a venomous spine.
11. Electric rays feed on fish that have been stunned with an electric shock that may reach over 300 volts.
12. Sawfish rays have a large anterior "saw" that they use to slash through schools of fish.
F. Bony Fishes
1. About 25,000 species of bony fishes are in the class Osteichthyes.
2. Bony fishes have a skeleton of bone; most are ray-finned with thin, bony rays supporting the fins.
3. A few lobe-finned fishes are related to ancestors of amphibians.
4. The ray-finned fishes include our familiar fishes.
a. They are the most successful and diverse of vertebrates.
b. They vary from filter feeders to predaceous carnivores.
c. Their skin is covered by scales formed of bone.
d. The gills do not open separately but instead are covered by an operculum.
e. The swim bladder is a gas-filled sac whose pressure can be altered to regulate buoyancy and depth.
f. Salmon, trout, and eels migrate between fresh and salt water but adjust their kidney and gill function.
g. Fish sperm and eggs are usually shed into water.
h. For most fish, the fertilization and embryonic development occur outside the female's body.
5. The lobe-finned fishes include six species of lungfishes and one species of coelacanth.
a. Their fleshy fins are supported by central bones.
b. Lungfishes live in stagnant water or ponds that dry up; found in Africa, South America and Australia.
c. Coelacanths live in deep oceans; once considered extinct, more than 200 have been captured since 1938.
G. Amphibians
1. All animals studied from this point on are tetrapods (have four limbs).
2. The lobe-finned fishes of Devonian are ancestral to amphibians.
3. Land animals use limbs to support their body since the air is less buoyant than water.
4. Some lobe-finned fishes and the early amphibians had lungs and internal nares to breathe air.
5. About 4,200 species of amphibians belong to class Amphibia.
6. Two hypotheses describe evolution of amphibians from lobe-finned fishes.
a. Lobe-finned fishes that could move from pond-to-pond had an advantage over those that could not.
b. The supply of food on land and the absence of predators promoted adaptation to land.
7. The first amphibians diversified during the Carboniferous Period which is known as the Age of the Amphibians.
8. Diversity of Amphibians
a. Modern amphibians include three groups: frogs and toads, salamanders and newts, and the caecilians.
b. Salamanders and newts have a long body and tail, and two pairs of legs; they resemble the earliest fossil amphibians.
c. Their S-shaped locomotion is similar to fish movements.
d. Salamanders and newts are carnivorous, feeding on insects, snails, etc.
e. Salamanders practice internal fertilization; the males produce a spermatophore that females pick up with the cloaca (the common receptacle for the urinary, genital, and digestive canals).
f. Frogs and toads are tailless as adults; the hind limbs are specialized for jumping.
g. Frogs and toads have the head and trunk fused; frogs live near or in fresh water while toads live in damp places away from water.
h. Caecilians are legless; most burrow in soil and feed on worms, etc.
i. Reproduction involves a return to the water; "amphibian" refers to this need to return to water from land.
i. They shed eggs into the water for external fertilization.
ii. Generally, amphibian eggs are protected by a coat of jelly but not by a shell.
iii. The young hatch into aquatic larvae with gills (tadpoles).
iv. The aquatic larvae usually undergo metamorphosis to develop into a terrestrial adult.
9. Anatomy and Physiology of Amphibians
a. A tongue is used for catching prey.
b. The eyelids keep their eyes moist.
c. Amphibian ears are adapted for detecting sound waves; in turn, the larynx produces sounds.
d. The amphibian brain is larger than that of fishes; their cerebral cortex is more developed.
e. Amphibians usually have small lungs supplemented by gas exchange across porous skin.
f. The single-loop circulatory path of fish is replaced by a closed double-loop circulatory system; however oxygen-rich blood mixes with some oxygen-poor blood.
g. A three-chambered heart with a single ventricle pumps mixed blood before and after it has gone to the lungs.
h. Amphibian skin is thin, smooth, and nonscaly, and contains numerous mucous glands; this skin plays an active role in osmotic balance and respiration.
i. Some skin glands secrete poisons; those tropical species often have brilliant warning coloration.
j. Amphibians are ectothermic, depending upon the environment to regulate body temperature.
k. If winter temperature drops too low, temperate ectotherms become inactive and enter torpor.
H. Reptiles
1. Reptiles were the first vertebrates to practice internal fertilization through copulation and to lay eggs that are protected by a leathery shell.
2. The amniotic egg contains extraembryonic membranes.
3. Extraembryonic membranes are not a part of the embryo and are disposed of after development.
4. They protect the embryo, remove nitrogenous wastes, and provide oxygen, food, and water.
5. The amnion is one extraembryonic membrane; it fills with fluid to provide a "pond" for embryo to develop.
6. About 8,000 species of reptiles are known; they are in the class Reptilia.
7. Reptiles evolved from amphibian ancestors by the Permian Period.
8. The first reptiles (stem reptiles) gave rise to several lineages; each was adapted to a different way of life.
a. The pelycosaurs or sail lizards are related to therapsids, mammallike reptiles ancestral to mammals.
b. Some lineages returned to aquatic environments; the ichthyosaurs were fishlike and plesiosaurs had a long neck.
c. The pterosaurs of the Mesozoic Era had a keel for attachment of flight muscles and air spaces in bones to reduce weight.
9. Dinosaurs varied in size and behavior; some had a bipedal stance and gave rise to birds.
10. Reptiles dominated earth for about 170 million years during the Mesozoic Era; then most died out.
11. One theory of mass extinction:
a. A large meteorite or comet at the end of the Cretaceous Period could have set off earthquakes and fires, raising enough dust and smoke to block out the sun.
b. An iridium layer, a mineral common in meteorites, occurs in rocks at the end of this period.
12. Diversity of Reptiles
a. Most reptiles today live in the tropics or subtropics; lizards and snakes live on soil; turtles, crocodiles and alligators live in water.
b. Tuataras are lizardlike and identical to fossils 200 million years old.
c. Crocodiles and alligators are largely aquatic, feeding on fishes and other animals.
i. Their powerful jaws have numerous teeth; a muscular tail is both a paddle to swim and a weapon.
ii. Male crocodiles bellow to attract mates; males of some species protect the eggs and young.
d. Turtles have a heavy shell fused to the ribs and to the thoracic vertebrae.
i. Turtles lack teeth but use a sharp beak.
ii. Sea turtles must return to lay eggs onshore.
e. Lizards have four clawed legs and are carnivorous.
i. Marine iguanas on the Galapagos are adapted to spend long times in the sea.
ii. Chameleons live in trees, have a long sticky tongue to catch insects, and change color.
iii. Australian frilled lizards have a collar to scare predators.
f. Snakes evolved from lizards and lost legs as an adaptation to burrowing.
i. Their jaws can readily dislocate to engulf large food.
ii. A tongue collects airborne molecules to transfer them to Jacobson's organ for tasting.
iii. Some snakes are poisonous and have special fangs to inject venom.
13. Anatomy and Physiology of Reptiles
a. Reptiles have a thick, scaly skin that is keratinized and is impermeable to water.
i. Keratin is the protein that is also found in hair, fingernails, and feathers.
ii. Reptile's protective skin prevents water loss but it also requires several molts a year.
b. Reptile lungs are more developed than in amphibians; air rhythmically moves in and out of the lungs due to an expandable rib cage, except in turtles.
c. Most have a nearly four-chambered heart, except in the crocodile it is completely four-chambered; oxygen-rich blood is more fully separated from oxygen-poor blood.
d. Well-developed kidneys excrete uric acid; therefore, less water is lost in excretion.
e. Reptiles are ectothermic.
i. They require a fraction of the food per body weight of birds and mammals.
ii. They are behaviorally adapted to warm their body temperature by sunbathing.
I. Birds
1. About 9,000 species of birds (class Aves) are known.
2. The ancestry of birds is in dispute; some biologists consider them related to bipedal dinosaurs.
3. Birds lack teeth and have only a vestigial tail but their relationship to reptiles shows in the scales on their legs, claws on their toes, and a horny beak.
4. Birds' eggs are hard-shelled rather than leathery.
5. Diversity of Birds
a. Most birds can fly; some, however are flightless.
b. Bird classification is based on beak and foot types, and some on habitats and behaviors.
i. Birds of prey have notched beaks and sharp talons.
ii. Shorebirds have long slender bills and long legs.
iii. Waterfowl have webbed toes and broad bills.
6. Anatomy and Physiology of Birds
a. Bird anatomy is closely related to its ability to fly.
b. Bird forelimbs are modified as wings for flying with hollow, light bones laced with air cavities.
c. A beak composed of keratin has replaced jaws equipped with teeth.
d. A keeled breastbone anchors muscles used in flight.
e. Bird respiratory air sacs are extensive, even extending into some larger bones.
i. Using a one-way flow of air, air sacs maximize gas exchange and oxygenation of blood.
ii. Efficient supply of oxygen to muscles is vital for the level of muscle activity needed for flight.
f. Birds possess a four-chambered heart; this double-loop circulatory system separates oxygen-rich and oxygen-poor blood.
g. Birds are endothermic; they have the ability to maintain a constant, relatively high body temperature.
i. Homeothermy enables an animal to be continuously active in cold weather.
ii. Feathers may have evolved for insulation and secondarily became adapted for flight.
h. Flight requires well-developed sense organs and nervous system.
i. Birds have very acute vision.
ii. Bird muscle reflexes are excellent.
iii. Bird migration allows use of widespread food sources; an enlarged portion of the brain is responsible for instinctive behaviors.
J. Mammals
1. Over 4,800 species belong to the class Mammalia.
2. Mammals evolved during the Mesozoic Era from therapsids, extinct mammal-like reptiles.
3. The mammal skull is bigger than reptiles', their teeth are differentiated into molars and premolars, and the vertebral column provides more movement.
4. True mammals appeared during the Jurassic period, about the same time as the first dinosaurs.
a. The first mammals were small, about the size of mice.
b. Some of the earliest mammalian groups were monotremes and marsupials.
c. Placental mammals evolved later to occupy habitats vacated by dinosaurs.
5. The chief characteristics of mammals are hair and mammary glands.
6. Mammals are endothermic; they produce heat and maintain a constant body temperature.
7. Many adaptations of mammals are related to temperature control; hair provides insulation against heat loss and allows mammals to be active in cold weather.
8. Gas exchange is efficiently accomplished by lungs.
9. Mammals possess a four-chambered heart and a double-loop circulatory system.
10. Mammary glands enable females to feed young without deserting them to obtain food.
11. Nursing creates a bond between mother and offspring to ensure parental care while the young are helpless.
12. In most mammals, the young are born alive after a period of development in uterus.
13. Mammals That Lay Eggs
a. Monotremes are mammals that have a cloaca and lay hard-shelled amniote eggs.
b. They are represented by the duckbill platypus and the spiny anteater of Australia.
c. A female duckbill platypus lays her eggs in a burrow in the ground where she incubates them.
d. After hatching, the young lick milk seeping from modified sweat glands on the abdomen.
e. The spiny anteater has a pouch formed by swollen mammary glands and muscle; the egg moves from cloaca to pouch and hatches; the young remain for 53 days and live in the burrow where the mother feeds them.
14. Mammals That Have Pouches
a. Marsupials begin development inside the mother's body but are then born in a very immature state.
b. The newborns crawl up into a pouch on their mother's abdomen.
c. Inside a pouch they attach to the nipples of the mother's mammary glands and continue to develop.
d. Today, most marsupials are found in Australia where they underwent adaptive radiation for several million years without competition from the placental mammals, only introduced recently.
15. Mammals That Have Placentas
a. Developing placental mammals are dependent on a placenta, an organ of exchange between maternal and fetal blood.
b. The placenta supplies nutrients to and removes wastes from the blood of developing offspring.
c. A placenta also allows a mother to move about while the offspring develop.
d. The placenta enables young to be born in a relatively advanced stage of development.
e. Placental mammals are very active animals; they possess acute senses and a relatively large brain.
f. The brains of placental animals have cerebral hemispheres proportionately larger than other animals.
g. The young go through a long period of dependency on their parents after birth.
h. Today, placental mammals populate all of the continents except Antarctica; they are the dominant group of mammals on Earth.
i. Most are terrestrial, but some are aquatic, and bats can fly.
16. Classification of mammals in the various taxonomic orders is based on mode of locomotion and the method of obtaining food.
a. The order Perissodactyla includes 17 species of horses, zebras, tapirs, and rhinoceroses and the order Artiodactyla includes 185 species of pigs, cattle, deer, buffaloes, giraffes, etc.
i. Both orders are hoofed animals.
ii. They have elongated limbs adapted for running across open grassland.
iii. They are herbivorous and have large grinding teeth.
b. About 270 species are in order Carnivora.
i. Carnivores include the dogs, cats, bears, raccoons, and skunks.
ii. The canines of meat eaters are large and conical.
iii. Aquatic carnivores such as seals and sea lions must return to land to reproduce.
c. The order Primates contains 180 species of lemurs, monkeys, gibbons, chimpanzees, gorillas, and humans.
i. Typical primates are tree-dwelling fruit eaters; some are ground dwellers.
ii. Their digits have nails, not claws; the thumb is more opposable.
iii. Primates, particularly humans, have well-developed brains.
d. The order Cetacea includes about 80 species of whales and dolphins.
i. They lack substantial hair or fur.
ii. Blue whales are the largest animal ever to live on this planet; baleen whales that strain plankton from the water.
iii. Toothed whales feed on fish and squid.
e. The order Chiroptera contains 925 species of nocturnal bats.
i. Wings are layers of skin and connective tissue stretched between the elongated bones of all fingers but the first.
ii. Many species use echolocation to locate their usual insect prey.
iii. Some bats also eat birds, fish, frogs and plant tissues.
f. The order Rodentia contains rodents (e.g., mice, rats, squirrels, beavers, and porcupines).
i. This is largest order with 1,760 species.
ii. Rodents have incisors that grow continuously.
iii. Most rodents eat seeds but some are omnivorous or eat mainly insects.
g. Only two extant species are in order Proboscidea: the elephants.
i. The upper lip and nose are elongated and muscularized forming a prehensile trunk.
ii. They are herbivores and are the largest living land mammals.
h. Order Lagomorpha includes 65 species of rabbits, hares, and pikas.
i. They resemble rodents but have two pairs of continuously growing incisors.
ii. Their hind legs are longer than their front legs and they are herbivores.
i. Order Insectivora (419 species) includes the shrews and moles, mammals with short snouts that live underground.
j. The 4 species of mammals and sea cows are in the order Sirenia.
i. These are large aquatic mammals with large heads and forelimps modified into flippers.
k. Order Pinnipedia (34 species) includes the walruses, seals, and sea lions—aquatic mammals with all four limbs modified into flippers.
l. The order Xenarthra includes the 29 species of anteaters, sloths, and armadillos; they are toothless or have peglike teeth.
32.1 Evolution of Primates
A. Primate Characteristics
1. Primates differ from other mammals by being adapted for arboreal life (living in trees).
2. Mobile Forelimbs and Hindlimbs
a. In primates, the limbs are mobile and the hands and feet have five digits each.
b. In most primates, flat nails replace claws and sensitive pads develop on the underside of fingers and toes.
c. Many primate hands have an opposable (i.e., can touch each of the other digits) thumb; some also have an opposable big toe.
d. These features allow the free grasping of tree limbs and easy harvesting of fruit.
3. Binocular Vision
a. Primates have a reduced snout and the face is relatively flat.
b. The sense of smell is generally reduced.
c. The eyes are moved to the front of the face for overlapping views that provide stereoscopic vision.
d. Cone cells provide greater visual acuity and color vision but require bright light.
4. Large, Complex Brain
a. Better senses requires both sense organs and a more complex brain to process the input.
b. More of the brain becomes devoted to processing information received from the hands and thumb, less to smell.
5. Reduced Reproductive Rate
a. Primates have more single births, which reduces the need for care for several offspring.
b. The period of parental care is extended with an emphasis on learned behavior and complex social interactions.
B. Sequence of Primate Evolution
1. All primates at one time shared one common ancestor; prosimians were an early group to diverge and African apes were the last group to diverge from our lineage.
2. Prosimians diverged first and are most closely related to the original primate.
3. Anthropoids
a. Surviving anthropoids are classified into three superfamilies: New World monkeys, Old World monkeys and hominoids (apes and humans).
b. New World monkeys reside in South America and Old World monkeys evolved in Africa.
c. New World monkeys (e.g., spider monkey and capuchin) have long prehensile tails and flat noses.
d. Old World Monkeys (e.g., baboon and rhesus monkey) lack prehensile tails and have protruding noses.
e. It is hypothesized that a common ancestor must have arisen earlier than the Oligocene when a narrower Atlantic would have made dispersal possible.
4. Hominoid Evolution
a. About 15 MYA, dozens of hominoid species arose.
b. Proconsul was one species that lived at this time, and it is believed to be the ancestral ape.
c. About 10 MYA, Africa rabia joined with Asia, and the hominoids migrated into Europe.
d. During this period, two ancestral groups of primates were the dryomorphs and the ramamorphs, the latter now believed to be the ancestral orangutan.
e. Dryopithecus was a tree-dweller that moved similar to orangutans but did not walk along tree limbs as did Proconsul.
32.2 Evolution of Early Hominids
1. The designation hominid includes humans and several extinct species related to humans.
2. Fossil and anatomical data indicate ancestors of African apes and the human lineage diverged about 7 MYA.
3. When such changes accumulate at a constant rate, it constitutes a molecular clock to indicate relatedness; these data indicate we diverged about 6 MYA.
B. Comparing Humans to Chimpanzees
1. Humans and chimpanzees have many traits in common.
2. Several distinct differences exist. In humans:
a. The skull is in the midline of the body.
b. The longer curved spine places the center of gravity over the feet.
c. The broader pelvis and hip joints prevent swaying when walking.
d. A longer neck on the femur in humans causes the femur to angle inward at the knees.
e. The human knee joint is modified to support the body's weight.
f. The human toe is not opposable but the foot has an arch for long distance walking.
C. The Early Hominids
1. Until recently, science thought that the climate changed forests into savannas; there is little evidence of a shift in vegetation at 6 MYA.
2. Additional advantages of bipedalism include reduction of heat stroke and carrying food back to females.
3. While still living in trees, the first hominids may have walked upright on two feet (bipedalism) to collect overhead fruit.
4. Early Hominid Fossils
a. The braincase of Sahelanthropus tchadensis has been dated at 7 MYA.
b. Skull fragments from Ardipithecus ramidus have been found; it was likely bipedal.
32.3 Evolution of Later Hominids
1. Australopithecines (genus Australopithecus) evolved in Africa 4 MYA.
2. Expanding fossil records show it is not an orderly sequence between forms.
3. Australopithecines evolved and diversified in Africa with gracile and robust forms with varied diets; they show adaptations to different ways of life.
4. They were apelike above the waist and humanlike below the waist; human characteristics probably did not evolve all together at the same time. This is an example of mosaic evolution.
5. Australopithecus africanus, described by Raymond Dart in the 1920s, is a gracile type from southern Africa.
6. Abundant fossils of A. africanus date about 2.8 MYA.
7. Australopithecus robustus was a robust type; it had a brain size of 500 cc similar to A. africanus and dated from 2 to 1.5 MYA.
8. Both had forelimbs longer than hindlimbs but probably walked upright.
9. Africanus had a larger brain and is the best candidate as ancestor to early Homo.
10. Australopithecus afarensis is based on many skeletal fragments (Lucy) dated at 3.18 MYA.
a. Its brain was small at 400 cc.
b. This may have been ancestral to the robust types, A. aethiopicus and A. boisei, that later died out.
c. They may be the species that left the Laetoli footprints in volcanic ash about 3.7 MYA.
d. This species is thought to have stood upright and walked bipedally.
e. It is possible that Australopithecus afarensis is ancestral to early Homo.
32.4 Evolution of Early Homo
1. Fossils are assigned to the genus Homo based on the following traits:
a. brain size 600 cc or greater;
b. jaw and teeth are human-like; and
c. tool use seems evident.
2. Homo habilis and Homo rudolfensis
a. The oldest fossils to be classified in the genus Homo aredated at around 2 mya.
b. H. habilis and H. rudolfensis had a brain size as large as 8oo cc and smaller cheek teeth; H. rudolfensis was the larger of the two.
c. Cut marks on bones suggest the use of tools to prepare meat and possible scavenging.
d. Tools associated with these two species include flakes used to scrape away hide or remove meat; they were likely omnivores.
e. The skulls indicate that this hominid may have had speech to help in cooperation and sharing.
f. Culture is dependent on the ability to speak and transmit knowledge; it is thought that the advantages of a culture to these hominids may have hastened the extinction of the austropithecines.
3. Homo ergaster and Homo erectus
a. Eugene DuBois, a Dutch anatomist, unearthed the first H. erectus bones in Java in 1891.
b. Fossils found in Africa, Asia, and Europe date between 1.9 and 0.3 million years ago.
c. The African and Asian types may be different species.
d. H. erectus had a brain capacity of 1000 cc, was taller than H. habilis, and had a striding gait.
e. H. erectus fossils found in Java and the Republic of Georgia at 1.9 MYA and 1.6 MYA indicates an early migration from Africa, followed by H. erectus evolving in Asia and spreading to other areas.
f. These are the first hominids to use fire, fashion more advanced tools, to be systematic game hunters, and possibly to use home bases.
g. Fossil remains of Homo floresiensis were discovered in 2004 on the island of Flores in the South Pacific; it was the size of a three-year-old human being but with a braincase only one-third the size.
i. Researchers believe this species evolved from normal sized H. erectus, but underwent "island dwarfing."
32.5 Evolution of Later Homo
• Two contradicting hypotheses are suggested about the origin of modern humans:
1. The multiregional continuity hypothesis proposes that modern humans originated separately in Asia, Europe, and Africa.
a. If valid, then a distinctive continuity in anatomy and genetic variation is expected in each location.
b. Evolution of modern humans would be essentially similar in several different places.
2. The out-of-Africa hypothesis states that modern humans originated only in Africa and after migrating into Europe and Asia, they replaced the archaic Homo species found there; current evidence leans toward this hypothesis.
a. All extant humans are descended from a few individuals from about 100,000 years ago.
b. Mitochondrial DNA analyses indicate a close genetic relationship among all Europeans; although the first analysis was flawed, the data tend to support the out-of-Africa hypothesis.
A. Neandertals
1. Neandertals were named for Neander Valley in Germany where skeletons were dated as early as 200,000 years ago.
2. Neandertals are classified as Homo neandertalensis.
3. Classic Neandertal anatomy includes massive brow ridges; a nose, jaws, and teeth that protruded forward; a low sloping forehead; a lower jaw sloping back without a chin; a longer pubic bone; a slightly larger brain than that of modern humans; shorter and thicker limb bones; and heavier muscles in the shoulder and neck.
4. It is speculated that a larger brain than that of modern humans was required to control the extra musculature.
5. The sturdy build of Neandertals was likely an adaptation to cold climate; they lived in Eurasia during the last Ice Age.
6. The Neandertals give evidence of being culturally advanced.
a. Most lived in caves, but those who lived in the open may have built houses.
b. They manufactured a variety of stone tools, including spear points, scrapers, and knives.
c. They used and could control fire, which probably helped in cooking frozen meat and in keeping warm.
d. They buried their dead with flowers and tools and may have had a religion.
B. Cro-Magnons
1. Cro-Magnons are the olderst fossils to be designated H. sapiens; they were found in Eurasia 100,000 years ago.
2. Cro-Magnons are named for a fossil location in France and had a thoroughly modern appearance.
3. They had advanced stone tools and may have been the first to throw spears.
4. Cro-Magnons hunted cooperatively, and perhaps were the first to have had a language.
5. They may have been responsible for the extinction of large mammals during the late Pleistocene.
6. Cro-Magnon culture included figurines carved out of bone and antler, and cave paintings.
C. Human Variation
1. Some human variation evolved as adaptation to local environmental conditions: darker skin to protect from UV light, lighter skin for vitamin D production, etc.
2. A bulkier body also benefits in colder regions while hot climates favor a slight build and longer limbs.
3. Hair texture, eyelid fold, and other traits are not explained as adaptations.
4. Variation among modern populations is considerably less than among archaic human populations of 250,000 years ago.
5. Comparative studies of mDNA indicate that human populations had a common ancestor no more than a million years ago.
6. The great majority of genetic variation, about 85%, occurs within ethnic groups, not among them.
7. Genetic Evidence for a Common Ancestry
a. The multiregional hypothesis suggests that different human populations came into existence as long as a million years ago, giving time for ethnic differences to accumulate despite gene flow.
b. The out-of-Africa hypothesis suggests that modern humans have a relatively recent common ancestor who evolved in Africa then spread to other regions.
c. Studies with mitochondrial DNA show that differences among human populations are consistent with their having a common ancestor no more than a million years ago..
33.1 Types of Tissues
• A tissue is composed of specialized cells of the same type that perform a common function in the body.
A. Four Major Types of Tissue
1. Epithelial tissue covers body surfaces and lines body cavities.
2. Connective tissue binds and supports body parts.
3. Muscular tissue causes body parts to move.
4. Nervous tissue responds to stimuli and transmits impulses.
B. Epithelial Tissues
1. Epithelial tissue forms a continuous layer over the body surfaces including inner cavities.
2. Epithelial tissue cells are packed tightly; they join to one another in one of three ways:
a. Tight junctions have plasma proteins extending between neighboring cells to bind cells tightly.
b. Adhesion junctions have cytoskeletal elements joining internal plaques in neighboring cells.
c. Gap junctions form when two identical plasma membrane channels of neighboring cells join so that ions and small molecules pass between cells.
3. Epithelial cells are exposed to the environment on one side; the other side is basement membrane, a thin layer of various types of proteins that anchors the epithelium to underlying connective tissue.
4. Simple Epithelia
a. Squamous epithelium is composed of flat cells (e.g., air sac linings of lungs, walls of capillaries).
b. Cuboidal epithelium has cube-shaped cells.
c. Columnar epithelium has elongated cells that resemble pillars or columns (e.g., small intestine).
d. Simple epithelium has one cell layer; all cells contact a basement membrane.
e. Pseudostratified epithelium appears layered, but actually all cells contact the basement membrane.
5. Stratified epithelia is composed of more than one layer of cells.
a. The outer layer of skin is stratified squamous epithelium, but the cells have been reinforced by keratin.
6. Glandular epithelia
a. A gland can be a single epithelial cell or a group of cells that secrete products into the lumen of or onto the lining of a tube or cavity, into blood, or to outside of the body; they are classified in two types:
i. Exocrine glands secrete their products into ducts or directly into a tube or cavity.
ii. Endocrine glands secrete their product directly into the bloodstream.
C. Connective Tissue
1. Connective tissue binds structures together, provides support and protection, fills spaces, stores fat, and forms blood cells.
2. Connective tissue provides source cells for muscle and skeletal cells in animals that regenerate parts.
3. Connective tissue cells are separated widely by a matrix, a noncellular material between cells.
4. Fibrous Connective Tissue
a. Cells of loose and fibrous connective tissues are fibroblasts.
b. Fibroblasts are spaced apart and are separated by a jelly matrix of white collagen fibers and yellow elastic fibers.
c. Collagen fibers provide flexibility and strength; elastic fibers provide elasticity.
d. Loose fibrous connective tissue supports epithelium and provides support, flexibility, and protective covering encasing many internal organs.
e. Dense fibrous connective tissue contains closely packed collagenous fibers; it is found in tendons, which attach muscles to bone, and ligaments, which bind bones to other bones at joints.
f. Adipose Tissue
i. This is loose connective tissue that insulates the body, provides protective padding, and stores fat.
ii. In mammals, adipose tissue is beneath the skin, around the kidneys, and on surface of the heart.
5. Supportive Connective Tissue
a. Cartilage and bone are rigid connective tissues.
b. Structural proteins (cartilage) or calcium salts (bone) are deposited in an intercellular matrix.
c. Cartilage cells or chondrocytes lie in small chambers or lacunae embedded in a strong, flexible matrix.
i. Hyaline cartilage is the most common type of cartilage; it is found in the nose, the ends of long bones and the ribs, the rings of the respiratory passages, and the human fetal skeleton.
ii. Elastic cartilage is more elastic than hyaline cartilage; it is found in the outer ear.
iii. Fibrocartilage has a matrix of strong collagen fibers; it is found in areas that must withstand tension and pressure (e.g., pads between vertebrae, knee joint).
6. Bone
a. In bone, a matrix of calcium salts is deposited around protein fibers.
b. Calcium salts give bone rigidity while protein fibers provide elasticity and strength.
c. Compact bone has cells called osteocytes that lie within lacunae arranged in concentric circles within osteons (Haversian systems) around tiny tubes called central canals.
d. These canals contain nerve fibers and blood vessels.
e. Nutrients brought by the blood reach all of the cells via minute canals (canaliculi) containing thin processes of osteocytes that connect them with one another and with the central canals.
f. Spongy bone at end of long bones is designed for strength, and has many long bony bars and plates.
7. FluidConnectiveTissue
a. Blood transports nutrients and oxygen to cells and removes CO2 and wastes; blood also has a role in fluid, ion and pH balance and distributes heat.
b. Blood is a connective tissue with cells separated by liquid plasma.
c. In vertebrates, the blood cells are mainly of two types.
i. Red blood cells (erythrocytes) carry oxygen.
ii. White blood cells (leukocytes) aid in fighting infection.
d. Platelets present in plasma are fragments of giant cells found in bone marrow and play a role in blood clotting.
e. Lymph is a fluid connective tissue located in lymphatic vessels.
i. Lymphatic vessels absorb excess tissue fluid and transport it to vessels of the cardiovascular system.
ii. Lacteals, special lymphatic capillaries, absorb fat molecules from the small intestine.
iii. White blood cells congegrate in the lymph nodes; lymph is cleansed as it passes through.
D. Muscular Tissue
1. Muscular (contractile) tissue is composed of cells called muscle fibers.
2. Muscle fibers contain actin and myosin filaments; interactions result in animal movement.
3. The three types of vertebrate muscle tissue are skeletal, cardiac, and smooth muscle.
4. Skeletal muscle attaches by tendons to the bones of the skeleton.
a. Skeletal muscle moves body parts, is under voluntary control, and contracts faster than other types.
b. Skeletal muscle fibers are long, cylindrical, multinucleate cells arising from the fusion of several cells.
c. Skeletal fibers are striated due to the light and dark bands of overlapping actin and myosin filaments.
5. Smooth (visceral) muscle is not striated.
a. Spindle-shaped fibers form layers with the thick middle portion of one fiber opposite the thin ends of adjacent fibers.
b. The nuclei form an irregular pattern in the tissue.
c. Smooth muscle is not under voluntary control; it is therefore involuntary.
d. Smooth muscle is found in the walls of viscera (e.g., intestine, stomach, etc.) and blood vessels.
e. Smooth muscles drive the intestinal contractions and blood vessel constrictions.
6. Cardiac muscle is found only in the heart wall and powers the heartbeat that pumps blood.
a. Cardiac muscle combines the features of both smooth and skeletal muscle.
b. Unlike skeletal muscles with many nuclei, cardiac muscles have one centrally placed nucleus.
c. Although it appears to be one mass of muscle fibers, the cardiac muscle fibers are individual cells.
d. Cardiac muscle cells are bound end-to-end at intercalated disks where the folded membranes between two fibers contain desmosomes and gap junctions
e. Impulses move from cell to cell so the heartbeat is coordinated.
E. Nervous Tissue
1. Nervous tissue contains neurons in the brain, spinal cord, and nerves.
2. A neuron has three parts.
a. Dendrites receive a stimulus and conduct signals to the cell body.
b. The cell body contains most of the cytoplasm and the nucleus of the neuron.
c. The axon conducts nerve impulses away from the cell body; long axons are covered by myelin.
d. Long axons and dendrites form neuron fibers; bound together by connective tissue, they form nerves.
e. Outside the brain and spinal cord, fibers bound by connective tissue form nerves.
3. Neuroglia
a. There are several types of neuroglia in the central nervous system.
b. Neuroglia outnumber neurons 50 to 1, and were once thought to only support or nourish neurons.
c. Microglial cells support neurons and also phagocytize bacterial and cellular debris.
d. Astrocytes provide nutrients and produce a growth factor known as glia-derived growth factor that someday may be used to cure diseases of neural degeneration.
e. Oligodendrocytes form the myelin around an axon.
f. Neuroglia lack long processes but communicate among themselves and with neurons.
33.2 Organs and Organ Systems
Organs are combinations of two or more different tissues performing common functions.
1. An organ system contains many different organs that cooperate to carry out a process (e.g., digestion).
2. The integumentary system is composed of the skin and accessory organs (i.e., nails, hair, glands, and sensory receptors).
B. Skin as an Organ
1. Human skin protects the underlying tissues from trauma, desiccation, radiation damage, and microbial invasion.
2. The skin produces a precursor molecule that is converted to vitamin D after exposure to UV light.
3. The skin also helps regulate body temperature.
4. Laden with sensory receptors, the skin collects information about the external environment.
C. Regions of Skin
1. The skin has both an outer epidermal layer (epidermis) and a deeper layer (dermis); a subcutaneous layer (hypodermis) is found between the skin and underlying structures.
2. The epidermis is the outer, thinner layer of skin.
a. The epidermis is composed of stratified squamous epithelium.
b. Epidermal cells are derived from the basal layer of stem cells that undergo continuous cell division underneath.
c. The newly formed cells push to the surface away from their blood supply; they flatten and harden as they accumulate keratin, a hard, waterproof protein.
d. Eventually, the keratinized cells die and are sloughed off.
e. Melanocytes located in the basal layer produce a melanin pigment that absorbs UV light, protecting deeper cells from radiation damage; certain cells in the epidermis convert a steroid related to cholesterol into vitamin D, a chemical required for proper bone growth.
f. Skin Cancer
i. Too much ultraviolet radiation is dangerous and can lead to skin cancer.
ii. Excessive exposure to UV radiation can convert cells in the basal layer of the epidermis into cancer cells (basal cell carcinoma); melanoma is skin cancer derived from melanocytes.
3. The dermis is fibrous connective tissue that forms a thicker and deeper layer of skin.
a. The dermis contains both elastic fibers and collagen fibers; these run parallel with the skin surface.
b. The dermis contains blood vessels that can constrict and dilate.
c. Many small sensory receptors are present in the dermis.
i. There are separate receptors for pressure, touch, temperature, and pain.
4. The subcutaneous layer is not technically a part of the skin; it is composed of loose connnective tissue and adipose cells, which store fat.
a. The subcutaneous layer lies below dermis.
b. This is composed of loose connective tissue, including adipose tissue.
c. Adipose tissue helps insulate the body by minimizing both heat gain and heat loss.
d. This layer of adipose gives a rounded appearance to the body.
e. The excessive development of adipose tissue occurs with obesity.
D. Accessory Organs of the Skin
1. Nails grow from special epidermal cells at the base of the nail in a region called the nail root.
a. The visible portion of a nail is the nail body.
b. Cells become keratinized as they grow out over the nail bed.
c. The vascular dermal tissue under the nail provides the pink color; the white half-moon area is the thicker germinal area.
2. A hair follicle contains a nonliving hair shaft and the living hair root that produced it.
a. The hair shaft is formed of dead, keratinized epidermal cells that protect the surface of the skin.
b. The arrector pili muscle is a smooth muscle attached to the hair follicle; contracting it causes the hair to erect.
c. Follicles have oil (sebaceous) glands producing sebum, an oil secreted to lubricate both the hair and the skin.
3. The sweat (sudoriferous) glands are coiled tubules present in most of the regions of skin that secrete a fluid (sweat) onto the surface of the skin.
E. Organ Systems
o In most animals, individual organs function as part of organ systems.
o The organ systems carry out life processes common to organisms.
3. Body Cavities
a. The human body has two main cavities: the dorsal cavity holds the brain and spinal cord, and the larger ventral cavity.
b. The ventral cavity located on the front side of the body develops from the coelom and is divided by a muscular diaphragm in humans and other mammals.
c. The upper (thoracic or chest) cavity is located in the upper part of the ventral cavity, above the diaphragm, and contains the heart and lungs.
d. The lower (abdominal) cavity is located in the lower part of the ventral cavity; it contains the major portions of the digestive and excretory systems, and much of the reproductive system.
33.3 Homeostasis
• The cells of the body live in an internal environment, tissue fluid that bathes the cells of an animal's body.
1. This concept was first proposed by Claude Bernard, a famous French physiologist in 1859.
2. The internal environment (e.g., composition and temperature) must stay within normal range.
3. This relative internal stability allows animals to tolerate considerable external variation.
4. The American physiologist Walter Cannon first used the term "homeostasis."
• Homeostasis is the maintenance of internal conditions in a cell or organism by means of self-regulating mechanisms that curtail fluctuations above and below a normal range.
1. The organ systems of the human body contribute to homeostasis.
a. The respiratory system adds oxygen and removes carbon dioxide; the amounts are altered to meet needs.
b. The liver removes and stores glucose as glycogen and then replaces the blood glucose levels when they lower.
c. The hormone insulin is secreted by the pancreas to regulate glucose levels.
d. The kidneys are under hormonal control to excrete wastes and salts and to maintain blood pH.
2. Although homeostasis is controlled by hormones, it is ultimately controlled by the nervous system.
3. The brain contains centers that regulate temperature and blood pressure.
4. Regulation requires a receptor that detects unacceptable levels and signals a regulator center that can direct an adaptive response; once normalcy is obtained, the receptor is no longer stimulated.
A. Negative Feedback
1. A negative feedback mechanism involves a response in which a variable is kept close to a particular set point.
a. The process involves a sensor and a control center.
b. The sensor detects a change in the internal environment.
c. The control center brings about an effect to bring conditions back to normal.
d. Example: the pancreas detects that the blood glucose level is too high; it secretes insulin which causes cells to take up glucose; blood glucose level returns to normal.
e. A home heating system is a mechanical example of a negative feedback mechanism.
f. HumanExample: Regulation of Body Temperature
i. The sensor and control center are located in the hypothalamus.
ii. When body temperature is above normal, the control center directs blood vessels in the skin to dilate—heat is lost to the environment.
iii. When body temperature is below normal, the the control center directs blood vessels in the skin to constrict—heat is conserved in the body.
B. Positive Feedback
1. A positive feedback mechanism involves output that intensifies and increases the input, thereby increasing the process; an ever-greater change in the same direction occurs.
2. Once childbirth begins, each event amplifies; the process continues until birth occurs.
3. Positive feedback mechanisms can be harmful, e.g., when a fever causes metobolic changes that push the fever even higher.
34.1 Transport in Invertebrates
1. Unicellular protozoa exchange directly with the environment across the plasma membrane.
2. Some multicellular animals lack an internal transport system.
3. The larger invertebrates usually have circulatory systems—either an open system or a closed system.
B. Invertebrates Without a Circulatory System
1. Sea anemones and planaria are organisms with a saclike body plan that makes a circulatory system unnecessary.
2. Sea anemone cells are part of an external layer or gastrovascular cavity and diffusion supplies all the nutrients.
3. Planaria have a trilobed gastrovascular cavity and a small, flat body where nutrients diffuse from cell to cell.
4. Pseudocoelomates, such as nematodes, use the coelomic fluid of the body cavity to transport fluids.
5. Echinoderms rely on movement of coelomic fluid as a circulatory system.
C. Invertebrates with an Open or a Closed System Circulatory System
1. In a circulatory system, a pumping heart moves one of two types of circulatory fluids.
a. Blood is a circulatory fluid and is always contained within blood vessels
b. Hemolymph is a circulatory fluid which flows into the hemocoel of certain arthropods and molluscs; it is a mixture of blood and interstitial fluid.
2. Certain arthropods and molluscs have an open circulatory system.
a. Hemolymph is pumped by the heart into the body cavity or saclike sinuses.
b. Hemolymph bathes the internal organs and then drains back to the heart.
c. In grasshoppers, a dorsal heart pumps hemolymph into an aorta, which empties into the hemocoel.
d. Hemolymph is colorless (it lacks hemoglobin or other respiratory pigments); a system of tracheae provides oxygen.
3. Some invertebrates, including earthworms and cephalopods, have a closed circulatory system in which blood never leaves the heart or vessels.
a. Valves prevent any backward flow of the blood as it moves through vessels.
b. Earthworms have five pairs of anterior lateral vessels that pump blood to every segment.
c. Blood moves in capillaries where an exchange with tissue fluid takes place before returning in veins.
d. Earthworms have a red respiratory pigment hemoglobin dissolved in the blood, not inside blood cells.
e. With no special cavity for gas exchange, the gas must diffuse across a moist body wall.
34.2 Transport in Vertebrates
1. Vertebrates have a closed circulatory system called a cardiovascular system.
2. The muscular heart keeps blood circulating through the blood vessels.
a. The atria are the chambers of the heart that receive blood.
b. The ventricles pump blood into arteries.
3. There are three kinds of blood vessels: arteries carry the blood away from the heart, capillaries are where the exchange with tissue fluid takes place, and veins return the blood to the heart.
a. Arteries
i. have thick walls and are resilient.
ii. expand to accommodate sudden increase in blood volume that results after heart contraction.
iii. divide into small arterioles.
b. Arteriole constriction and dilation is regulated by the nervous system to affect blood pressure.
c. Capillaries are microscopic blood vessels with a wall formed of one layer of simple squamous cells.
i. Capillary beds are so prevalent that, in humans, all cells are within 60–80 μm of a capillary.
ii. Only 5% of the capillaries are open at one time; after an animal has eaten, the capillary beds of the digestive system open.
iii. Capillaries are so narrow that red blood cells must pass through them in single file.
iv. Gas, nutrient, and waste exchange occurs across the thin capillary walls.
d. The venules are vessels that take blood from capillaries and join to form a vein.
e. Veins transport blood toward the heart.
i. The walls of a veins are much thinner than those of arteries; there is a lower blood pressure in veins than in arteries.
ii. One-way valves open in the direction of the heart, then close to prevent backflow.
B. Comparison of Circulatory Pathways
1. In vertebrates, there are three different types of circulatory pathways.
2. Fishes have a one-circuit (single-loop) circulatory pathway.
a. The heart has a single atrium and a single ventricle and pumps the blood under pressure to the gills.
b. Blood is oxygenated in the gills.
c. After passing through gills, blood is returned to the dorsal aorta, which distributes the blood throughout the body.
3. Other vertebrates have a two-circuit (double-loop) circulatory pathway to breathe air on land.
a. The systemic circuit transports the blood to tissues.
b. The pulmonary circuit pumps the blood to lungs.
4. In amphibians and most reptiles, the heart has two atria and a single ventricle.
5. In most reptiles, and in all birds and mammals, the heart is divided into left and right halves.
a. With two atria and two ventricles, the oxygenated blood is always separate from the deoxygenated blood.
b. The right ventricle pumps blood to the lungs; the left ventricle pumps blood to the rest of the body.
c. This arrangement provides adequate blood pressure for both the pulmonary and the systemic circulations.
34.3 Transport in Humans
A. The Human Heart
1. The pumping of the heart keeps the blood moving in arteries.
2. Skeletal muscle contraction is responsible for the blood movement in veins.
3. The heart is a cone-shaped, muscular organ about the size of a fist.
4. It is located between the lungs directly behind the sternum and is tilted so that the apex is oriented to the left.
5. The myocardium is a major portion of the heart consisting mostly of cardiac muscle; its muscle fibers are branched and tightly joined together.
6. The heart lies within the pericardium, a sac that secretes a lubricating fluid.
7. The endocardium lines the inner surface of the heart; it consists of connective tissue and endothelial tissue.
8. An internal wall called the septum separates the heart into right and left halves.
9. The heart has two upper, thin-walled atria and two lower, thick-walled ventricles.
10. Heart valves direct the flow of blood and prevent any backward movement.
a. Valves are supported by strong fibrous tendons (chordae tendineae) which support the valves and prevent them from inverting when the heart contracts.
b. Atrioventricular valves between the atria and ventricles prevent any back flow from the ventricle to the atrium.
c. The right atrioventricular (tricuspid) valve on right side of the heart consists of three cusps or flaps.
d. The left atrioventricular (bicuspid or mitral) valve on left side consists of two cusps or flaps.
e. Semilunar valves are located between the ventricles and their attached vessels.
i. The pulmonary semilunar valve lies between the right ventricle and the pulmonary trunk.
ii. The aortic semilunar valve lies between the left ventricle and the aorta.
B. Path of Blood Through the Heart
1. The route of blood through the heart is as follows:
a. Oxygen-poor blood enters the right atrium from both the superior vena cava and the inferior vena cava.
b. The right atrium sends blood through the right atrioventricular (tricuspid) valve to the right ventricle.
c. The right ventricle sends blood through the pulmonary semilunar valve into the pulmonary trunk and arteries to the lungs.
d. Oxygen-rich blood returns from the lungs through pulmonary veins and is delivered to the left atrium.
e. The left atrium sends blood through the left atrioventricular (bicuspid or mitral) valve to the left ventricle.
f. The left ventricle sends blood through the aortic semilunar valve into the aorta and on to the body proper.
2. The heart is therefore a double pump serving the lungs and body circulations simultaneously; O2-poor blood and O2-rich blood never mix.
3. Since the left ventricle has the harder job of pumping blood throughout the body, its walls are thicker; accordingly, blood pressure is greatest in the aorta.
4. Blood pressure decreases as the cross-sectional area of the arteries and arterioles increases.
C. The Heartbeat
1. The human heart contracts (beats) about 70 times a minute (2.5 billion times in a lifetime); each heartbeat lasts about 0.85 seconds.
2. The heartbeat or cardiac cycle consists of phases.
3. The atria contract first while the ventricles relax (0.15 sec.), then the ventricles contract while atria relax (0.30 sec.), and then all chambers rest (0.40 sec.).
4. Systole refers to the contraction of heart chambers and diastole refers to the relaxation of the heart chambers.
5. The heart is in diastole about 50% of the time.
6. The short systole of the atria is needed only to send blood into the ventricles.
7. When the term "systole" is used alone, it refers to the left ventricle systole; the volume of blood that the left ventricle pumps per minute into the systemic circuit is called the cardiac output.
8. When the heart beats, the familiar lub-dub sound is heard as the valves of the heart close.
a. Lub is caused by the vibrations of the heart when the atrioventricular valves close.
b. Dub is heard when the vibrations occur due to the closing of semilunar valves.
9. The pulse is a wave effect that passes down the walls of arterial blood vessels when the aorta expands and then recoils following ventricular systole.
10. Since there is one arterial pulse per ventricular systole, the arterial pulse rate can be used to determine the heart rate.
11. Rhythmic contraction of the heart is due to the cardiac conduction system.
a. The sinoatrial (SA) node is the "pacemaker" found in the upper dorsal wall of the right atrium; it initiates the heartbeat by sending out an excitatory impulse every 0.85 seconds to cause the atria to contract.
b. The atrioventricular (AV) node is found in the base of the right atrium very near the septum; when stimulated by impulses from the SA node, it sends out impulses through the septum to cause the ventricles to contract.
c. Although the beat of the heart is intrinsic, it is regulated by the nervous system which can increase or decrease the heartbeat rate.
d. The SA node is called the cardiac pacemaker because it usually keeps the heartbeat regular.
12. An electrocardiogram (ECG) is a recording of the electrical changes that occur in the myocardium during a cardiac cycle; it is used as a diagnostic tool to identify abnormal cardiac function.
13. Normal Cardiac Cycle
a. The P wave represents excitation and occurs just before atrial contraction.
b. The QRS complex signals that the ventricles are about to contract.
c. The electrical changes that occur as the ventricular muscle fibers recover produce the T wave.
14. Ventricular fibrillation is uncoordinated contraction of the ventricles; with the application of a strong electric current, the SA node may reestablish a coordinated beat.
D. Vascular Pathways
o The human cardiovascular system has two major circular pathways.
2. The Pulmonary Circuit
a. The pulmonary circuit circulates blood to the lungs.
b. Oxygen-poor blood from the body collects in the right ventricle, which pumps it to pulmonary trunk.
c. The pulmonary trunk divides into right and left pulmonary arteries to carry blood to each lung.
d. In the lungs, carbon dioxide (CO2) is unloaded and O2 is picked up by blood.
e. Oxygen-rich blood from the lungs is returned through pulmonary veins to the left atrium.
3. The Systemic Circuit
a. The aorta and vena cavae are main pathways for blood in the systemic circuit.
b. Transport of oxygenated blood moves from the left ventricle through the aorta out to all tissues.
c. Deoxygenated blood returns from all tissues via the vena cavae.
d. In a systemic circuit, arteries contain bright red oxygen-rich blood; the veins contain dull red oxygen-poor blood that appears blue when viewed through the skin.
4. The coronary arteries serve the heart muscle itself.
a. Coronary arteries originate from the base of the aorta just above the aortic semilunar valve.
b. Coronary arteries lie on the external surface of the heart; they branch into arterioles and capillaries.
c. Capillary beds enter the venules that join to form the cardiac veins.
d. Coronary veins collect oxygen-poor blood from the capillaries and empty it into the right atrium.
5. The portal system is a pathway of blood flow that begins and ends in capillaries.
a. The hepatic portal vein transports blood from capillaries in the small intestinal villi to capillaries in the liver.
b. The hepatic vein leaves the liver and enters the inferior vena cava.
c. In the liver, substances absorbed by the intestine are modified, toxins and bacteria are removed, and the normal composition of blood is monitored.
E. Blood Pressure
0. Systolic pressure results from blood being forced into the arteries during ventricular systole.
1. Diastolic pressure is the pressure in arteries during ventricular diastole.
2. Human blood pressure is measured as the force pushing against the wall of the brachial artery of the upper arm.
a. Blood pressure is measured by a sphygmomanometer which has a pressure cuff.
b. Clinical blood pressure measures pressures produced by contraction and relaxation of the left ventricle.
c. Blood pressure is stated in millimeters of mercury (e.g., 120/80 mm Hg) for systolic/diastolic.
3. As blood flows from the aorta into arteries and arterioles, the blood pressure falls.
4. The difference in pressure between systolic and diastolic pressures gradually diminishes.
5. Capillaries have a slow, even blood flow due to the high total cross-sectional area.
6. Blood pressure in the veins is low and cannot move blood back to heart, especially from the limbs.
7. Skeletal muscle contraction on the walls of veins with valves, preventing backflow of blood, is responsible for the flow of blood in veins.
8. Varicose veins are abnormal dilations that develop when the valves become weak and ineffective.
34.4 Cardiovascular Disorders
1. Cardiovascular disease (CVD) is the leading cause of untimely death in Western countries.
2. The risk of CVD can be reduced by following guidelines for a heart-healthy life-style.
B. Hypertension
1. An estimated 20% of Americans suffer from hypertension (high blood pressure).
2. Women have this condition if their blood pressure is significantly higher than 160/95; men under the age of 45 if over 130/90, and beyond the age of 45 if above 140/95.
3. The diastolic pressure is what is emphasized when medical treatment is considered.
4. Hypertension may not be detected until a stroke or heart attack occurs; for this reason, hypertension is often called a silent killer.
5. Two genes are involved in hypertension for some individuals.
a. One gene codes for angiotensinogen, a plasma protein converted to a vasoconstrictor by the product of a second gene.
b. Persons with this form of hypertension may one day be cured by gene therapy.
C. Atherosclerosis
1. Hypertension is seen in individuals with atherosclerosis (formerly called arteriosclerosis).
2. Soft masses of fatty materials, mostly cholesterol, accumulate beneath the inner linings of arteries.
3. As this plaque accumulates, it protrudes into the vessel and interferes with blood flow.
4. Atherosclerosis develops in early adulthood but the symptoms may not appear until age 50 or older.
5. Plaque can cause a blood clot to form on irregular arterial walls.
6. As long as a clot remains stationary, it is a thrombus.
7. If a clot dislodges, it is an embolus, a blood clot that moves in the blood.
8. In some families, atherosclerosis is inherited as familial hypercholesterolemia.
D. Stroke and Heart Attack
1. Stroke, heart attack, and aneurysm are associated with hypertension and atherosclerosis.
2. A stroke can result in paralysis or death; a small cranial arteriole bursts or is blocked by an embolus.
a. A stroke is also called a cardiovascular accident (CVA).
b. Whether paralysis or death occurs depends on the extent of the portion of the brain that lacks O2.
c. Warning symptoms that foretell stroke include: numbness in hands or face, difficulty speaking, blindness in one eye, etc.
3. A myocardial infarction (MI) is also called heart attack.
a. This occurs when a portion of heart muscle dies due to a lack of O2; this may be caused by a thromboembolism blocking a coronary artery.
b. A partially blocked coronary artery causes angina pectoris causing pains or a flash of burning.
c. Nitroglycerin and related drugs dilate the blood vessels and relieve pain.
34.5 Blood, a Transport Medium
• The blood of mammals has two components: plasma and formed elements (cells and platelets).
1. Plasma contains water and many types of molecules, including nutrients, wastes, salts, and proteins.
2. Salts and proteins buffer the blood.
a. They effectively keep the blood pH near 7.4.
b. They maintain the blood osmotic pressure so water has a tendency to enter capillaries.
3. Some plasma proteins are involved in blood clotting.
4. Some plasma proteins assist in transporting large organic molecules in the blood.
a. Lipoproteins that transport cholesterol are globulins.
b. Albumin, a common plasma protein, transports bilirubin, a breakdown product of hemoglobin.
A. Formed Elements
1. Formed elements are of three types: red blood cells (RBCs), white blood cells (WBCs), and platelets.
2. Red Blood Cells
a. Red blood cells (erythrocytes) are small biconcave disks.
b. When mature, RBCs lack a nucleus and contain hemoglobin.
c. There are 6 million RBCs per mm3 of whole blood.
d. Each RBC contains about 250 million hemoglobin molecules.
i. Hemoglobin contains four globin protein chains, each with an iron-containing heme group.
ii. The iron atom of a heme group loosely binds with an O2 molecule; thus, blood carries oxygen.
iii. Anemia is either a lack of enough RBC or insufficient hemoglobin; an individual suffers from a tired, run-down feeling.
e. RBCs are manufactured in the red bone marrow of the skull, ribs, vertebrae, and the ends of long bones.
f. The growth factor erythropoietin is produced when an enzyme from the kidneys acts on a precursor made by the liver and stimulates production of red blood cells; as a drug it helps people with anemia.
g. Before being released from bone marrow, the RBCs lose their nucleus and synthesize hemoglobin.
h. Red blood cells have a life span of about 120 days; then they are destroyed chiefly in the liver and spleen.
i. When the RBCs are destroyed, the hemoglobin is released; the iron is recovered and returned to the bone marrow where it is reused.
j. The heme portions undergo chemical degradation and are excreted by the liver as bile pigments; it colors the feces.
3. White Blood Cells
a. White blood cells (leukocytes) differ from RBCs in being larger and in having a nucleus.
b. WBCs lack hemoglobin and appear translucent without staining.
c. Granular leukocytes contain conspicuous granules in their cytoplasm and have a lobed nucleus.
i. Neutrophils have granules that stain slightly pink; they are amoeboid, spherical cells that readily squeeze through capillary walls and phagocytize foreign material.
ii. Eosinophils have granules that take up the red dye eosin.
iii. Basophils have granules that take up a basic dye, staining them deep blue.
d. A newly discovered stem cell growth factor (SGF) increases the production of all WBCs, which helps patients with low immunity.
e. Agranular leukocytes lack granules in their cytoplasm and have a circular or indented nucleus.
i. Monocytes are amoeboid and able to enter tissues where they transform into macrophages.
ii. Macrophages release white blood cell growth factors that increase the number of leukocytes.
iii. Pus is a thick, yellowish fluid that contains a large proportion of dead WBCs that have fought infection.
iv. Lymphocytes play a key role in fighting infection and include two types.
1. T cells are lymphocytes that directly attack virus-infected cells.
2. B cells can be stimulated to produce one type of antibody specific for one type of antigen.
v. An antigen is any substance stimulating production of antibodies; antigen is foreign to the body.
vi. Antibodies combine with antigens to promote their being phagocytized by a macrophage.
vii. A person is actively immune when many B cells produce a specific antibody for an infection.
4. Platelets
a. Platelets (thrombocytes) result from fragmented giant cells (megakaryocytes) in the bone marrow.
b. 200 billion platelets are produced a day; blood contains 150,000–300,000 platelets per mm3.
c. Platelets are involved in blood clotting, or coagulation.
d. At least 12 clotting factors in the blood participate in blood clotting.
e. Hemophilia is an inherited disorder where the liver is unable to produce one of the clotting factors.
f. In hemophilia, minor bumps can cause internal bleeding; bleeding into the brain causes death.
g. Vitamin K is necessary to produce prothrombin; deficiency of vitamin K causes hemorrhagic disorders.
5. Blood Clotting
a. When a blood vessel is damaged, platelets clump at the site of the puncture and partially seal the leak.
b. The platelets and damaged tissue cells release a clotting factor called prothrombin activator.
c. With calcium ions, prothrombin activator catalyzes a reaction converting prothrombin to thrombin.
d. Thrombin acts as an enzyme to sever two amino acid chains from each fibrinogen molecule.
e. These activated fragments join end-to-end forming long threads of fibrin.
f. Fibrin threads wind around the platelet plug and provide a framework for a clot.
g. RBCs are trapped within the fibrin threads, making the clot appear red.
h. When blood vessel repair is initiated, plasmin destroys the fibrin network and restores plasma fluidity.
i. When clotting occurs in a test tube, a fluid serum collects above a clot; it has the same composition as plasma except fibrinogen.
B. Capillary Exchange
1. Two forces control the movement of fluid through the capillary walls.
a. Osmotic pressure tends to cause water to move from tissue fluid to the blood.
b. Blood pressure tends to cause water to move from the blood to tissues.
c. At the arterial end of a capillary, blood pressure is higher than osmotic pressure: water exits and moves into tissues.
d. Along the capillary, O2 and nutrients diffuse out into the tissue fluid, while CO2 and other metabolic wastes diffuse into the capillaries from the tissue fluid.
2. Midway along a capillary, there is no net movement of water.
3. The tissue fluid is intercellular fluid that surrounds the cells; the circulatory system exchanges materials with this fluid.
4. The exchange between the blood and tissue fluid occurs by diffusion through the one-cell-thick capillary walls.
a. At the venule end, osmotic pressure is higher than blood pressure and water moves back into the blood.
b. Almost the same amount of fluid that left the capillary returns to it; there is always some excess tissue fluid collected by the lymphatic capillaries.
5. The tissue fluid within lymphatic vessels is lymph.
6. Lymph returns to the systemic venous blood when lymphatic vessels enter the subclavian veins in the shoulder.
7. Not all capillary beds are open at the same time; precapillary sphincters shunt blood along various pathways.
8. Through capillary dilation and constriction, blood also distributes heat to body parts and conserves heat when cold.
35.1 Lymphatic System
1. The mammalian lymphatic system consists of lymphatic vessels and lymphoid organs.
2. This system is closely associated with the cardiovascular system and has three main functions.
a. Lymphatic vessels absorb excess tissue fluid and return it to the bloodstream.
b. Lacteals receive lipoproteins at the intestinal villi and the lymphatic vessels transport these fats to the bloodstream.
c. The lymphatic system is responsible for the production, maintenance, and distribution of lymphocytes.
d. The lymphatic system helps defend the body against disease.
B. Lymphatic Vessels
1. Lymphatic vessels are extensive; most regions have lymphatic capillaries.
2. The structure of the larger lymphatic vessels resembles veins, including the presence of valves.
3. The movement of fluid is dependent upon skeletal muscle contraction; when the muscles contract, fluid is squeezed past a valve that closes, preventing it from flowing backwards.
4. The lymphatic system is a one-way system that begins with lymphatic capillaries.
a. They take up fluid that has diffused out of the blood capillaries and has not been reabsorbed.
b. If excess tissue fluid is produced or not absorbed, it will accumulates and result in edema.
c. Edema is the swelling caused by buildup of fluid from excessive production or inadequate drainage.
5. Once tissue fluid enters the lymphatic capillaries, it is called lymph.
6. Lymphatic capillaries join as lymphatic vessels that merge before entering one of two ducts.
a. The thoracic duct is larger than the right lymphatic duct.
i. It serves the lower extremities, abdomen, left arm, left side of the head and neck, and the left thoracic region.
ii. It then delivers lymph to the left subclavian vein of the cardiovascular system.
b. The right lymphatic duct is smaller.
i. It serves the right arm, the right side of the head and neck, and the right thoracic region.
ii. It then delivers lymph to the right subclavian vein of the cardiovascular system.
C. Lymphoid Organs
1. The lymphatic (lymphoid) organs contain large numbers of lymphocytes: B cells and T cells.
2. Primary Lymphatic Organs
a. The red bone marrow is the origin for all blood cells including all leukocytes that function in immunity.
i. Stem cells are continually being produced; these cells differentiate into the various blood cells.
ii. Most bones of a child have red bone marrow but in adults, red bone marrow is only in the skull, sternum, ribs, clavicle, pelvic bones and vertebral column.
iii. Red bone marrow consists of reticular fibers produced by reticular cells packed around thin-walled sinuses.
iv. Differentiated blood cells enter the bloodstream at these bone sinuses.
v. B cells also mature in the thymus, whereas T cells mature in the thymus.
b. The thymus gland is located along the trachea behind the sternum in the upper thoracic cavity.
i. The thymus gland is larger in children than in adults and may disappear completely in old age.
ii. It is divided into lobules by connective tissue; lobules are the site of T lymphocyte maturation.
iii. The interior (medulla) of each lobule consists mostly of epithelial cells which produce thymic hormones (e.g., thymosin) that aid maturation of T lymphocytes.
3. Secondary Lymphatic Organs
a. The spleen is located in the upper left abdominal cavity just below the diaphragm.
i. The spleen is similar to a lymph node but it is much larger, about the size of a fist.
ii. Instead of cleansing the lymph, the spleen cleanses the blood.
iii. A capsule divides the spleen into lobules which contain sinuses filled with blood.
iv. Red pulp consists of blood vessels and sinuses where macrophages remove old and defective blood cells; lymphocytes cleanse the blood of foreign particles.
v. White pulp consists of little lumps of lymphatic tissue.
vi. If the spleen ruptures due to injury, it can be removed; its functions are assumed by other organs.
vii. However, a person without a spleen is more susceptible to infections and may require antibiotic therapy.
b. Lymph nodes are small (about 1–25 mm) ovoid structures located along lymphatic vessels.
i. A lymph node contains nodules, each packed with B lymphocytes and contain a sinus.
ii. Lymph is filtered as it moves through the sinuses and macrophages engulf pathogens (disease-causing agents such as bacteria and viruses); T lymphocytes fight infections and attack cancer cells.
iii. Lymph nodes cluster in certain regions of the body and are named accordingly: inunal nodes in the groin and axillary nodes in the armpits.
4. Other Lymphatic Organs
a. The tonsils are patches of lymphatic tissue located in a ring around the pharynx.
i. The tonsils perform the same function as lymph nodes, and are the first to encounter pathogens and antigens that enter the body by way of the nose and mouth.
b. Peyer's patches are located in the intestinal wall and the vermiform appendix is attached to the cecum—both encounter pathogens that enter the body by way of the intestinal tract.
35.2 Nonspecific and Specific Defenses
• Immunity is the ability to repel infectious agents, foreign cells, and cancer cells.
1. Immunity includes both nonspecific and specific defenses.
2. The four nonspecific defenses include barrier to entry, inflammatory reaction, natural killer cells, and protective proteins.
A. Nonspecific Defenses
1. Barriers to Entry
a. Skin and the mucous membranes lining the respiratory, digestive, and urinary tracts are mechanical barriers.
b. Oil gland secretions inhibit the growth of bacteria on the skin.
c. Ciliated cells lining the upper respiratory tract sweep mucous and particles up into the throat to be swallowed.
d. The stomach has a low pH (1.2–3.0) that inhibits the growth of many bacteria.
e. The normal harmless bacteria that reside in the intestine or vagina prevent pathogens from colonizing.
2. Inflammatory Reaction
a. If tissue is damaged, a series of events known as the inflammatory response, occurs.
b. The inflamed area has four symptoms: redness, pain, swelling, and heat.
c. Chemical signals, e.g. histamine, and mast cells, a type of white blood cell, cause vasodilation and increased permeability of capillaries.
d. Enlarged capillaries produce redness and a local increase in temperature.
e. The swollen area stimulates free nerve endings, causing pain.
f. Neutrophils and monocytes migrate by amoeboid movement to the site of the injury; they escape from the blood by squeezing through the capillary wall.
g. Dendritic cells and macrophages recognize the presence of pathogens and respond by releasing cytokines.
h. The cytokines stimulate other immune cells.
i. Neutrophils, dendritic cells, and macrophages engulf pathogens.
j. As phagocytic cells die, they, along with dead bacteria, dead tissue cells, and living white blood cells, form pus.
k. Dendritic cells and macrophages move to the lymph nodes and spleen, where they activate B and T lymphocytes.
l. An inflammatory response may also involve the production of a fever, which serves to inhibit the growth of microorganisms and stimulates immune cells.
3. Chronic inflammation, one that persists for weeks or longer, is thought to play a role in many human ailments, including Alzheimer's, diabetes, and various autoimmune diseases.
4. Protective Proteins
a. Complement is composed of a number of plasma proteins designated by the letter C and a subscript.
b. It "complements" certain immune responses, which accounts for its name.
c. It amplifies an inflammatory reaction by triggering histamine release and by attracting phagocytic cells to the site of infection.
d. Some complement binds to antibodies already on the surface of pathogens, thereby increasing the probability that pathogens will be phagocytized by a neutrophil or macrophage.
e. Some complement proteins form a membrane attack complex that produces holes in bacterial cell walls and plasma membranes; fluids and salts then enter to the point where the cell bursts.
f. Interferons are proteins produced by virus-infected animal cells.
5. Natural Killer Cells
a. Natural killer (NK) cells kill virus-infected cells and tumor cells by cell-to-cell contact.
b. After stimulation by dendritic cells, they food for a self protein on the body's cells.
c. NK cells are not specific; they have no memory and their numbers do not increase after stimulation.
B. Specific Defenses
1. If nonspecific defenses fail to prevent an infection, specific defenses activate against a specific antigen.
2. We do not ordinarily become immune to our own cells; the immune system can tell "self" from "nonself."
3. Specific immunity is primarily the result of the action of B lymphocytes ( B cells) and T lymphocytes (T cells).
a. B cells and T cells recognize antigens because they have antigen receptors—plasma membrane proteins that allow them to combine with particular antigens.
b. B cells give rise to plasma cells that produce antibodies.
c. T cells differentiate into helper T cells, which regulate the immune response, or cytotoxic T cells, which kill virus-infected and tumor cells.
4. B Cells and Antibody-Mediated Immunity
a. Each type of B cell carries its specific receptor on its surface; this is called the B cell receptor (BCR).
b. When a B cell in a lymph node of the spleen encounters an appropriate antigen, it is activated to divide.
c. The resulting cells are plasma cells, mature B cells that produce antibodies in the lymph nodes and spleen; the antibodies are identical to the BCR of the B cell that produced them.
d. The clonal selection theory states that the antigen selects the B cell to produce a clone of plasma cells.
e. A B cell will not clone until its antigen is present; it recognizes the antigen directly.
f. However, B cells are stimulated to clone by helper T cell secretions.
g. Some cloned B cells do not participate in antibody production but remain in the blood as memory B cells.
h. Once the threat of infection has passed, development of new plasma cells ceases; those present die.
i. Apoptosis is the process of programmed cell death; apoptosis is critical to maintaining tissue homeostasis.
j. Defense by B cells is called antibody-mediated immunity.
k. It is also called humoral immunity because antibodies are present in the blood and lymph; a humor is a body fluid.
5. Structure of an Antibody (Immunoglobulin)
a. An antibody molecule is a Y-shaped protein molecule with two arms.
b. Each arm has a "heavy" and "light" polypeptide chain.
c. These chains have constant regions and variable regions.
d. The constant regions have amino acid sequences that do not change; the constant regions are not identical among all antibodies.
e. The variable regions have portions of polypeptide chains whose amino acid sequence changes providing antigen specificity; it forms the antigen binding sites of antibodies—their shape is specific to antigen.
f. The antigen binds with a specific antibody at the antigen-binding site.
g. The antigen-antibody complex (or immune complex) marks the antigen for destruction by being engulfed by neutrophils or macrophages, or it may activate complement.
h. If complement attaches to antigens on the surface of pathogens, it renders them more easily phagocytized.
i. Types of Antibodies
 There are five different classes of circulating antibodies or immunoglobulins (Igs).
ii. IgG Antibodies
1. These are the major type in blood; less is in the lymph and tissue fluid.
2. IgG antibodies bind to pathogens and toxins.
iii. IgM Antibodies
0. These are pentamers; they contain five Y-shaped structures.
1. IgM appears in blood soon after an infection begins and disappears before it is over.
2. They are good activators of the complement system.
iv. IgA Antibodies
0. IgA contains two Y-shaped structures.
1. They attack pathogens before they reach the blood.
2. They are the main type of antibody in bodily secretions.
v. The role of IgD antibodies is to serve as receptors for antigens on immature B cells.
vi. IgE antibodies are involved in immediate allergic reactions, and in response to parasites.
6. T Cells and Cell-Mediated Immunity
a. Like B cells, T cells have unique antigen receptors, called the T cell receptor, or TCR..
b. However, the receptors of cytotoxic and helper T cells cannot recognize antigen present in the tissues, lymph, or blood.
c. Instead, antigen must be presented to them by an antigen-presenting cell (APC).
d. When an APC presents a viral or cancer cell antigen, the antigen is first linked to an MHC (major histocompatibility complex) protein; together they are presented to a T cell.
i. The importance of the MHC was recognized when it was discovered it contributes to the difficulty of transplanting tissues from one person to another.
ii. When a donor and recipient are histocompatible, it is likely a transplant will be successful.
e. When a macrophage antigen is presented to a T cell, the T cell recognizes the antigen.
i. Once a helper T cell recognizes the antigen, it undergoes clonal expansion and produces cytokines stimulating immune cells to remain active and perform their functions.
ii. Once a cytotoxic T cell is activated, it undergoes clonal expansion and destroys any cell that possesses antigen if the cell bears the correct HLA antigen presented earlier.
iii. As the infection disappears, the immune reaction wanes and few cytokines are produced.
f. Apoptosis occurs in the thymus if the T cell bears a receptor to recognize a self antigen; if apoptosis does not occur, T-cell cancers result (i.e., lymphomas and leukemias).
g. Types of T Cells
i. Cytotoxic T Cells
0. They destroy antigen-bearing cells (e.g., virus-infected or cancer cells).
1. They have storage vacuoles that contain perforin molecules.
2. Perforin molecules perforate a plasma membrane; water, salts, and enzymes (called granzymes) then enter causing the cell to burst.
3. This is called cell-mediated immunity.
ii. Helper T Cells regulate immunity by enhancing the response of other immune cells.
0. When exposed to an antigen, they enlarge and secrete cytokines.
1. Cytokines stimulate the helper T cells to clone and other immune cells to perform their functions.
C. Immunity in Other Animals
1. In 1882, the Russian Elie Metchnikoff observed phagocytes gathered around a thorn in a starfish.
2. In 1979, the Swedish Hans G. Boman discovered antibacterial peptides in silkmoths.
3. Sea stars have cells similar to macrophages that release interleukinlike chemicals.
4. Using the polymerase chain reaction (PCR), Gary W. Litman studied sharks that rely on both antibody diversity and inherited immunity to familiar pathogens.
35.3 Induced Immunity
1. Immunity is acquired naturally through infection or artificially by medical intervention.
a. Active immunity is where an individual alone makes antibodies.
b. Passive immunity is where an individual receives prepared antibodies.
B. Active Immunity
1. Active immunity sometimes develops naturally after a person is infected.
2. However, active immunity is often induced when a person is well so that future infection is prevented.
3. Immunization uses vaccines to provide the antigen to which the immune system responds.
4. To prepare vaccines, usually pathogens are treated so they are no longer virulent.
5. Genetically engineered bacteria can also produce antigen proteins from pathogens; the protein is then used as a vaccine.
6. After a vaccine is given, the immune response is measured by the antibody level in serum—the antibody titer.
a. After the first exposure, a primary response occurs from no antibodies to a slow rise in titer.
b. After a brief plateau, a gradual decline follows as antibodies bind to antigen or simply break down.
c. After a second exposure, a secondary response occurs and the antibody titer rises rapidly to a level much greater than before; this is a "booster."
d. The high antibody titer is now expected to prevent any disease symptoms if the individual is infected.
e. Active immunity depends on memory B and memory T cells responding to lower doses of antigen.
f. Active immunity is usually long-lived although a booster may be required every so many years.
C. Passive Immunity
1. Passive immunity occurs when an individual is given prepared antibodies to combat a disease.
2. It is short-lived because antibodies are not made by an individual's own B cells.
3. Newborn infants are immune to some diseases because the mother's antibodies have crossed the placenta.
4. Breast-feeding also promotes passive immunity—the antibodies are in the mother's milk.
5. Passive immunity is also needed when a patient is in immediate danger from an infectious disease or toxin.
6. A person may be given a gamma globulin injection (serum that contains antibodies against the agent) taken from an individual or animal who has recovered from it.
7. If antibodies are made in an animal (e.g., a horse), and used for passive immunization, some individuals become sick with serum sickness.
8. In some cases, passive and active immunization can be used concurrently to combat a pathogen, e.g., rabies.
D. Cytokines and Immunity
1. Cytokines are signaling molecules produced by either lymphocytes, monocytes or other cells.
2. Cytokines stimulate white blood cell formation; they may work as adjunct therapy for cancer and AIDS.
3. Interferon and interleukins are used to improve the ability of an individual's T cells to fight cancer.
4. Cancer cells with altered proteins on their cell surface should be attacked by cytotoxic T cells.
5. Cytokines may awaken the immune system and lead to the destruction of cancer.
a. Researchers withdraw T cells from a patient and culture them in the presence of interleukin.
b. The T cells are re-injected into the patient; doses of interleukin then maintain the killer activity of the T cells.
6. Interleukin antagonists may help prevent skin or organ rejection, autoimmune diseases, and allergies when used as adjuncts for vaccines.
E. Monoclonal Antibodies
1. Every plasma cell derived from the same B cell secretes antibodies against the same antigen; these are monoclonal antibodies.
2. Monoclonal antibodies can be produced in vitro.
a. B lymphocytes are removed from the body (usually mice are used) and exposed to a particular antigen.
b. Activated B lymphocytes are fused with myeloma cells (malignant plasma cells that divide indefinitely).
c. Fused cells are called hybridomas because they result from two different cells (hybrid) and one is cancerous, therefore the suffix "-oma."
3. Monoclonal antibodies are used for quick, reliable diagnosis of various conditions such as pregnancy.
4. They identify infections, sort out different T cells, and distinguish between normal and cancer cells.
5. They can distinguish cancerous from normal cells and can be used to carry isotopes or toxic drugs to kill tumors.
35.4 Immunity Side Effects
A. Allergies
1. Allergies are hypersensitivities to substances such as pollen and other everyday substances.
2. A response to these antigens, called allergens, usually involves tissue damage.
3. Immediate and delayed allergic responses are two of four possible responses.
a. Immediate allergic responses occur within seconds of contact with an allergen.
b. Coldlike symptoms are common.
c. IgE antibodies are attached to the plasma membrane of mast cells in tissues and basophils in blood.
d. When an allergen attaches to IgE antibodies on these mast cells, they release large amounts of histamine and other substances, which cause the cold symptoms or even anaphylactic shock.
e. Anaphylactic shock is an immediate allergic response that occurs because the allergen has entered the blood stream; it is characterized by a sudden, life-threatening drop in blood pressure.
4. Allergy shots sometimes prevent the onset of allergic symptoms.
a. Injections of the allergen cause the body to build up high quantities of IgG antibodies.
b. These combine with allergens received from the environment before they have a chance to reach IgE antibodies located on the plasma membrane of mast cells and basophils.
5. Delayed Allergic Response
a. Delayed allergic responses are initiated by sensitized memory T cells at the site of allergen in the body.
b. The allergic response is regulated by the cytokines secreted by both T cells and macrophages.
c. The tuberculin skin test is an example: positive test shows prior exposure to TB bacilli but requires some time to develop reddening of tissue.
B. Blood-Type Reactions
1. ABO System
a. The presence or absence of type A and B antigens on red blood cells determine a person's blood type.
b. If the person has type A blood, the A antigen is on the red blood cells; if the person is type B, the B antigen is on the red blood cells.
c. In the ABO system, there are four blood types: A, B, AB, O
i. Individuals have naturally-occurring antibodies to blood type antigens not present on their blood cells; these are called anti-A and anti-B
ii. RBCs with a particular antigen agglutinate when exposed to corresponding antibodies, e.g., type A RBCs will agglutinate in the presence of anti-A antibody (as would be found in the blood of a type B individual).
iii. Agglutination is the clumping of red blood cells due to a reaction between antigens on the red blood cells.
iv. To receive blood, the recipient's plasma must not have an antibody that causes donor cells to agglutinate.
1. Recipients with type AB blood can receive any type blood; they are the universal recipient.
2. Recipients with type O blood cannot receive A, B, or AB; but they are a universal donor.
3. Recipients with type A blood cannot receive B or AB.
4. Recipients with type B blood cannot receive A or AB.
C. Rh System
1. Rh factor is an important antigen in human blood types.
2. Rh positive (Rh+) has the Rh factor on red blood cells; Rh negative (Rh-) lacks the Rh antigen on RBCs
3. Rh-negative individuals do not have antibodies to Rh factor but make them if exposed to Rh+ blood.
4. Rh factor is particularly important during pregnancy.
a. Hemolytic disease of the newborn is possible if the mother is Rh negative and the father is Rh positive.
b. Rh positive is a genetically dominant trait; an Rh negative mother and an Rh positive father pose a Rh conflict.
c. The child's Rh positive RBCs can leak across the placenta into the mother's circulatory system when the placenta breaks down.
d. The presence of the "foreign" Rh positive antigens causes the mother to produce anti-Rh antibodies.
e. Anti-Rh antibodies pass across the placenta and destroy the RBCs of the Rh positive child.
f. The Rh problem has been solved by giving Rh- women an Rh immunoglobulin injection (called Rho-Gam) either midway through the first pregnancy or no later than 72 hours after giving birth to an Rh+ child.
i. The injection includes anti-Rh antibodies that attack a child's RBCs before they trigger the mother's immune system.
ii. The injection is not effective if the mother has already produced antibodies; timing is important.
D. Tissue Rejection
1. Tissue rejection occurs because cytotoxic T cells cause disintegration of foreign tissue; this is a correct distinguishing between self and nonself.
2. Selection of compatible organs and administration of immunosuppressive drugs prevent tissue rejection.
3. Transplanted organs should have the same type antigens as in the recipient.
4. Cyclosporine and tacrolimus both act by inhibiting the response of T cells to cytokines.
E. Autoimmune Diseases
1. Autoimmune diseases result when cytotoxic T cells or antibodies mistakenly attack the body's own cells as if they bear foreign antigens.
2. The cause is not known but autoimmune diseases sometimes appear following recovery from an infection.
3. In myasthenia gravis, the neuromuscular junctions do not work properly and muscular weakness results.
4. In multiple sclerosis (MS), the myelin sheath of nerve fibers is attacked.
5. Persons with systemic lupus erythematosus present many symptoms before dying from kidney damage.
6. Heart damage following rheumatic fever and type I diabetes are also autoimmune diseases.
7. There are no cures for autoimmune diseases, but they are controlled by drugs.
36.1 Digestive Tracts
1. Most animals need to digest food into small molecules that can cross plasma membranes.
2. Digestion provides the energy needed to carry out routine metabolic activities and maintain homeostasis.
3. The digestive tract ingests food, breaks down food into small molecules that can cross plasma membranes, absorbs these nutrient molecules, and eliminates nondigestible remains.
B. Incomplete Versus Complete Tracts
1. Planarians are organisms with an incomplete digestive tract, a single opening (usually called a mouth).
a. Planaria are carnivorous and feed largely on smaller aquatic animals.
b. The digestive system contains only a mouth, a pharynx, and an intestine.
c. To feed, its pharynx extends far beyond the mouth to suck up minute quantities at one time.
d. Digestive enzymes in the tract allow some extracellular digestion.
e. Digestion is finished intracellularly by cells that line the digestive cavity; food then diffuses to nearby cells.
f. The digestive system lacks regions of specialized function.
g. The tapeworm, relatives of planaria, lack a digestive system altogether; they absorb food through a body wall with modified microscopic projections that absorb nutrients from the host.
2. In contrast, the earthworm has a complete digestive tract.
a. The digestive system is composed of a tube with a mouth and an anus.
b. Earthworms feed on decayed organic matter in the soil.
c. Different regions of the gut have specialized functions (e.g., ingestion, mechanical digestion, etc.).
d. A muscular pharynx draws in food with sucking action.
e. The crop is storage area with expansive walls.
f. The gizzard has thick muscular walls to grind food.
g. Digestion occurs in the intestine, outside of cells or "extracellular."
h. The surface area for absorption is increased by an intestinal fold called the typhlosole.
i. The undigested remains exit the body at the anus.
C. Continuous Versus Discontinuous Feeders
1. Clams are continuous feeders, or filter feeders.
a. Water moves into a mantle cavity through an incurrent siphon and deposits particles on gills.
b. Ciliary action moves particles to the labial palps which direct them into the mouth and into the stomach.
c. Digestive enzymes from a digestive gland help amoeboid cells in the tract complete digestion.
2. Marine fanworms are sessile filter feeders; only small particles are consumed while large particles are rejected.
3. Baleen whales are active filter feeders; baleen (fringe) filters small krill from water.
4. Squids are an example of discontinuous feeders.
a. The head of a squid has ten arms; two arms seize the prey and bring it to the squid's mouth.
b. Beaklike jaws and a radula (toothy tongue) reduce the food to pieces.
c. The esophagus leads to a stomach that holds food until digestion is complete.
d. Discontinuous feeders require a storage region in the gut.
D. Adaptation to Diet
1. Animals are herbivores (eat plants only) or carnivores (eat only other animals) or omnivores (eat both).
2. Invertebrates demonstrate a wide variety of diets.
3. Mammal dentition differs according to their mode of nutrition.
a. Omnivores, including humans, have dentition that accommodates both a vegetable and meat diet.
b. Omnivore teeth include incisors (cutting and shearing), canines (tearing), premolars (crushing), and molars (grinding).
c. Herbivores have large, flat premolars and molars for grinding plant matter.
d. Grazers (e.g., horses) have sharp incisors for clipping off grass and leaves.
e. Hard-to-digest plant material requires extensive grinding to disrupt the plant cell walls.
f. Animals that feed on plants may have long and complex digestive tracts and bacteria in their digestive tracts that can digest cellulose, producing nutrients (glucose) that the animal can use.
g. Some grazers have a rumen to digest chewed grasses; partially digested cud is then rechewed.
h. Carnivores' pointed incisors and canines tear off pieces small enough to swallow.
i. Meat is rich in protein and fatty acids and is easier to digest than plant material.
j. Carnivores have fewer molars for grinding and a shorter digestive tract with less specialization.
36.2 Human Digestive Tract
1. The human digestive tract is a complete tube-within-a-tube system.
2. Each part of the digestive system has a specific function.
3. Food is never found within the accessory glands, only within the tract itself.
4. The digestion of food in humans is an extracellular process.
5. Enzymes are secreted into the digestive tract by nearby glands which never contain food themselves.
6. Digestion requires a cooperative effort by the production of hormones and the actions of the nervous system.
B. Mouth
1. Food is chewed in the mouth (oral cavity) and mixed with saliva; the mouth is the beginning of the digestive tract.
a. Three pairs of salivary glands secrete saliva by way of ducts into the mouth.
b. Salivary amylase is the enzyme that begins starch digestion; maltose is the common end product.
c. Food is manipulated by a muscular tongue containing both touch and pressure receptors.
d. Taste buds are located primarily on the tongue but also on the surface of the mouth; these chemical receptors are stimulated by the chemical composition of food.
e. Food is chewed and mixed with saliva to form a bolus in preparation for swallowing.
C. The Pharynx and the Esophagus
1. The digestive and respiratory passages come together in the pharynx, and then separate.
a. During swallowing, the pathway of air to the lungs could be blocked if food entered the trachea.
b. The epiglottis covers the opening into the trachea as muscles move a bolus of food through the pharynx into the esophagus.
2. The esophagus is a muscular tube that moves swallowed food to the stomach by peristalsis, a rhythmical contraction that moves the contents along in tubular organs.
D. Stomach
1. The stomach stores liters of partially digested food, freeing humans from continual eating.
2. William Beaumont revealed much of the stomach's functions in the mid-1800s.
a. Alexis St. Martin had an opening (fistula) into the stomach, received from a gunshot, through which Dr. Beaumont could observe stomach activity.
b. Beaumont collected the gastric juice produced by cells of gastric glands.
c. Walls of the stomach contract vigorously and mix food with juices secreted when the food enters.
d. Beaumont found that gastric juice contains hydrochloric acid and another digestive substance, pepsin.
e. He discovered gastric juices are produced independently of the protective mucous secretions.
f. His careful work pioneered the study of the physiology of digestion.
3. Hydrochloric acid (HCl) lowers pH of the gastric contents to about 2.
a. The epithelial lining of the stomach has millions of gastric pits leading to gastric glands.
b. This acid kills most bacteria and other microorganisms.
c. The low pH also stops the activity of salivary amylase and promotes the activity of pepsin, an enzyme that digests large proteins to smaller peptides.
4. A thick layer of mucus protects the wall of the stomach from the HCl and pepsin.
5. Ulcers develop when the lining is exposed to digestive action; recent research indicates this is usually due to infection by Helicobacter pylori bacteria.
6. Stomach contents, a thick, soupy mixture, is called chyme.
7. At the base of the stomach is a narrow opening controlled by a sphincter (a circular muscle valve).
a. When the sphincter relaxes, chyme enters the first part of the small intestine, called the duodenum; a neural reflex causes the sphincter to contract, closing off the opening.
b. The sphincter relaxes and allows more chyme to enter the duodenum.
c. The slow, rhythmic pace with which chyme exits the stomach allows for thorough digestion.
E. The Small Intestine
1. The human small intestine is a coiled muscular tube about three meters long.
2. As chyme enters the duodenum, proteins and carbohydrates are partly digested but no fat digestion occurs.
3. Additional digestion is aided by secretions from the liver and the pancreas (below).
a. Bile is a secretion of the liver temporarily stored in the gallbladder before being sent to duodenum; bile emulsifies fat (allows fat droplets to disperse in water).
4. The lining of the small intestine has ridges and furrows; these surfaces are covered by villi (sing. villus); the small intestine is specialized for absorption by the huge number of villi that line the intestinal wall.
a. Villi are fingerlike projections; their surface cells are covered by microvilli.
b. Microvilli are minute projections, called the "brush border," on the surface of the cells of the intestinal villi.
c. Ridges, furrows, villi, and microvilli greatly increase the effective surface area of the small intestine.
5. Each villus contains blood vessels and a lymphatic capillary, called a lacteal..
a. A lacteal aids in the absorption of fats.
6. Sugars and amino acids enter villi cells and are absorbed into bloodstream.
7. Glycerol and fatty acids enter villi cells; reassembled into fat molecules, they move into lacteals.
8. After nutrients are absorbed, they are eventually carried throughout the body by the bloodstream.
F. Large Intestine
1. The large intestine has four parts: the cecum, colon, rectum, and anal canal.
2. It is larger in diameter but shorter in length than the small intestine and is the region following the small intestine.
3. Appendix
a. The appendix is a fingerlike projection extending from the cecum, a blind sac at the junction of the small and large intestine.
b. It may play a role in fighting infections.
c. If an infected appendix bursts, it results in general abdominal infection , called peritonitis.
4. The colon is subdivided into the ascending, transverse, descending, and sigmoidal colon.
5. About 1.5 liters of water enter the digestive tract daily from ingestion and another 8.5 liters enter from various secretions.
a. About 95% of this total liquid is reabsorbed by the small intestine; most of the remainder is absorbed by cells of the colon.
b. If the water is not reabsorbed, it causes diarrhea which can cause a serious dehydration and ion loss.
6. In addition to water, the large intestine absorbs salts and some vitamins, including the vitamin K produced by intestinal bacteria.
7. The large intestine terminates at the anus, the opening of the anal canal.
8. Feces
a. Feces consists of about 75% water and 25% solid matter.
b. One-third of the the solid matter is intestinal bacteria.
c. The remainder is undigested wastes, fats, organic material, mucus, and dead cells from the intestinal lining.
9. Intestinal polyps are small growths arising from the epithelial lining.
a. Whether they are benign or cancerous, polyps can be removed surgically.
b. A low-fat, high-fiber diet promotes regularity and may provide protection against mutagenic agents.
G. Three Accessory Organs
1. The Pancreas
a. The pancreas lies deep within the abdominal cavity, just below the stomach, and rests on the posterior abdominal wall.
b. It is an elongated and somewhat flattened organ.
c. As an endocrine gland, it secretes glucagon and insulin hormone into the bloodstream.
d. As an exocrine gland, it secretes pancreatic juice.
i. Pancreatic juice contains sodium bicarbonate that neutralizes acidic chyme.
ii. Pancreatic enzymes digest carbohydrates, fats and proteins.
2. The Liver
a. The liver, the largest gland in the body, fills the top of the abdominal cavity, just under the diaphragm.
b. The liver has numerous functions:
i. It detoxifies blood by removing and metabolizing poisonous substances.
ii. It makes plasma proteins including albumin and fibrinogen.
iii. It destroys old red blood cells and converts hemoglobin to bilirubin and biliverdin in bile.
iv. It produces bile stored in the gallbladder before it enters the duodenum to emulsify fats.
v. It stores glucose as glycogen and breaks down glycogen to maintain a constant blood glucose concentration.
vi. The liver produces urea from amino groups and ammonia.
c. Blood vessels from both the large and small intestines lead to the liver as the hepatic portal vein.
d. The liver maintains the blood glucose level at 0.1% by removing glucose from the hepatic portal vein to store as glycogen; when needed, glycogen is broken down and glucose re-enters the hepatic vein.
e. Amino acids can be converted to glucose but deamination (removal of amino groups) must occur.
f. By a complex metabolic pathway, the liver converts amino groups to urea, the nitrogenous breakdown product of amino acids.
g. Urea is the most common human nitrogenous waste; it is transported by the blood to the kidneys.
3. Liver Disorders
a. Jaundice is a symptom involving a yellowish skin due to a large amount of bilirubin in blood.
i. Jaundice can also result from hepatitis, inflammation of the liver.
b. Viral hepatitis is a viral liver infection.
i. Hepatitis A results from eating contaminated food.
ii. Hepatitis B and C are spread by blood transfusions, kidney dialysis, and unsterile needle use.
iii. All three can be caused from sexual contact.
c. Cirrhosis is a chronic disease where the liver tissue is replaced by fatty tissue and then scar tissue; alcoholics provide too much alcohol for the liver to break down.
4. The Gallbladder
a. The gallbladder is a pear-shaped, muscular sac attached to the surface of the liver.
b. Bile, produced in the liver, is stored in the gallbladder.
c. When needed, bile leaves the gallbladder and goes into the duodenum via the common bile duct.
36.3 Digestive Enzymes
1. Salivary amylase is the enzyme that begins starch digestion; maltose is the common end product.
a. Maltose cannot be absorbed in the small intestine; additional digestive action breaks the maltose into glucose, which can be absorbed.
2. Protein digestion begins in the stomach.
a. Pepsinogen is converted to pepsin when exposed to the HCl in the stomach; pepsin breaks proteins into smaller peptides.
3. Starch, proteins, nucleic acids, and fats are enzymatically broken down in the small intestine.
a. Pancreatic amylase digests starch to maltose.
b. Trypsin, also a pancreatic enzyme, digests protein to peptides.
c. Peptidases and maltase, produced by the small intestine, complete the digestion of protein to amino acids and starch to glucose, respectively.
d. Lipase, another pancreatic enzyme, digests fat droplets to glycerol and fatty acids.
36.4 Nutrition
1. Nutrients in the diet provide energy, promote growth and development, and regulate cellular metabolism.
2. Three eating disorders:
a. Obesity (body weight >19% above ideal weight for a person's height) can lead to diabetes, cardiovascular (CV) problems, and perhaps cancer.
b. In bulimia nervosa, individuals binge eat then purge themselves with laxitives or self-induced vomiting.
c. Individuals with anorexia nervosa have a fear of weight gain; they literally starve themselves to appear thin—this leads to bone density decrease, CV problems, and possibly a general "shut down" of the body.
B. Carbohydrates
1. Complex carbohydrates (e.g., whole grain cereals, breads, etc.) are recommended because they are digested to sugars and contain fiber.
a. Insoluble fiber (e.g., as found in wheat bran) has a laxative effect and may guard against colon cancer.
b. Soluble fiber (e.g., as found in oat bran) combines with bile acids and cholesterol in the intestine and prevents their absorption.
2. Simple sugars (e.g., candy) and the sugars obtained from the starch in potatoes and white bread, have a high glycemic index (GI) because the blood glucose response to these foods is high.
C. Proteins
1. Of the 20 different amino acids required for protein synthesis, 8 (9 in children) cannot be synthesized by the body and are thus termed essential amino acids.
2. Some foods do not provide all the essential amino acids—vegetarians should combine two or more plant products to acquire all the essential amino acids.
3. A high-protein diet can harm the body.
a. Dehydration can occur.
b. Calcium loss in the urine can occur, leading to kidney stones.
D. Lipids
1. Saturated fats (i.e., solid at room temperature) usually have an animal origin.
2. Oils contain unsaturated fatty acids, which do not promote CV disease; omega-3 fatty acids, found in some cold-water fishes and in flax seed oil, are believed to prevent heart disease.
3. Fats That Cause Disease
a. CV disease is often due to arteries being blocked by plaque, which contains saturated fats and cholesterol.
b. Trans fatty acids are more likely to cause CV disease than saturated fats—any packaged goods that contain partially hydrogenated vegetable oils ("shortening") will likely contain trans fat.
E. Vitamins
1. Vitamins are essential organic compounds the body cannot make but still requires for metabolic activities.
2. Many vitamins are portions of coenzymes: e.g., niacin is part of NAD+ and riboflavin is a part of FAD.
3. Coenzymes are needed in small amounts because they are used repeatedly.
4. Vitamin A is not a coenzyme but a precursor for the visual pigment that prevents night blindness.
5. Lack of vitamins results in vitamin deficiencies.
6. The 13 vitamins are divided into those that are fat soluble and those that are water soluble.
F. Antioxidants
1. Cell metabolism generates free radicals, unstable molecules with an extra electron; O2- is a common free radical.
2. Free radicals stabilize by eventually donating electrons to another molecule; this damages cellular molecules.
3. Free radicals damage DNA, proteins, and other molecules by donating an electron; this may cause cancer or plaque in arteries.
4. Vitamins C, E, and A—abundant in fruits and vegetables—are antioxidants that defend against free radicals.
5. Supplements do not replace fruits and vegetables that also contain many other beneficial compounds.
G. Minerals
1. Humans require certain minerals (e.g., calcium, phosphorus) in amounts of over 100 mg per day.
a. They are constituents of cells and body fluids and structural components of tissues.
b. Calcium is needed to build bones and teeth and for nerve conduction and muscle contraction.
2. Certain other minerals are elements (e.g., zinc, iron) recommended in amounts less than 20 mg per day.
a. These microminerals are more likely to have very specific functions.
b. Iron is needed to produce hemoglobin; adult females need more due to loss of menstrual blood.
c. Iodine is used to produce thyroxin, a hormone of the thyroid glands.
d. Minute amounts of molybdenum, selenium, chromium, nickel, vanadium, silicon, and arsenic are essential.
e. Some individuals may not receive enough calcium; stress can cause a magnesium deficiency, and a vegetarian diet may be short on zinc.
H. Calcium
1. Calcium supplements counteract the osteoporosis that afflicts 25% of older men and 50% of older women.
2. Porous bones break easily due to lack of calcium.
3. After menopause, bone-eating cells called osteoclasts are more active than bone-forming osteoblasts.
4. Calcium supplements have been shown to slow bone loss in the elderly.
5. Intake of 1,000–1,500 mg calcium/day is recommended; therefore supplemental calcium is usually necessary.
6. Exercise is also effective in building bone mass.
I. Sodium
1. The recommended daily intake of sodium is 400–3,300 mg; the average American intake is 4,000–4,700 mg.
2. A high sodium intake has been linked to hypertension in some people.
3. One third of our sodium intake is found naturally in foods; another third is added in processing.
4. We add one-third of our salt intake in cooking or as table salt.
37.1 Gas Exchange Surfaces
• Respiration is the sequence of events that results in gas exchange between the environment and the body's cells.
1. Ventilation (breathing) includes inspiration (bringing air in) and expiration (moving air out).
2. External respiration involves gas exchange with the external environment in the lungs.
3. Internal respiration involves gas exchange between the blood and tissue fluid.
4. An effective gas exchange region must be moist, thin, and large in relation to the size of the body.
5. Some animals are small and shaped to allow their surface to be an adequate gas-exchange surface.
6. Larger animals are complex and have a specialized gas-exchange surface.
7. Diffusion improves with vascularization; gas delivery to cells is promoted if the blood contains hemoglobin.
A. Water Environments
1. It is more difficult for animals to obtain O2 from water than from air.
a. Water fully saturated with air contains only a fraction of the O2 as the same volume of air.
b. Water is more dense than air; therefore aquatic animals must use more energy to respire.
c. Fishes use up to 25% of their energy to respire, whereas land mammals use only 1–2% of their energy output.
2. Hydras and planaria have a large surface area in comparison to their size.
a. Gas exchange occurs directly across their body surface.
b. The hydra's outer cell layer contacts the environment; an inner layer exchanges gases with the water in the gastrovascular cavity.
c. The flat body of planaria permits cells to exchange gases with the external environment.
3. A tubular shape and vascularized parapodia extensions in polychaete worms provide surface areas for diffusion.
4. Aquatic animals often pass water over gills.
a. Gills are finely divided and vascularized outgrowths of either an outer or inner body surface.
b. Among clams, water is drawn into the mantle cavity and flows over gills.
c. Decapod gills are located in brachial chambers under the exoskeleton; water is circulated by special mouthparts.
d. Fish gills are outward extensions of the pharynx organized into arches.
e. Ventilation is the result of the combined action of the mouth and gill covers.
f. When the mouth is open, the opercula are closed and water is drawn in; the mouth then closes and the opercula open, drawing water from the pharynx through gill slits located between the gill arches.
g. To the outside of the gill arches are gill filaments folded into platelike lamellae, each of which contains capillaries; the result is a tremendous surface area for gas exchange.
i. Blood in capillaries of gill lamellae flows in a direction opposite to that of water.
ii. This countercurrent flow of water and blood increases the amount of O2 and CO2 exchanged.
iii. Such a countercurrent mechanism extracts about 80–90% of the initial dissolved O2 in the water.
B. Land Environments
1. Air is a richer source of O2 than water, but air dries the wet respiratory surfaces; humans lose 350 ml of water per day at 50% relative humidity.
2. The earthworm is an invertebrate that uses its body surface for respiration.
a. An earthworm expends energy to secrete mucus and release fluids from excretory pores.
b. The earthworm is also behaviorally adapted to stay in the moist soil during the day when air is driest.
3. Insects and certain terrestrial arthropods utilize air tubes called tracheae.
a. Oxygen enters a tracheal system at spiracles, which are valvelike openings at each side of the body.
b. The tracheae branch and rebranch to end in tiny tracheoles that are in direct contact with body cells.
c. Larger insects have air sacs located near major muscles to keep air moving in and out of the trachea.
d. The tracheae effectively deliver adequate oxygen to the cells of insects; the circulatory system has no role in gas transport.
4. Terrestrial vertebrates have evolved lungs for gas exchange.
a. Lungs are vascularized outgrowths of the lower pharyngeal region.
b. Amphibian lungs are simple, saclike structures, that connect to the external environment by way of two bronchi which connect to a short trachea.
i. Amphibian gas exchange occurs through a skin kept moist by mucus produced by surface glands.
ii. In the winter, amphibians burrow in mud and all gas exchange occurs across the skin.
iii. Frogs use positive pressure to force air in; nostrils shut and the floor of the mouth forces air into the lungs.
c. The lining of the lung becomes progressively more finely divided as we move from amphibians to reptiles to birds and mammals.
d. Human lungs have at least 50 times the skin's surface area.
e. Reptiles, birds, and mammals use negative pressure to move air into lungs.
i. Jointed ribs are raised and the muscular diaphragm is flattened to expand the lungs.
ii. As the thoracic cavity expands, the lung volume increases; air flows in due to the difference in air pressure.
iii. By lowering the ribs, pressure is exerted on the lungs, which forces air out.
iv. All terrestrial vertebrates except birds use a tidal ventilation mechanism: air moves in and out by the same route.
f. The lungs of reptiles, amphibians and mammals are not completely emptied during each breathing cycle.
i. With incomplete ventilation, entering air mixes with used air in the lungs.
ii. This conserves moisture but decreases gas-exchange efficiency.
g. The high oxygen requirements of flying birds requires a one-way ventilation mechanism.
i. Incoming air is carried past the lungs by a trachea that takes it to a set of posterior air sacs.
ii. Air then passes forward through the lungs into a set of anterior air sacs and is finally expelled.
iii. The one-way flow means that oxygen-rich air does not mix with used air; this maximizes gas exchange.
iv. The ventilation mechanism of birds results in a higher partial pressure of oxygen in the lungs; thus the uptake of oxygen is greater than in other vertebrates.
37.2 Human Respiratory System
1. The human respiratory system includes everything that conducts air to and from the lungs; the lungs lie deep within the thoracic cavity for protection against drying.
2. Air moves into the nose, crosses the pharynx, flows through the glottis (an opening into the larynx or voice box) to the trachea, bronchi, bronchioles, and finally the alveoli, where gas exchange occurs.
a. This process filters debris, warms the air, and adds moisture.
b. When the air reaches the lungs, it is at body temperature and is saturated with water.
c. The trachea and bronchi are lined with cilia that beat upward carrying mucus, dust, and any food particles that went the wrong route.
d. The hard and soft palates separate the nasal cavities from the mouth.
e. When food is being swallowed, the glottis is closed by the epiglottis, and the soft palate covers the entrance of the nasal passages into the pharynx.
f. At the edges of the glottis are vocal cords; as air passes across them, these tissues vibrate creating sounds.
g. From the larynx, air flows down the trachea to the bronchi.
i. The larynx is held open by cartilage that forms the Adam's apple.
ii. The trachea walls are reinforced with C-shaped rings of cartilage.
h. The trachea divides into two bronchi; C-shaped rings of cartilage diminish as bronchi branch.
i. Within the lungs, each bronchus branches into numerous bronchioles that conduct air to alveoli.
j. Alveoli are microscopic air sacs.
B. Ventilation
1. Humans respire using a tidal ventilation mechanism; negative pressure in the lungs allows for air flow during inspiration.
2. During inhalation, lowering the diaphragm and raising the ribs forms a negative pressure by increasing the volume of the thoracic cavity; the air–under greater outside pressure–flows into the lung.
3. Increases in the CO2 and H+ concentrations in the blood are the primary stimuli increasing the breathing rate.
a. The chemical content of blood is monitored by chemoreceptors sensitive to increases in CO2 and H+ concentrations of the blood, but minimally sensitive to decreases in O2 concentration.
b. The aortic bodies are chemoreceptors located in the wall of the aortic arch.
c. Carotid bodies are chemoreceptors located in the wall of the carotid arteries.
4. Information from these goes to a respiratory center in the brain, which in turn increases the ventilation rate when CO2 or H+ concentrations increase.
C. Gas Exchange and Transport
1. Gas exchange between the air in the alveoli and the blood in the pulmonary capillaries is primarily by diffusion.
2. Atmospheric air contains little CO2, but blood flowing in the pulmonary capillaries has a higher concentration of CO2.
3. CO2 diffuses from higher concentration in the blood across the walls of alveolar capillaries to lower concentration in the air in the alveoli.
4. The blood coming into pulmonary capillaries is oxygen poor and the alveolar air is oxygen-rich.
5. Oxygen diffuses from higher concentration in alveoli across the walls of the alveolar capillaries to the lower concentration in the blood.
D. Transport of Oxygen and Carbon Dioxide
1. Most O2 entering the pulmonary capillaries combines with hemoglobin (Hb) to form oxyhemoglobin (HbO2).
Hb + O2 → HbO2
deoxyhemoglobin oxygen oxyhemoglobin
2. Each hemoglobin molecule has four polypeptide chains; each chain folds over an iron-containing heme.
a. Each RBC has 250 million hemoglobin molecules.
b. Each RBC can carry a billion molecules of O2 oxyhemoglobin.
c. The iron atom of a heme group loosely binds with an O2 molecule.
3. The oxygen-binding ability of hemoglobin can be graphed.
a. The percentage of oxygen-binding sites of hemoglobin carrying O2 varies with partial pressure of O2 in the immediate environment.
b. The partial pressure is the amount of pressure exerted by a particular gas among all of the gases present.
c. At a normal partial pressure of O2 in lungs, hemoglobin becomes practically saturated with O2.
d. At the O2 partial pressures in the tissues, oxyhemoglobin quickly unloads much of its O2.
HbO2 → Hb + O2
e. The acid pH and warmer temperature of the tissues also promote this dissociation.
4. In tissues, some hemoglobin combines with CO2 to form carbaminohemoglobin.
5. However, most CO2 is transported in the form of bicarbonate ion (HCO3-).
a. First, CO2 combines with water, forming carbonic acid (H2CO3).
b. This then dissociates to a H+ and a HCO3-
CO2 + H2O → H2CO3 → H+ + HCO3-
carbonic acid bicarbonate ion
c. Carbonic anhydrase, an enzyme in red blood cells, speeds this reaction.
d. Release of H+ ions could drastically lower blood pH; however, the hydrogen ions are absorbed by the globin portions of hemoglobin and the HCO3- diffuses out of the RBCs and into the plasma.
e. Hemoglobin combines with H+ ions as reduced hemoglobin (HHb); HHb plays a vital role in maintaining normal blood pH.
f. As blood enters the pulmonary capillaries, most of the CO2 is in plasma as HCO3-.
g. The little free CO2 remaining diffuses out of the blood across the walls of the pulmonary capillaries and into alveoli.
h. Any decrease in plasma CO2 concentration causes the following reaction also catalyzed by carbonic anhydrase:
H+ + HCO3- → H2CO3 → CO2 + H2O
i. At the same time, hemoglobin unloads H+ and HHb becomes Hb.
37.3 Respiration and Health
A. Upper Respiratory Tract Infections
1. The entire respiratory tract has a warm, wet, mucous membrane lining exposed to environmental air.
2. The upper respiratory tract consists of the nose, pharynx and larynx.
3. "Strep throat" is an infection caused by the bacteria Streptococcus pyogenes resulting in a high fever and difficulty swallowing—it can lead to a systemic infection.
4. Sinusitis is infection of the sinuses; 1–3% of upper respiratory infections are accompanied by sinusitis.
5. Tonsillitis occurs when the tonsils and adenoids of the pharynx are inflamed as a first line of defense.
6. Laryngitis is an infection of the larynx causing hoarseness and an inability to talk.
7. Persistent hoarseness without any upper respiratory infection is one of the warning signs of cancer.
B. Lower Respiratory Tract Disorders
1. Acutebronchitis is an infection of the primary and secondary bronchi and is usually preceded by a viral upper respiratory infection.
2. Pneumonia
a. Pneumonia is usually caused by a bacterial or viral lung infection.
b. The bronchi and alveoli fill with fluid.
c. Pneumonia can be localized in specific lobules.
d. AIDS patients are subject to a rare form of pneumonia caused by the protozoan Pneumocystis carinii.
3. Pulmonary Tuberculosis
a. Pulmonarytuberculosis is caused by the tubercle bacillus, a type of bacterium.
b. A TB skin test is a highly diluted extract of the bacilli injected into the patient's skin; if a person has been exposed, the immune response will cause an area of inflammation.
c. Bacilli that invade lung tissue are isolated by the lung tissue in tiny capsules called tubercles.
d. If the person is highly resistant, the imprisoned bacteria die.
e. If resistance is low, the bacteria can eventually be liberated.
f. A chest X ray detects active tubercles.
g. Appropriate drug therapy can ensure localization and the eventual destruction of live bacteria.
h. Resurgence has accompanied increases in AIDS, homeless, and poor.
i. The new strains are resistant to standard antibiotics.
C. Disorders
1. Pulmonary Fibrosis
a. Inhaling particles of silica, coal dust, fiberglass and asbestos can lead to pulmonaryfibrosis.
b. These agents result in a build up of fibrous connective tissue—the lungs then cannot inflate properly.
c. Asbestos was used widely for fireproofing and widespread exposure occurred; it is estimated that a possible 2 million deaths could be caused by asbestos between 1990 and 2020.
2. Chronic Bronchitis
a. In chronicbronchitis, airways are inflamed and filled with mucus; often a cough brings mucus up.
b. The bronchi degenerate, losing cilia and normal cleansing action and making an infection likely.
c. Smoking cigarettes and cigars is the most common cause, but other pollutants are also involved.
3. Emphysema
a. Emphysema is a chronic and incurable disorder; it involves distended and damaged alveoli.
b. The lungs often balloon due to trapped air and ineffective alveoli.
c. Emphysema is often preceded by chronic bronchitis.
d. The elastic recoil of the lungs is reduced and the airways are narrowed, making expiration difficult.
e. Since the surface area for gas exchange is reduced, insufficient O2 reaches the heart and the brain.
f. This triggers the heart to work furiously to force more blood through the lungs; this can then lead to a heart condition.
g. Lack of oxygen to the brain makes the patient feel depressed, sluggish, and irritable.
h. Exercise, drug therapy, and supplemental oxygen may relieve the symptoms and slow the progress.
4. Asthma
a. Asthma is a disease of the bronchi and bronchioles; it causes wheezing, breathlessness, and a cough.
b. The airways are sensitive to specific allergens (e.g., pollen, dust, cold air, etc.)
c. Exposure to the irritant causes the smooth muscle in bronchi to spasm; chemical mediators given off by the immune cells in the bronchioles result in the spasms.
d. Bronchial inflammation reduces the diameter of the airways.
e. Special inhalers can control the inflammation and sometimes prevent an attack; other inhalers can stop muscle spasms.
5. Lung Cancer
a. Formerly more common in men, lung cancer now surpasses breast cancer as the cause of death in women due to smoking.
b. Lungcancer develops in the lung tissue in steps.
i. First, a thickening and callusing of the cells lining the bronchi appears.
ii. Cilia are lost so it becomes impossible to prevent dust and dirt from settling in the lungs.
iii. Next, cells with atypical nuclei appear in the callused lining.
iv. A tumor consisting of disordered cells with atypical nuclei develops as cancer in situ (cancer at one location).
v. When some tumor cells break free and penetrate other tissue (metastasis), the cancer spreads.
vi. A tumor may grow until the bronchus is blocked, cutting off the air supply to the lungs.
vii. The entire lung then collapses; the trapped secretions become infected causing pneumonia or lung abscess.
c. The only treatment is surgery (pneumonectomy) where a lobe or whole lung is removed before metastasis occurs.

38.1 Body Fluid Regulation
1. The excretory system regulates body fluid concentrations by regulating the water and ions in body fluids.
2. This regulation depends on the concentration of mineral ions (i.e., Na+, CI-, K+, and HCO3- ).
3. Body fluids gain mineral ions when animals eat food and drink fluids; the body then loses ions by excretion.
4. Water enters animals via ingestion (eating and drinking) and by metabolism (cellular respiration produces water).
5. Water is lost by evaporation from skin and lungs, through feces, and by excretion.
6. To be in fluid balance, the water entering the body must equal the water lost.
7. If osmolarity (i.e., solute concentration) differs between two regions, water moves into the region with the higher solute concentration.
8. Marine environments are high in salt and promote the loss of water and the gain of ions by drinking water.
9. Fresh water promotes a gain of water by osmosis and a loss of ions as this excess water is excreted.
10. Terrestrial animals tend to lose both water and ions to the environment.
B. Aquatic Animals
1. Only cartilaginous fishes (sharks, rays) and marine invertebrates (molluscs, arthropods) have body fluids that are isotonic to seawater, yet those fluids do not contain the same amount of salt as seawater.
a. Their blood has high concentrations of urea to match the tonicity of seawater; for some unknown reason, this is not toxic to the fishes.
2. Bony Fishes
a. Marine bony fishes have a moderate salt level compared to seawater; their common ancestor probably evolved in fresh water.
i. They are prone to water loss and could become dehydrated, therefore they constantly drink seawater.
ii. They swallow water equal to 1% of their weight every hour to counteract dehydration.
iii. To remove excess salt, they actively transport Na+ and CI- ions (salt) at the gills.
b. The body fluids of freshwater bony fish are hypertonic to freshwater; they passively gain water.
i. Freshwater fishes never drink water.
ii. They take in salts at the gills and pass large quantities of dilute, hypotonic urine.
iii. They discharge a quantity of urine equal to one-third their body weight each day.
iv. Because this causes them to lose salts, they actively import Na+ and CI- ions into the blood at the gills.
c. Some fish can move between marine and freshwater environments.
i. Salmon, for example, begin their life in freshwater streams, mature in ocean and return to freshwater to breed.
ii. Salmon alter their behavior and gill and kidney functions in response to osmotic changes.
C. Terrestrial Animals
1. Some terrestrial animals that live near oceans are able to drink seawater despite its high osmolarity.
2. Such birds and reptiles have a salt gland that excretes concentrated salt solution.
3. Water Loss Prevention
a. Some animals excrete a rather insoluble nitrogenous waste.
b. Animal skin is adapted to moist (moist, thin, permeable) or dry (dry, thick, impermeable) environments.
c. Some animals have other unique adaptations to prevent water loss; e.g., some nematodes literally dry up under adverse conditions—this is called anhydrobiosis.
d. The camel and kangaroo rat have convoluted nasal passages that absorb moisture from exhaled air.
4. Most terrestrial animals must drink fresh water often; however, the kangaroo rat completely avoids drinking water.
a. It forms a very concentrated urine.
b. It defecates fecal matter that is almost completely dry.
c. It meets its water requirements with the metabolic water derived from aerobic respiration.
38.2 Nitrogenous Waste Products
1. The breakdown of nucleic acids and amino acids results in nitrogenous wastes.
2. Amino acids derived from protein synthesize body proteins or nitrogen-containing molecules.
3. Unused amino acids are oxidized to generate energy or are stored as fats or carbohydrates.
4. In both cases, amino groups (—NH2) must be removed.
5. Nitrogenous wastes are excreted as ammonia, urea, or uric acid, depending on the species.
6. The removal of amino groups requires a fairly constant amount of energy that differs for each conversion.
B. Excretion of Nitrogenous Wastes
1. Amino groups removed from amino acids form ammonia (NH3) by adding a third hydrogen ion (H+).
a. This requires little or no energy.
b. Ammonia is quite toxic but it is water soluble; it requires the most water to wash it away from the body.
c. Bony fishes, aquatic invertebrates, and amphibians excrete this ammonia through gills and skin surfaces.
2. Terrestrial amphibians and mammals usually excrete urea as their main nitrogenous waste.
a. Urea is much less toxic than ammonia; excreted in a moderately concentrated solution, it also conserves body water.
b. Production of urea requires energy; it is produced in the liver as a product of the energy-requiring urea cycle.
c. In the urea cycle, carrier molecules take up carbon dioxide and two molecules of ammonia, finally releasing urea.
3. Insects, reptiles, and birds excrete uric acid as their main nitrogenous waste.
a. Uric acid is not very toxic and is poorly soluble in water; uric acid is readily concentrated for water conservation.
b. In reptiles and birds, a dilute solution of uric acid passes from the kidneys to the cloaca, a common reservoir for products of the digestive, urinary, and reproductive systems.
c. After any water is absorbed by the cloaca, the uric acid passes out with the feces.
d. Reptiles and bird embryos are enclosed in eggshells; the uric acid that builds up is nontoxic in storage.
e. Uric acid is synthesized by enzymatic reactions using even more ATP than urea synthesis.
f. Therefore, there is a trade-off between water conservation and energy expenditure.
38.3 Organs of Excretion
• Most animals have tubular organs to regulate salt-water balance and excrete metabolic wastes.
A. Flame Cells in Planarians
1. Planaria have two strands of branching excretory tubules that open to the outside through excretory pores.
2. Located along the tubules are flame cells (photonephridia) containing tufts of cilia that appear to flicker.
3. The cilia beat back and forth, propelling a hypotonic fluid through canals emptying at the body surface.
4. This system functions in the excretion of excess water and wastes.
B. Nephridia in Earthworms
1. An earthworm's body is divided into segments and nearly every segment has a pair of nephridia.
2. A nephridium is a tubule with a ciliated opening, the nephridiostome, and an excretory nephridiopore.
3. Fluid from the body cavity is propelled through this tubule by cilia.
4. Nutrients are reabsorbed and carried away by the network of capillaries surrounding the tubules.
5. The nephridia form urine that contains only metabolic wastes, salts, and water.
6. Each day, an earthworm produces urine equal to 60% of its body weight, enough water that it can safely excrete ammonia.
C. Malpighian Tubules in Insects
1. Insects have a unique excretory system consisting of long, thin Malpighian tubules attached to the gut.
2. The Malpighian tubules take up metabolic wastes and water from the hemolymph—these follow the salt gradient established by active transport of K+ ions.
3. At the rectum, water and other useful substances are reabsorbed.
4. Uric acid remains and eventually passes out the anus.
5. Insects that live in the water or that eat large quantities of moist food reabsorb very little water.
6. Insects in dry climates reabsorb most of the water and excrete a dry, semisolid mass of uric acid.
7. Crayfish (an arthropod) have antennal glands (green glands) in the head which perform selective secretion of certain salts.
8. In shrimp and pillbugs, the excretory organs, called maxillary glands, are located in the maxillary segments.
9. Arachnids (spiders, scorpions, etc.) have coxal glands used in excretion near the joint of one or more appendages.
38.4 Urinary System in Humans
• The human urinary system is an organ system with four parts.
1. The human kidneys are two bean-shaped, reddish brown organs, each about the size of a fist.
a. They are located on either side of the vertebral column, below the diaphragm, and partly protected by the lower rib cage.
b. The kidneys are the sites of urine formation.
2. Each kidney is connected to a ureter; each conducts urine from a kidney to the urinary bladder.
3. The urinary bladder stores urine until it is voided from the body through the urethra.
4. A single urethra conducts urine from the urinary bladder to the exterior of the body.
5. The male urethra runs through the penis and also carries semen; in females, it opens the ventral to the vaginal opening.
A. Kidneys
1. If a kidney is sectioned longitudinally, three major regions can be distinguished.
a. The renal cortex is the thin, outer region and it appears granular.
b. The renal medulla consists of the striped, pyramid-shaped regions that lie on the inner side of the cortex.
c. The renal pelvis is the innermost hollow chamber.
2. Each human kidney is composed of about one million tiny tubules called nephrons.
3. Some nephrons are located primarily in the cortex but others dip down into the medulla.
B. Nephrons
1. Each nephron is comprised of several parts.
2. The end of a nephron pushes in to form a cuplike structure called the glomerular capsule.
a. The outer layer is composed of simple squamous epithelium.
b. The inner layer is made of specialized cells that allow easy passage of molecules.
3. Nearest the glomerular capsule is the proximal convoluted tubule, lined by cells with many mitochondria and tightly packed microvilli.
4. Simple squamous epithelium forms the loop of the nephron, the middle portion of the nephron tubule with a descending and ascending limb.
5. The distal convoluted tubule is the distal portion of the nephron tubule; several join to deliver the urine into collecting ducts.
6. The loop of nephron and the collecting duct give pyramids of the medulla their striped appearance.
7. Each nephron has its own blood supply.
a. The renal artery branches into smaller arteries, which then branch into afferent arterioles, one for each nephron.
b. Each afferent arteriole divides to form a capillary bed or glomerulus.
c. The glomerular capillaries drain into an efferent arteriole which branches into a peritubular network.
d. The peritubular capillaries drain into a venule; the venules from many nephrons drain into a small vein; small veins join to form the renal vein, a vessel that enters the inferior vena cava.
C. Urine Formation
1. Urine production requires three distinct processes.
2. Glomerular filtration occurs at the glomerular capsule.
3. Tubular reabsorption occurs at the proximal convoluted tubule.
4. Tubular secretion occurs at the distal convoluted tubule.
D. Glomerular Filtration
1. When blood enters the glomerulus, blood pressure moves small molecules from the glomerulus across the inner membrane of the glomerular capsule into the lumen of the glomerular capsule; this is glomerular filtration.
2. The glomerular walls are 100 times more permeable than the walls of most capillaries.
3. Molecules that leave the blood and enter the glomerular capsules are the glomerular filtrate.
4. Plasma proteins and blood cells are too large to be part of the glomerular filtrate.
5. Failure to restore fluids would soon cause death from loss of water, nutrients, and low blood pressure.
E. Tubular Reabsorption
1. Tubular reabsorption of fluids from the nephron back to the blood occurs through the walls of the proximal convoluted tubule.
2. Reabsorption recovers much of the glomerular filtrate.
a. The osmolarity of the blood equals the filtrate so osmosis of water does not occur.
b. Sodium ions are actively reabsorbed; chlorine ions follow passively.
c. This changes the osmolarity of the blood so that water moves passively from the tubule back to the blood.
d. About 60–70% of salt and water are reabsorbed at the proximal convoluted tubule.
3. Cells of the proximal convoluted tubule have numerous microvilli, increasing the surface area for absorption, and numerous mitochondria, which supply the energy needed for active transport.
4. Only molecules with carrier molecules for them are reabsorbed.
5. If there is more glucose, for example, than carriers, excess glucose will appear in the urine.
6. In diabetes mellitus, there is a too much glucose because the liver fails to store glucose as glycogen.
F. Tubular Secretion
1. Tubular secretion moves substances from the blood to the tubular lumen by other than glomerular filtration.
2. Secretion back into the filtrate is primarily associated with the distal convoluted tubule.
3. This helps rid the body of potentially harmful compounds that were not filtered into the glomerular capsule.
4. Uric acid, hydrogen ions, ammonia, and penicillin are eliminated this way.
G. Urine Formation and Homeostasis
1. The kidneys regulate the water balance of the blood, thereby maintaining the blood volume and blood pressure.
H. Maintaining the Salt-Water Balance
1. The long loop of nephron is comprised of a descending limb and an ascending limb.
2. Salt (NaCl) passively diffuses out of the lower portion of the ascending limb, but the upper, thick portion of the limb actively transports salt out into the tissue of the outer renal medulla.
3. Less salt is available for transport from the tubule as fluid moves up the thick portion of the ascending limb; the ascending limb is impermeable to water.
4. Urea leaks from the lower portion of the collecting ducts, causing the concentration in the lower medulla to be highest.
5. Because of the solute concentration gradient within the renal medulla, water leaves the descending limb of the loop of nephron along its length.
6. The decreasing water concentration in the descending limb encounters an increasing solute concentration; this is a countercurrent mechanism.
7. Fluid received by a collecting duct from the distal convoluted tubule is isotonic to the cells of the cortex; as this fluid passes through the renal medulla, water diffuses out of the collecting duct into the renal medulla.
8. The urine finally delivered to the renal pelvis is usually hypertonic to the blood plasma.
9. Antidiuretic hormone (ADH) is released from the posterior lobe of the pituitary.
a. ADH acts on the collecting ducts by increasing its permeability to H2O, thereby increasing H2O retention.
b. When ADH is released, more water is reabsorbed and there is less urine.
c. When ADH is not released, more water is excreted and more urine forms.
d. If an individual does not drink much water, the pituitary releases ADH; if hydrated, ADH is not released.
e. Diuresis means increased urine; antidiuresis means a decreased amount of urine.
10. Reabsorption of Salt
a. Usually more than 99% of the sodium filtered out at the glomerulus is returned to the blood.
b. Most is reabsorbed at the proximal tubule, 25% is extruded by the ascending limb of the loop of nephron and the rest is from the distal convoluted tubule and collecting duct.
c. Aldosterone is a hormone secreted by the adrenal cortex.
i. It acts on the distal convoluted tubules to increase the reabsorption of Na+ and the excretion of K+.
ii. Increased Na+ in the blood causes water to be reabsorbed, increasing blood volume and pressure.
iii. If the blood pressure is insufficient to promote glomerular filtration, the afferent arteriole cells secrete renin.
iv. Renin catalyzes the conversion of angiotensinogen (a protein produced by the liver) into angiotensin I.
v. Later, angiotensin I is converted to angiotensin II by angiotensin-converting enzyme.
vi. Angiotensin II stimulates cells in the adrenal cortex to produce aldosterone.
vii. Angiotensin II increases the blood pressure as a vasoconstrictor.
11. The Atrial natriuretic hormone (ANH) is produced by the atria of the heart when cardiac cells stretch.
a. When blood pressure rises, the heart produces ANH to inhibit the secretion of renin and the release of ADH.
b. Therefore, this hormone decreases blood volume and pressure.
I. Maintaining the Acid-Base Balance
1. The bicarbonate buffer system and breathing work together to maintain blood pH.
2. The excretion of H+ ions and NH3, and reabsorption of bicarbonate ions (HCO3-) is adjusted.
a. If the blood is basic, fewer hydrogen ions are excreted and fewer sodium and bicarbonate ions are reabsorbed.
H+ + HCO3- ⇔ H2CO3 ⇔ H2O + CO2
b. If the blood is acidic, H+ ions are excreted with ammonia, while Na+ and HCO3- ions are reabsorbed; Na+ ions promote formation of hydroxide ions and bicarbonate takes up H+ ions when carbonic acid is formed.
NH3 + H+ → NH4+
c. Ammonia is produced in the tubule cells by deamination of amino acids.
3. Reabsorption or excretion of ions by the kidneys is a homeostatic function that maintains the pH of the blood and osmolarity.
39.1 Evolution of the Nervous System
A. Invertebrate Nervous Organization
1. Comparative study shows the evolutionary steps leading to the centralized nervous system of vertebrates.
2. Even primitive sponges, with only a cellular level of organization, respond by closing the osculum.
3. Hydra (cnidarians) possess two cell layers separated by mesoglea.
a. The hydra can contract, extend, and move tentacles to capture prey and even turn somersaults.
b. A simple nerve net extends throughout the hydra body within the mesoglea.
c. The hydra nerve net is composed of neurons in contact with one another and with contractile epitheliomuscular cells.
d. The more complex cnidaria (sea anemones and jellyfish) may have two nerve nets.
i. A fast-acting nerve net enables major responses, particularly in times of danger.
ii. Another nerve net coordinates slower and more delicate movements.
4. The planarian nervous system is bilaterally symmetrical.
a. It has two lateral nerve cords that allow rapid transfer of information from anterior to posterior.
b. The nervous system of planaria is called a ladderlike nervous system.
c. The nervous system of planaria exhibits cephalization; at their anterior end, planaria have a simple brain composed of a cluster of neurons or ganglia.
d. Cerebral ganglia receive input from photoreceptors in eyespots and sensory cells in auricles.
e. The transverse nerve fibers between the sides of the ladderlike nerve cords keep the movement on both sides of a planarian body coordinated.
f. Bilateral symmetry plus cephalization are important trends in nervous system development.
g. The organization of the planarian nervous system foreshadows both the central and peripheral system of vertebrates.
5. The annelids, arthropods, and molluscs are complex animals with true nervous systems.
a. The nerve cord has a ganglion in each segment of the body that controls muscles of that segment.
b. The brain still receives sensory information and controls the activity of the ganglia so the entire animal is coordinated.
c. The presence of a brain and other ganglia indicate an increased number of neurons among invertebrates.
B. Vertebrate Nervous Organization
1. Vertebrate nervous systems exhibit cephalization and bilateral symmetry.
a. The vertebrate nervous system is composed of both central and peripheral nervous systems.
i. The central nervous system develops a brain and spinal cord from the embryonic dorsal nerve cord.
ii. The peripheral nervous system consists of paired cranial and spinal nerves.
b. Paired eyes, ears, and olfactory structures gather information from the environment.
c. A vast increase in number of neurons accompanied evolution of the vertebrate nervous system; an insect may have one million neurons while vertebrates may contain a thousand to a billion times more.
2. The Vertebrate Brain
a. The vertebrate brain is at the anterior end of the dorsal tubular nerve cord.
b. The vertebrate brain is customarily divided into the hindbrain, midbrain, and forebrain.
i. A well-developed hindbrain regulates organs below a level of consciousness; in humans it regulates lung and heart function even when sleeping; also, it coordinates motor activity.
ii. The optic lobes are part of a midbrain which was originally a center for coordinating reflex responses to visual input.
iii. The forebrain receives sensory input from the other two sections and regulates their output.
iv. The cerebrum is highly developed in mammals and is associated with conscious control; the outer layer, called the cerebral cortex, is large and complex.
C. The Human Nervous System
1. Three specific functions of the nervous system are to:
a. receive sensory input,
b. perform integration, and
c. generate motor output to muscles and glands.
2. The central nervous system (CNS) consists of the brain (in the skull) and the spinal cord (in the vertebral column).
3. The peripheral nervous system (PNS) lies outside the CNS and contains the cranial and spinal nerves.
4. The PNS is divided into the somatic and autonomic systems.
a. The somatic system controls the skeletal muscles.
b. The autonomic system controls the smooth muscles, cardiac muscles, and glands.
5. The CNS and PNS of the human nervous system are connected and work together to perform the functions of a nervous system.
39.2 Nervous Tissue
• Nervous tissue is made up of neurons (nerve cells) and neuroglia (which support and nourishe the neurons).
A. Neurons and Neuroglia
1. Neurons vary in appearance, depending on their function and location, but they all have three parts.
a. The cell body contains the nucleus and other organelles.
b. Dendrites receive information and conduct impulses toward the cell body.
c. A Single axon conducts impulses away from the cell body to stimulate or inhibit a neuron, muscle, or gland.
i. A long axon is called a nerve fiber.
ii. The long axons are covered by a white myelin sheath.
2. Types of Neurons
a. Motor (efferent) neurons have many dendrites and a single axon; they conduct impulses from the CNS to muscles or glands.
b. Sensory (afferent) neurons are unipolar; they conduct impulses from the periphery toward the CNS.
i. The process that extends from the cell body divides into two processes, one going to the CNS and one to periphery.
c. Interneurons (association neurons) are multipolar
i. They have highly-branched dendrites within the CNS.
ii. Interneurons convey messages between the various parts of the CNS.
iii. They form complex brain pathways accounting for thinking, memory, language, etc.
B. Transmission of the Nerve Impulses
1. In 1786, Luigi Galvani discovered that a nerve can be stimulated by an electric current.
2. An impulse is too slow to be due to simply an electric current in an axon.
3. Julius Bernstein (early 1900s) proposed that the nerve impulse is the movement of unequally distributed ions on either side of an axonal membrane, the plasma membrane of an axon.
4. L. Hodgkin and A. F. Huxley later confirmed this theory.
a. They and other researchers inserted a tiny electrode into the giant axon of a squid.
b. The electrode was attached to a voltmeter and an oscilloscope to trace a change in voltage over time.
c. The voltage measured the difference in the electrical potential between the inside and outside of the membrane.
d. An oscilloscope indicated any changes in polarity.
5. Resting Potential
a. When an axon is not conducting an impulse, an oscilloscope records a membrane potential equal to negative 70 mV, indicating that the inside of the neuron is more negative than the outside.
b. This is the resting potential because the axon is not conducting an impulse.
c. This polarity is due to the difference in electrical charge on either side of the axomembrane.
i. The inside of the plasma membrane is more negatively charged than the outside.
ii. Although there is a higher concentration of K+ ions inside the axon, there is a much higher concentration of Na+ ions outside the axon.
iii. The plasma membrane is more permeable to K+ ions, so this gradient is less and the K+ ion potential is less.
iv. The sodium-potassium pump maintains this unequal distribution of Na+ and K+ ions.
d. The sodium-potassium (Na+-K+) pump is an active transport system that moves Na+ ions out and K+ ions into the axon.
e. The pump is always working because the membrane is permeable to these ions and they tend to diffuse toward the lesser concentration.
f. Since the plasma membrane is more permeable to potassium ions than to sodium ions, there are always more positive ions outside; this accounts for some polarity.
g. The large negatively charged proteins in the cytoplasm of the axon also contribute to the resting potential of – 70 mV.
6. Action Potential
a. When an axon conducts a nerve impulse, the rapid change in the membrane potential is the action potential.
b. Protein-lined channels in the axomembrane open to allow either sodium or potassium ions to pass; these are sodium and potassium gated ion channels.
c. The action potential is generated only after the occurrence of a threshold value.
d. The oscilloscope goes from –70 mV to +40 mV in a depolarization phase, indicating the cytoplasm is now more positive than the tissue fluid.
e. The trace returns to –70 mV again in the repolarization phase, indicating the inside of the axon is negative again.
7. Propagation of Action Potentials
a. If an axon is unmyelinated, an action potential stimulates an adjacent axomembrane to produce an action potential.
b. In myelinated fibers, the action potential at one neurofibril node causes action potential at the next node.
i. The myelinated sheath has neurofibril nodes, gaps where one neurolemmocyte ends and the next begins.
ii. The action potential "leaps" from one neurofibril node to another—this is called saltatory conduction.
iii. Saltatory conduction may reach rates of over 100 meters/second, compared to 1 meter/second without it.
c. As each impulse passes, the membrane undergoes a short refractory period before it can open the sodium gates again.
d. The conduction of a nerve impulse is an all-or-nothing event.
e. This ensures a one-way direction to the impulse; during a refractory period, sodium gates cannot open.
C. Transmission Across a Synapse
1. The minute space between the axon bulb and the cell body of the next neuron is the synapse.
2. A synapse consists of a presynaptic membrane, a synaptic cleft, and the postsynaptic membrane.
a. Synaptic vesicles store neurotransmitters that diffuse across the synapse.
b. When the action potential arrives at the presynaptic axon bulb, synaptic vesicles merge with the presynaptic membrane.
c. When vesicles merge with the membrane, neurotransmitters are discharged into the synaptic cleft.
d. The neurotransmitter molecules diffuse across the synaptic cleft to the postsynaptic membrane where they bind with specific receptors.
e. The type of neurotransmitter and/or receptor determines if the response is excitation or inhibition.
f. Excitatory neurotransmitters use gated ion channels and are fast acting.
g. Other neurotransmitters affect the metabolism of the postsynaptic cells and are slower.
3. Neurotransmitters and Neuromodulators
a. At least 100 different neurotransmitters have been identified.
b. Acetylcholine (ACh) and norepinephrine (NE), dopamine, and serotonin are present in both the CNS and the PNS.
i. ACh can have either an excitatory or an inhibitory effect, depending on the tissue.
ii. NE is important to dreaming, waking, and mood.
iii. Dopamine is involved in emotions, learning, and attention.
iv. Serotonin is involved in thermoregulation, emotions, and perception.
c. Once a neurotransmitter is released into a synaptic cleft, it initiates a response and is then removed from the cleft.
d. In some synapses, the postsynaptic membrane contains enzymes that rapidly inactivate the neurotransmitter.
e. Acetylcholinesterase (AChe) breaks down acetylcholine.
f. In other synapses, the presynaptic membrane reabsorbs the neurotransmitter for repackaging in synaptic vesicles or for molecular breakdown.
g. The short existence of neurotransmitters in a synapse prevents continuous stimulation (or inhibition) of postsynaptic membranes.
h. Many drugs that affect the nervous system act by interfering with or potentiating the action of neurotransmitters.
i. Neuromodulators are molecules that block the release of a neurotransmitter or modify a neuron's response to one.
i. Substance P is released by sensory neurons when pain is present; endorphins block the release of substance P and therefore act as natural painkillers.
D. Synaptic Integration
1. A neuron has many dendrites and may have one to ten thousand synapses with other neurons.
2. A neuron receives many excitatory and inhibitory signals.
3. Excitatory neurotransmitters produce a potential change (signal) that drives the neuron closer to an action potential; inhibitory signals produce a signal that drives the neuron further from an action potential.
4. Thus excitatory signals have a depolarizing effect and inhibitory signals have a hyperpolarizing effect.
5. Integration is the summing up of excitatory and inhibitory signals.
a. If a neuron receives many excitatory signals, or at a rapid rate from one synapse, the axon will probably transmit a nerve impulse.
b. If both positive and inhibitory signals are received, the summing may prohibit the axon from firing.
39.3 Central Nervous System: Brain and Spinal Cord
1. The central nervous system (spinal cord and brain) is where sensory impulses are received and motor control is initiated.
2. Both the brain and the spinal cord are protected by bone.
3. Both are wrapped in three connective tissue coverings called meninges; meningitis is a disease caused by many different bacteria or viruses that invade the meninges.
4. The spaces between the meninges are filled with cerebrospinal fluid to cushion and protect the CNS.
5. The cerebrospinal fluid is contained in the central canal of the spinal cord and within the ventricles of the brain.
6. The ventricles are interconnecting spaces that produce and serve as reservoirs for the cerebrospinal fluid.
B. The Spinal Cord
1. The spinal cord has two main functions.
a. It is the center for many reflex actions.
b. It provides the means of communication between the brain and the spinal nerves.
2. The spinal cord is composed of white and gray matter.
a. Gray Matter
i. The unmyelinated cell bodies and short fibers give gray matter its color.
ii. In a cross section, the gray area looks like a butterfly or the letter H.
iii. It contains portions of sensory neurons and motor neurons; short interneurons connect them.
b. White Matter
i. Myelinated long fibers of interneurons run together in tracts and give the white matter its color.
ii. Tracts conduct impulses between the brain and the spinal nerves; ascending tracts are dorsal and descending tracts from the brain are ventral.
iii. Tracts cross over near the brain; therefore the left side of the brain controls the right side of the body.
c. If a spinal cord injury occurs in the cervical region, the condition of quadriplegia (paralysis of all four limbs) results.
d. If the injury is in the thoracic region, the lower limbs may be paralyzed (paraplegia).
C. The Brain
1. The brain has four ventricles: two lateral ventricles and a third and fourth ventricle.
2. The cerebrum is associated with the two lateral ventricles, the diencephalon with the third, and the brain stem and cerebellum with the fourth.
3. The Cerebrum
a. The cerebrum, also called the telencephalon, is the largest part of the brain in humans.
b. It is the last center receiving sensory input and carrying out integration to command motor responses.
c. The cerebrum carries out higher thought processes for learning and memory, language and speech.
d. The right and left cerebral hemispheres (the two halves of the cerebrum) are connected by a bridge of nerve fibers, the corpus callosum; different functions are associated with the two hemispheres.
e. The outer portion is a highly convoluted cerebral cortex consisting of gray matter containing cell bodies and short unmyelinated fibers.
f. The cerebral cortex in each hemisphere contains four surface lobes: the frontal, parietal, occipital, and temporal lobes.
g. Different functions are associated with each lobe.
h. The cerebral cortex contains motor, sensory, and association areas.
i. The human hand takes up a large proportion of the primary motor area.
ii. The ventral to the primary motor area is a premotor area that organizes motor functions before the primary area sends signals to the cerebellum.
iii. The left frontal lobe has Broca's area for our ability to speak.
iv. Sensory information from the skin and skeletal muscles arrives at a primary somatosensory area.
v. The primary visual area in the occipital lobe receives information from the eyes; a visual association area associates new visual information with old information.
vi. The primary auditory area in the temporal lobe receives information from our ears.
vii. The primary taste area is in the parietal lobe.
viii. The somatosensory association area processes and analyzes sensory information from skin and muscles.
ix. A general interpretation area receives information from all of the sensory association areas and allows us to quickly integrate signals and send them to the prefrontal area for immediate response.
x. The prefrontal area in the frontal lobe receives input from other association areas and reasons and plans.
i. White Matter
i. White matter in the CNS consists of long myelinated axons organized into tracts.
ii. Descending tracts from the primary motor area communicate with lower brain centers.
iii. Ascending tracts from lower brain centers send sensory information up to the primary somatosensory area.
iv. These tracts cross over near the brain; therefore the left side of the brain controls the right side of the body.
j. Basal Nuclei
i. Aside from the tracts, there are masses of gray matter located deep within the white matter.
ii. These basal nuclei integrate motor commands; malfunctions cause Huntingdon and Parkinson disease.
4. The Diencephalon
a. The hypothalamus and thalamus are in a portion of the brain known as the diencephalon, where the third ventricle is located.
b. The hypothalamus forms the floor of the third ventricle.
c. The hypothalamus maintains homeostasis.
i. It is an integrating center that regulates hunger, sleep, thirst, body temperature, water balance, and blood pressure.
ii. It controls the pituitary gland and thereby serves as a link between the nervous and endocrine systems.
d. The thalamus consists of two masses of gray matter in the sides and roof of the third ventricle.
i. It is the last portion of the brain for sensory input before the cerebrum.
ii. It is a central relay station for sensory impulses traveling up from the body or from the brain to the cerebrum.
iii. Except for smell, it channels sensory impulses to specific regions of the cerebrum for interpretation.
e. The pineal gland, which secretes the melatonin hormone, is in the diencephalon.
5. The Cerebellum
a. The cerebellum is separated from the brain stem by the fourth ventricle.
b. The cerebellum is in two portions joined by a narrow median portion.
c. The cerebellum integrates impulses from higher centers to coordinate muscle actions, maintain equilibrium and muscle tone, and sustain normal posture.
d. It receives information from the eyes, inner ear, muscles, etc. indicating body position, integrates the information and sends impulses to muscles maintaining balance.
e. The cerebellum assists in the learning of new motor skills, as in sports or playing the piano; it may be important in judging the passage of time.
6. The Brain Stem
a. The brain stem contains the medulla oblongata, pons, and midbrain.
b. Besides acting as a relay station for tracts passing between the cerebrum and spinal cord or cerebellum, the midbrain has reflex centers for visual, auditory, and tactile responses.
c. The pons ("bridge") contains bundles of axons traveling between the cerebellum and rest of the CNS.
i. The pons functions with the medulla to regulate the breathing rate.
ii. It has reflex centers concerned with head movements in response to visual or auditory stimuli.
d. The medulla oblongata lies between the spinal cord and the pons, anterior to the cerebellum.
i. It contains vital centers for regulating heartbeat, breathing, and vasoconstriction.
ii. It contains reflex centers for vomiting, coughing, sneezing, hiccuping, and swallowing.
iii. It contains nerve tracts that ascend or descend between the spinal cord and the brain's higher centers.
7. The Limbic System
a. The limbic system is a complex network of tracts and nuclei that incorporate medial portions of cerebral lobes, subcortical nuclei, and diencephalon.
b. It blends higher mental functions and primitive emotions.
c. Its two major structures are the hippocampus and amygdala, essential for learning and memory.
i. The hippocampus makes prefrontal area aware of past experiences stored in association areas.
ii. The amygdala causes experiences to have emotional overtones.
iii. Inclusion of the frontal lobe in the limbic system allows reasoning to keep us from acting out strong feelings.
d. Learning and Memory
i. Memory is the ability to hold thoughts in the mind and to recall past events.
ii. Learning takes place when we retain and utilize past memories.
iii. The prefrontal area in the frontal lobe is active in short-term memory (e.g., telephone numbers).
iv. Long-term memory is a mix of semantic memory (numbers, words) and episodic memory (persons, events).
v. Skill memory is the ability to perform motor activities.
vi. The hippocampus serves as a go-between to bring memories to mind.
vii. The amygdala is responsible for fear conditioning and associates danger with sensory stimuli.
viii. Long-term potentiation (LTP) is an enhanced response at synapses within the hippocampus.
ix. LTP is essential to memory storage; excited postsynaptic cells may die due to a glutamate neurotransmitter.
x. Extinction of too many cells in the hippocampus is the underlying cause of Alzheimer disease.
39.4 Peripheral Nervous System
1. The peripheral nervous system lies outside the CNS.
a. Cranial nerves connect to the brain.
b. Spinal nerves lie on either side of the spinal cord.
2. Axons in nerves are called nerve fibers.
3. The cell bodies of neurons are found in the CNS or in ganglia.
4. Ganglia are collections of cell bodies in the PNS.
5. Humans have 12 pairs of cranial nerves attached to the brain.
a. Sensory nerves only contain sensory nerve fibers.
b. Motor nerves only contain motor nerve fibers.
c. Mixed nerves contain both sensory and motor nerve fibers.
d. Cranial nerves mostly connect to the head, neck, and facial regions.
e. The vagus nerve also branches to the pharynx, larynx, and some internal organs.
6. Humans have 31 pairs of spinal nerves emerging from the spinal cord.
a. The paired spinal nerves leave the spinal cord by two short branches, or roots.
b. The dorsal root contains fibers of sensory neurons conducting nerve impulses to the spinal cord; the cell body of a sensory neuron is in the dorsal root ganglion.
c. The ventral root contains the axons of motor neurons that conduct nerve impulses away from the spinal cord.
d. All spinal nerves are mixed nerves that conduct impulses to and from the spinal cord.
e. Spinal nerves are mixed nerves with sensory and motor fibers; each serves its own region.
B. Somatic System
1. The somatic system includes the nerves that carry sensory information to the CNS and motor commands away from the CNS to skeletal muscles.
2. Any voluntary control of muscles involves the brain; reflexes, involuntary responses to stimuli, involve the brain or just the spinal cord.
3. Outside stimuli can initiate reflex actions, some of which involve the brain.
C. The Reflex Arc
1. Reflexes are automatic, involuntary responses.
2. A reflex arc involves the following pathway:
a. Sensory receptors generate an impulse in a sensory neuron that moves along sensory axons toward the spinal cord.
b. Sensory neurons enter the cord dorsally and pass signals to interneurons.
c. Impulses travel along motor axons to an effector, which brings about a response to the stimulus.
d. The immediate response is that muscles contract to withdraw from source of pain.
3. Reflex response occurs because the sensory neuron stimulates several interneurons.
4. Some impulses extend to the cerebrum, which makes a person conscious of the stimulus and the reaction.
D. Autonomic System
1. The autonomic system is a part of the PNS and regulates cardiac and smooth muscle and glands.
2. There are two divisions: the sympathetic and parasympathetic systems.
a. Both function automatically and usually in an involuntary manner.
b. Both innervate all internal organs.
c. Both utilize two neurons and one ganglion for each impulse.
i. The first neuron has a cell body within the CNS and a preganglionic fiber.
ii. The second neuron has a cell body within the ganglion and a postganglionic fiber.
d. Breathing rate and blood pressure are regulated by reflex actions to maintain homeostasis.
3. Sympathetic Division
a. Most preganglionic fibers of the sympathetic system arise from the middle (thoracic-lumbar) portion of the spinal cord and almost immediately terminate in ganglia that lie near the cord (thoracic-lumbar portion).
b. Therefore the preganglionic fiber is short, but the postganglionic fiber that contacts an organ is long.
c. The sympathetic system is especially important during emergency situations (the "fight or flight" response).
d. To defend or flee, muscles need a supply of glucose and oxygen; the sympathetic system accelerates heartbeat, and dilates bronchi.
e. To divert energy from less necessary digestive functions, the sympathetic system inhibits digestion.
f. The neurotransmitter released by the postganglionic axon is mainly norepinephrine, similar to epinephrine (adrenaline) used as a heart stimulant.
4. Parasympathetic Division
a. The parasympathetic system consists of a few cranial nerves, including the vagus nerve, and fibers that arise from the bottom craniosacral portion of the spinal cord.
b. In this case, the preganglionic fibers are long and the postganglionic fibers are short.
c. This system is a "housekeeper system"; it promotes internal responses resulting in a relaxed state.
d. The parasympathetic system causes the eye pupil to constrict, promotes digestion, and retards heartbeat.
e. The neurotransmitter released is acetylcholine.
40.1 Chemical Senses
A. Chemoreceptors are responsible for taste and smell by being sensitive to chemicals in food, liquids, and air.
1. Chemoreception is found universally in animals; it is thought to be the most primitive sense.
2. Chemoreceptors are present all over a planarian but concentrated in the auricles at the side of the head.
3. Insects, such as houseflies, taste with their feet.
4. Crustacea have chemoreceptors on their antennae and appendages.
5. In amphibians, chemoreceptors are located in the nose, mouth, and all over the skin.
6. In mammals, receptors for taste are in the mouth, and receptors for smell are in the nose.
B. Sense of Taste
1. Human taste buds are located primarily on the tongue.
2. Many lie along the walls of papillae, the small elevations on the surface of the tongue.
3. Isolated ones are present on the surfaces of the hard palate, pharynx, and epiglottis.
4. Taste buds are embedded in tongue epithelium and open at a taste pore.
5. Taste buds have supporting cells and elongated taste cells that end in microvilli.
6. Microvilli bear receptor proteins for certain chemicals.
a. Molecules bind to receptor proteins and impulses are generated in associated sensory nerves
b. Nerve impulses go to the brain cortical areas which interpret them as tastes.
7. Humans have four primary types of taste.
a. Taste buds for each are concentrated in particular regions.
i. Sweet receptors are most plentiful near the tip of the tongue.
ii. Sour receptors occur primarily along the margins of the tongue.
iii. Salty receptors are most common on the tip and upper front portion.
iv. Bitter receptors are located near the back of the tongue.
v. A fifth type, called umami, may exist for certain flavors (cheese, beef broth, seafood).
b. The brain appears to take an overall "weighted average" of taste messages as the perceived taste.
C. Sense of Smell
1. The sense of smell depends on olfactory cells located in the olfactory epithelium high in the roof of the nasal cavity.
2. Olfactory cells are modified neurons.
3. Each cell has a tuft of five olfactory cilia that bear receptor proteins for an odor molecule.
a. There are around 1,000 different types of odor receptors; many olfactory cells carry the same type.
b. Nerve fibers from like olfactory cells lead to the same neuron in the olfactory bulb.
c. An odor activates a characteristic combination of cells; this information is pooled in the olfactory bulb.
d. Interneurons communicate this information via the olfactory tract to areas of the cerebral cortex.
4. Olfactory bulbs are directly connected with the limbic system; smells associate with emotions and memory.
5. Taste and smell supplement each other.
a. "Smelling" food also involves the taste receptors.
b. Losing taste when you have a cold is usually due to a loss of smell.
40.2 Sense of Vision
A. Animals lacking photoreceptors, sensory receptors sensitive to light, depend on their senses of hearing and smell rather than sight.
B. Photoreceptors vary in complexity.
1. In its simplest form, a photoreceptor indicates only the presence of light and its intensity.
2. "Eyespots" of planaria allow flatworms to determine direction of light.
3. Image-forming eyes occur in four invertebrate groups: cnidaria, annelids, molluscs, and arthropods.
4. Arthropods have compound eyes composed of many independent visual units (ommatidia), each possessing all of the elements needed for light reception.
a. The cornea and crystalline cone of each visual unit focus rays toward the photoreceptors.
b. Photoreceptors generate nerve impulses, which pass to the brain by way of optic nerve fibers.
c. The image resulting from all stimulated visual units is crude; the small size of compound eyes limits the number of visual units.
d. Insects have color vision but utilize a narrower range of the electromagnetic spectrum and can see some ultraviolet.
5. Some fishes, reptiles, and most birds are believed to have color vision, but among mammals, only humans and other primates have color vision; this is adaptive for day activity.
6. Vertebrates and certain molluscs (e.g., the squid and the octopus) have a camera-type eye.
a. Molluscs and vertebrates are not closely related; therefore this is convergent evolution.
b. A single lens focuses an image of the visual field on closely packed photoreceptors.
c. In vertebrates the lens changes shape to aid in focusing; in molluscs the lens move back and forth.
d. The human eye is considerably more complex than a camera.
7. Animals with two eyes facing forward have three-dimensional, or stereoscopic, vision.
8. Animals with eyes facing sideways (e.g., rabbits) have panoramic vision—the visual field is wide.
C. The Human Eye
1. The human eye is an elongated sphere 2.5 cm in diameter with three layers.
2. The sclera is the outer, white fibrous layer that covers most of the eye; it protects and supports the eyeball.
3. The cornea is a transparent part of the sclera at the front of the eye; it is the window of the eye.
4. The conjunctiva is a thin layer of epithelial cells that covers the sclera and keeps the eyes moist.
5. The middle, thin, dark-brown layer is the choroid containing many blood vessels and pigments absorbing stray light rays.
6. To the front of the eye, the choroid thickens and forms a ring-shaped ciliary body and finally becomes the iris that regulates the size of an opening called the pupil.
7. The lens divides the cavity into two portions: aqueous humor fills the anterior cavity and vitreous humor fills the posterior.
8. Retina
a. The inner layer is the retina that contains photoreceptors called rod cells and cone cells.
b. The fovea centralis is a small area of retina that contain only cones; this area produces acute color vision in daylight.
c. Cone cells are not very sensitive in low intensity light; at night, the rods are still active.
9. Focusing of the Eye
a. Light rays enter the eye through the pupil and are focused on the retina.
b. Focusing involves light passing through the cornea, the lens, and the humors.
c. Because of refraction, the image on the retina is inverted 180° from actual but is perceived righted in the brain.
d. The shape of the lens is controlled by the ciliary muscle.
e. The lens is flatter when the ciliary muscle is relaxed when we are viewing distant objects.
f. The lens is naturally elastic and becomes rounder for viewing near objects where light rays must bend to a greater degree.
g. This change is called visual accommodation.
h. An aging lens loses its ability to accommodate for near objects and we may need reading glasses by middle age.
i. The lens is also subject to cataracts, or becoming opaque; surgery, using a cryoprobe, is the only current treatment.
j. Persons who can see well close up but not far away are nearsighted (myopia).
i. They often have an elongated eyeball that focuses a distant image in front of the retina.
ii. They can wear corrective concave lenses to refocus the image on the retina.
iii. Radial keratotomy is a new treatment that surgically cuts and flattens the cornea.
k. Persons who can see far away but not up close are farsighted (hyperopia)
i. They often have a shortened eyeball that focuses near images behind retina.
ii. They can wear corrective convex lenses to refocus the image on retina.
l. When the cornea or lens is uneven, the image is fuzzy; this is astigmatism corrected by an unevenly ground lens to compensate for unevenness.
D. Photoreceptors of the Eye
1. Vision begins when light has been focused on photoreceptors in the retina.
2. Rod cells and cone cells have an outer segment joined to an inner segment by a stalk.
3. The outer segment contains stacks of membranous disks (lamellae) with many molecules of rhodopsin.
4. The rhodopsin molecules contain a protein opsin and the pigment molecule retinal derived from Vitamin A.
5. When a rod absorbs light, rhodopsin splits into opsin and retinal, leading to a cascade of reactions and the closure of ion channels in the rod cell plasma membrane.
6. This stops the release of inhibitory molecules from the rod's synaptic vesicles and starts signals that result in impulses to brain.
7. The rods are stimulated by low light and provide night vision.
8. Because rods are distributed throughout the retina, rods detect our peripheral vision and motion but not color or detail.
9. Cones located primarily in the fovea centralis are activated by bright light and detect detail and color.
10. The three kinds of cones contain either blue, green, or red pigment.
11. Each pigment is composed of retinal and opsin, but the structure of opsin varies among the three.
12. Combinations of cones are stimulated by intermediate colors; the combined nerve impulses are interpreted in the brain.
E. Integration of Visual Signals in the Retina
1. The retina has three layers of neurons.
a. The rods and cones are nearest the choroid.
b. Bipolar cells form the middle layer.
c. Ganglion cells, whose fibers become the optic nerve, form the innermost layer.
2. Since only rod and cone cells are sensitive to light, light must penetrate through the ganglion cells.
3. Rods and cones synapse with bipolar cells which pass the impulse to ganglion cells.
4. There are more rods and cones than nerve fibers leaving ganglionic cells.
5. Up to 150 rods may synapse with a ganglion cell; this results in indistinct vision.
6. Each cone synapses with one ganglionic cell; this accounts for the detailed images of cones, mostly found in the fovea.
7. Integration occurs as signals pass from the bipolar to the ganglion cells.
a. If all rod cells in a receptive field are stimulated, the ganglion cell is weakly stimulated or neutral.
b. If only the center is lit, it is stimulated; if only the edge is lit, it is inhibited.
c. Therefore considerable processing occurs in the retina before an impulse is sent to the brain.
8. The blind spot is an area where the optic nerve passes through the retina; it lacks rods and cones.
40.3 Senses of Hearing and Balance
1. The ear has two sensory functions: hearing and balance (equilibrium).
2. The sensory receptors for both are in the inner ear, and each consists of hair cells with stereocilia that are sensitive to mechanical stimulation; they are mechanoreceptors.
B. Anatomy of the Ear
1. The human ear has three divisions: an outer, middle, and inner ear.
2. The outer ear consists of the pinna (external flap) and the auditory canal.
a. The auditory canal opening is lined by fine hairs that filter air.
b. Modified sweat glands in the auditory canal secrete earwax to guard against foreign matter.
3. The middle ear begins at the tympanic membrane and ends at a bony wall with membrane-covered openings (the oval window and the round window).
a. It contains small bones called ossicles: malleus (hammer), incus (anvil), and stapes (stirrup).
b. The malleus adheres to the tympanum; the stapes touches the oval window.
c. The auditory (eustachian) tube extends from the middle ear to the pharynx to equalize the inside and outside air.
4. The inner ear has three regions: the semicircular canals, vestibule, and cochlea.
5. The cochlea resembles a snail shell because it spirals.
C. Process of Hearing
1. The process of hearing begins when sound waves enter the auditory canal, causing the ossicles to vibrate.
2. Sound is amplified about 20 times by the size difference between the tympanic membrane and the oval window.
3. The stapes strikes the membrane of the oval window, passing pressure waves to the fluid in the cochlea.
4. Three canals are located within the cochlea: vestibular canal, cochlear canal, and tympanic canal.
5. The vestibular canal connects with the tympanic canal, which leads to the oval window membrane.
6. Along the basilar membrane are hair cells whose stereocilia are embedded in a tectorial membrane.
7. The hair cells of the spiral organ (organ of Corti) synapse with nerve fibers of the cochlear (auditory) nerve.
8. When the stapes strikes the membrane of the oval window, pressure waves move from the vestibular canal to the tympanic canal and across the basilar membrane, and the round window bulges.
9. The basilar membrane vibrates up and down bending the stereocilia of hair cells embedded in the tectorial membrane.
10. This generates nerve impulses in the cochlear nerve that travel to the brain stem.
11. When they reach the auditory areas of the cerebral cortex, this is interpreted as sound.
12. The spiral organ is narrow at its base and widens at its tip; each part is sensitive to different pitches.
13. Nerve fibers from each region (high pitch base or low pitch tip) lead to slightly different regions of the brain, producing the sensation of pitch.
14. Sound volume is caused by more vibration; the increased stimulation is interpreted as louder sound intensity.
15. Tone is an interpretation by the brain based on the distribution of the hair cells stimulated.
D. Sense of Balance
1. The sense of balance is divided into:
a. rotational equilibrium (angular or rotational movement of the head), and
b. gravitational equilibrium (vertical or horizontal movement).
2. Rotational equilibrium utilizes the semicircular canals.
a. The semicircular canals are oriented at right angles to one another in three different planes.
b. The enlarged base of each semicircular canal is called an ampulla.
c. Fluid flowing over and displacing a cupula causes the stereocilia of the hair cells to bend; the pattern of impulses carried by the vestibular nerve to the brain changes.
d. Continuous movement of the fluid in the semicircular canals causes vertigo motion sickness.
e. By spinning and stopping, we see a room still spin; this indicates that vision is also involved in balance.
3. Gravitational equilibrium utilizes the utricle and saccule.
a. A vestibule or space between the semicircular canals and the cochlea contains the utricle and the saccule.
b. The utricle and saccule are small membranous sacs, each of which contains hair cells.
c. Hair cell stereocilia are embedded within a gelatinous material called the otolithic membrane.
d. Calcium carbonate granules (otoliths) rest on this membrane.
e. The utricle is sensitive to horizontal movements; the saccule responds best to up-down movements.
f. When the head is still, otoliths in the utricle and saccule rest on the otolithic membrane above the hair cells.
g. As the head bends or the body moves, otoliths are displaced and the otolithic membrane sags, bending the larger stereocilia (kinocillium) of hair cells beneath; this tells the brain the direction of movement.
E. Sensory Receptors in Other Animals
1. The lateral line system of fish and amphibians detects water currents and pressure waves.
2. Primitive fishes have the system on the surface; advanced fishes enclose it in a canal on the side.
3. The lateral line receptor is a collection of hair cells with cilia embedded in a mass of gelatinous material (cupula).
4. Static equilibrium organs called statocysts are found in cnidaria, molluscs, and crustacea.
5. A small particle called a statolith stimulates cilia that generate impulses, indicating the position of the head.
41.1 Diversity of Skeletons
1. Three types of skeletons occur in the animal kingdom.
2. A hydrostatic skeleton occurs in cnidarians, flatworms, roundworms and annelids.
3. An exoskeleton is found in molluscs and arthropods, respectively.
4. An endoskeleton is found in sponges, echinoderms, and vertebrates.
B. Hydrostatic Skeleton
1. A fluid-filled gastrovascular cavity or coelom can act as a hydrostatic skeleton.
2. It offers support and resistance to the contraction of muscles for motility.
3. Many animals have hydroskeletons.
a. Hydras use a fluid-filled gastrovascular cavity to support tentacles that rapidly contract.
b. Planaria easily glide over substrate with muscular contractions of body walls and cilia.
c. Roundworms have a fluid-filled pseudocoelom and move when their longitudinal muscles contract against it.
d. Earthworms are segmented with septa dividing the coelom into compartments; circular and longitudinal muscles contract in each segment to coordinate elongation and contraction.
e. Animals with exoskeletons or endoskeletons move selected body parts by means of muscular hydrostats, i.e., fluid contained within certain muscle fibers assists movement of that part.
C. Exoskeletons and Endoskeletons
1. An exoskeleton is an external skeleton.
a. Molluscs have exoskeletons that are predominantly calcium carbonate (CaCO3).
b. Insects and crustacea have jointed exoskeletons composed of chitin, a strong, flexible, nitrogenous polysaccharide.
c. The exoskeleton provides protection against damage frm enemies and also keeps tissues from drying out.
d. Although stiffness provides support for muscles, the exoskeleton is not as strong as an endoskeleton.
e. The clam and snail exoskeletons grow with the animals; their thick nonmobile CaCO3 shell is for protection.
f. The chitinous exoskeleton of arthropods is jointed and moveable.
g. Arthropods must molt when their exoskeleton becomes too small; a molting animal is vulnerable to predators.
2. Vertebrates have an endoskeleton composed of bone and cartilage that grows with the animal.
a. The endoskeleton does not limit the space available for internal organs and it can support greater weight.
b. Soft tissues surround the endoskeleton to protect it; injuries to soft tissue are easier to repair.
c. Usually an endoskeleton has elements that protect vital internal organs.
d. The jointed exoskeleton of arthropods and endoskeletons of vertebrates allow flexibility and helped arthropods and vertebrates colonize land.
41.2 The Human Skeletal System
1. Skeletons protect organs: skull (brain), vertebral column (spinal cord), and rib cage (heart and lungs).
2. The large, heavy leg bones support the body against the pull of gravity.
3. Leg and arm bones permit flexible body movement.
4. The flat bones of the skull, ribs, and breastbone contain red bone marrow that manufactures blood cells.
5. All bones store inorganic calcium and phosphorous salts.
B. Bone Growth and Renewal
1. The prenatal human skeleton is cartilaginous; cartilage structures serve as "models" for bone construction.
a. The cartilaginous models are converted to bones when calcium salts are deposited in the matrix, first by cartilaginous cells and later by bone-forming cells called osteoblasts.
b. Conversion of cartilaginous models to bones is called endochondral ossification.
c. Some bones (e.g., facial bones) are formed without a cartilaginous model.
2. During endochondral ossification, there is a primary ossification center at the middle of a long bone; latter secondary centers form at the ends.
3. A cartilaginous growth plate occurs between primary and secondary ossification centers.
4. As long as the growth plate remains between the two centers, bone growth occurs.
5. The rate of growth is controlled by hormones, including growth hormone (GH) and sex hormones.
6. Eventually plates become ossified and bone stops growing; this determines adult height.
7. In adults, bone is continually being broken down and built up again.
a. Bone-absorbing cells (osteoclasts) break down bone, remove worn cells, and deposit calcium in the blood.
b. Osteoblasts form new bone, taking calcium from the blood.
c. Osteoblasts become entrapped in the bone matrix and become osteocytes in the lacunae of osteons.
d. This continual remodeling allows bones to gradually change in thickness.
e. Osteoclasts also determine the calcium level in the blood; calcium level is important for muscle contraction and nerve conduction and levels are controlled by the hormones PTH and calcitonin.
8. Adults need more calcium in the diet than do children to promote the work of osteoblasts.
C. Anatomy of a Long Bone
1. A long bone illustrates the principles of bone anatomy.
a. A long bone consists of a central medullary cavity surrounded by compact bone.
b. Ends are composed of spongy bone surrounded by a thin layer of compact bone and covered with hyaline cartilage.
c. Compact bone contains many osteons (Haversian systems); bone cells in tiny chambers (lacunae) are arranged in concentric circles around central canals.
d. Central canals contain blood vessels and nerves.
e. The lacunae are separated by a matrix that contains protein fibers of collagen and mineral deposits.
2. Spongy bone has numerous plates and bars separated by irregular spaces.
a. Spongy bone is lighter but designed for strength; solid portions of bone follow the lines of stress.
b. Bone spaces are often filled with red bone marrow, a specialized tissue that produces blood cells.
D. The axial skeleton lies at the midline of the body and consists of the skull, vertebral column, sternum and ribs.
1. The Skull
a. The skull is formed by the cranium and the facial bones.
b. Newborns have membranous junctions called fontanels that usually close by the age of two.
c. The bones of the cranium contain sinuses, air spaces lined with mucous membrane that reduce the weight of skull and give a resonant sound to the voice.
d. Two mastoid sinuses drain into the middle ear; mastoiditis is an inflammation that can lead to deafness.
e. The cranium is composed of eight bones: a frontal, two parietal, an occipital, two temporal, a sphenoid, and an ethmoid.
f. The spinal cord passes through the foramen magnum, an opening at the base of the skull in the occipital bone.
g. Each temporal bone has an opening that leads to the middle ear.
h. The sphenoid bone completes the sides of the skull and forms the floors and walls of the eye sockets.
i. The ethmoid bone is in front of the sphenoid, part of the orbital wall, and a component of the nasal septum.
j. Fourteen facial bones include: mandible, two maxillae, two palatine, two zygomatic, two lacrimal, two nasal, and vomer.
k. The mandible or lower jaw is the only movable portion of the skull; it contains tooth sockets.
l. The maxilla forms the upper jaw and the anterior of the hard palate; it also contains tooth sockets.
m. The palatine bones make up the posterior portion of the hard palate and the floor of the nasal cavity.
n. The zygomatic gives us our cheekbone prominences.
o. Nasal bones form the bridge of the nose.
p. Other bones make up the nasal septum which divides the nose cavity into two regions.
q. The ears are elastic cartilage and lack bone; the nose is a mixture of bone, cartilage, and fibrous connective tissue.
2. The Vertebral Column and Rib Cage
a. The vertebral column supports the head and trunk and protects the spinal cord and the roots of the spinal nerves.
b. The vertebral column serves as an anchor for all of the other bones of the skeleton.
c. Seven cervical vertebrae are located in the neck.
d. Twelve thoracic vertebrae are in the thorax or chest.
e. The lumbar vertebrae are in the small of the back.
f. One sacrum is formed from five fused sacral vertebrae.
g. One coccyx is formed from four fused coccygeal vertebrae.
h. Normally, the spinal column has four normal curvatures that provide strength and resiliency in posture.
i. Scoliosis is an abnormal sideways curvature; hunchback and swayback are also abnormal.
j. Intervertebral disks between the vertebrae act as a padding to prevent the vertebrae from grinding against each other, and to absorb shock during running, etc.; they weaken with age.
k. Vertebral disks allow motion between vertebrae for bending forward, etc.
l. The rib cage: all twelve pairs of ribs connect directly to the thoracic vertebrae in back; seven attach directly to the sternum.
i. Three pairs connect via cartilage to the sternum at front.
ii. The two ribs totally unattached to the sternum are called "floating ribs."
iii. The rib cage protects the heart and lungs, yet is flexible enough to allow breathing.
3. The Appendicular Skeleton
a. The appendicular skeleton consists of the bones within the pectoral girdle and upper limbs and the pelvic girdle and lower limbs.
b. The pectoral girdle and arms are specialized for flexibility; the pelvic girdle and legs is built for strength.
c. The components of the pectoral girdle are only loosely linked by ligaments.
i. The clavicle ("collarbone") connects with the sternum in front and the scapula ("shoulderblade") in back.
ii. The scapula connects with the clavicle; it is freely movable and held in place only by muscles.
d. The humerus is the long bone of the upper arm; its smoothly rounded head fits into a socket of the scapula.
e. The radius is the more lateral of the bones of the lower arm; it articulates with the humerus at the elbow joint, a hinge joint, and the radius crosses in front of the ulna for easy twisting.
f. The ulna is the more medial of the two bones of the lower arm; its end is the prominence in your elbow.
g. The many hand bones increase its flexibility.
i. The wrist has eight carpal bones which look like small pebbles.
ii. Five metacarpal bones fan out to form the framework of the palm.
iii. The phalanges are the bones of fingers and thumb.
h. The pelvic girdle consists of two heavy, large coxal (hip) bones.
i. The coxal bones are anchored to the sacrum; together with the sacrum they form a hollow cavity that is wider in females than in males; it transmits weight from the vertebral column via the sacrum to the legs.
ii. The femur is the largest bone of the body; it is limited in the amount of weight that it can support.
iii. The tibia has a ridge called the "shin"; its end forms the inside of the ankle.
iv. The fibula is the smaller of the two bones; its end forms the outside of the ankle.
v. Seven tarsal bones are in each ankle; one receives the weight and passes it to the heel and ball of foot.
vi. The metatarsal bones form the arch of the foot and provide a springy base.
vii. The phalanges are the bones of the toes, which are stouter than the fingers.
E. Classification of Joints
1. Bones are joined at joints that are classified as fibrous, cartilaginous, or synovial.
2. Fibrous joints, such as those between the cranial bones, are immovable.
3. Cartilaginous joints, such as those between the vertebrae, are slightly moveable; the two hipbones are slightly movable because they are ventrally joined by cartilage and respond to pregnancy hormones.
4. Synovial joints are freely movable.
a. Most joints are synovial joints, with the two bones separated by a cavity.
b. Ligaments are fibrous connective tissue that bind bone to bone, forming a joint capsule.
c. In a "double-jointed" individual, the ligaments are unusually loose.
d. The joint capsule is lined with a synovial membrane that produces a lubricating synovial fluid.
e. The knee represents a synovial joint.
i. Knee bones are capped by cartilage; a crescent-shaped piece of cartilage, the meniscus, is between the bones.
ii. Athletes who injure the meniscus have torn this cartilage.
iii. The knee joint also contains 13 fluid-filled sacs called bursae to ease friction between the tendons and ligaments and tendons and bones.
iv. Inflammation of the bursae is bursitis; "tennis elbow" is a form of bursitis.
v. The knee and elbow are hinge joints; the shoulder and hip are ball-and-socket joints.
f. Synovial Joints
i. Synovial joints are subject to arthritis.
ii. In rheumatoid arthritis, the synovial membrane becomes inflamed and thickened.
iii. The joint degenerates and becomes immovable and painful.
iv. This is likely caused by an autoimmune reaction.
v. In osteoarthritis from old age, the cartilage at the ends of bones disintegrates; the bones then become rough and irregular.
41.3 The Human Muscular System
1. Skeletal muscle contraction assists homeostasis by helping maintain constant body temperature.
2. Skeletal muscle contraction also causes ATP breakdown, releasing heat that is distributed about the body.
B. Macroscopic Anatomy and Physiology
1. Skeletal muscles are attached to the skeleton by tendons made of fibrous connective tissue.
2. When muscles contract, they only shorten or pull; therefore, skeletal muscles must work in antagonistic pairs.
a. One muscle of an antagonistic pair bends the joint and brings a limb toward the body.
b. The other one straightens the joint and extends the limb.
3. If a muscle is given a rapid series of stimuli, it responds to the next stimulus before completely relaxing.
4. Muscle contraction summates until it reaches a maximal sustained contraction, called tetanus.
5. Even at rest, muscles maintain tone by some fibers contracting; this is essential to maintaining posture.
C. Microscopic Anatomy and Physiology
1. A whole skeletal muscle consists of a number of muscle fibers in bundles.
2. Each muscle fiber is a cell with some special features.
a. A plasma membrane called the sarcolemma forms a T (transverse) system.
i. Transverse (T) tubules penetrate down into the cell and contact with, but do not fuse with, the modified endoplasmic reticulum (the sarcoplasmic reticulum).
ii. Expanded portions or sacs of the sarcoplasmic reticulum are modified for Ca2+ ion storage; this encases hundreds and sometimes thousands of myofibrils.
b. The myofibrils are contractile portions of fibers that lie parallel and run the length of the fiber.
c. A light microscope shows light and dark bands called striations.
d. An electron microscope shows that these striations of myofibrils are formed by placement of protein filaments within sarcomeres.
e. The two protein filaments are either thick (made of myosin) or thin (made of actin).
f. A sarcomere has repeating bands of actin and myosin that occur between two Z lines in a myofibril.
i. The I band contains only actin filaments.
ii. The H zone contains only myosin filaments.
3. Sliding Filament Model
a. As a muscle fiber contracts, sarcomeres within the myofibrils shorten.
b. As a sarcomere shortens, actin filaments slide past the myosin; the I band shortens and the H zone disappears.
c. Sliding filament model: actin filaments slide past myosin filaments because myosin filaments have cross-bridges that pull actin filaments inward, toward their Z line.
d. The contraction process involves the sarcomere shortening although the filaments themselves remain the same length.
e. ATP supplies the energy for muscle contraction.
f. Myosin filaments break down ATP to form cross-bridges that attach to and pull the actin filaments toward the center of the sarcomere.
4. ATP
a. Muscle cells contain myoglobin that stores oxygen; cellular respiration does not immediately supply all of the ATP needed.
b. Muscle fibers rely on a supply of stored creatine phosphate (phosphocreatine), a storage form of high-energy phosphate.
c. Creatine phosphate does not directly participate in muscle contraction but regenerates ATP rapidly: creatine — P + ADP → ATP + creatine
d. This reaction occurs in the midst of sliding filaments and is speedy.
e. When all creatine phosphate is depleted, and if O2 is in limited supply, fermentation produces a small amount of ATP, but this results in a buildup of lactate.
f. The buildup of lactate partially accounts for muscle fatigue and represents oxygen debt.
g. Lactate is transported to the liver; 20% is completely broken down to CO2 and H2O in aerobic respiration.
h. The ATP gained from this respiration is then used to reconvert 80% of the lactate to glucose.
i. In persons who train, the number of mitochondria increases, reducing the need for fermentation.
D. Muscle Innervation
1. Muscles are stimulated to contract by motor nerve fibers.
2. The neuromuscular junction is a region where an axon bulb is in close association with the sarcolemma of a muscle fiber.
3. An axon bulb contains synaptic vesicles filled with the neurotransmitter acetylcholine.
4. When nerve impulses travel down a motor neuron to the axon bulb, vesicles merge with the presynaptic membrane and acetylcholine molecules are released into the synaptic cleft.
5. Acetylcholine rapidly diffuses to and binds with receptors on the sarcolemma.
6. The sarcolemma generates impulse spreading down the T tubule system to the sarcoplasmic reticulum where it triggers the release of Ca2+ ions out amongst the myofilaments.
7. The Ca2+ ions then initiate muscle contraction.
8. Ca2+ ions bind to troponin, which causes tropomyosin threads to shift position.
9. The change in the structure of tropomyosin exposes the myosin heads with ATP binding sites.
10. The myosin heads function as ATPase enzymes, splitting ATP into ADP and Pi .
11. After attaching to actin filaments, the myosin cross-bridges bend forward and the actin filament is pulled along.
12. While ATP and Ca2+ ions are available, cross-bridges attach; as ADP and P are released, the cross-bridges change their positions and cause a power stroke as filaments pull together.
13. When another ATP molecule binds to the myosin head, the cross-bridge detaches and the cycle begins again.
14. When a nerve impulse ceases, active transport proteins in the sarcoplasmic reticulum pump calcium ions back into calcium storage sites and muscle relaxation occurs.
42.1 Endocrine Glands
1. The endocrine system functions differently from the nervous system.
2. An endocrine system consists of glands that coordinate body activities through hormones.
3. In contrast to the exocrine glands, which have ducts leading to other organs or outside the body, the endocrine glands secrete their products into the bloodstream, which delivers them throughout the body.
4. Chemical signals are used between individuals, between body parts, and between cells.
5. Both the nervous system and the endocrine system rely on negative feedback mechnisms.
B. Hormones Are Chemical Signals
1. Chemical signals are a means of communication between cells, body parts, or individuals.
2. In general, they affect the metabolism of the cells that have receptors for them.
3. Most hormones act at a distance between body parts, traveling through the bloodstream from the gland to the target cell.
4. Some hormones (e.g., prostaglandins, growth factors) are localhormones—they affect only neighboring cells.
5. Pheromones are environmental signals that act at a distance between individual organisms.
a. Axillary secretions of men and women may have some effect on other people; women may synchronize their menstrual cycles with co-workers and some women may prefer the axillary odor of men with a different plasma membrane protein.
C. The Action of Hormones
1. The Action of Peptide Hormones
a. The term peptide hormone is used to include hormones that are peptides, proteins, glycoproteins, and modified amino acids.
b. Epinephrine is a peptide hormone that binds to a receptor protein in the target cell's plasma membrane; a relay system leads to conversion of ATP to cyclic AMP (cAMP).
c. Cyclic adenosine monophosphate (cAMP) is made from ATP; it has one phosphate group attached to adenosine at two locations.
d. Peptide hormones are the first messenger; cAMP and calcium are the second messenger.
e. The peptide hormone does not enter the cell; the second messenger sets an enzyme cascade in motion.
2. The Action of Steroid Hormones
a. Steroid hormones are lipids and cross cell membranes freely; they do not bind to plasma membrane receptors.
b. Inside the cytoplasm or a nucleus, steroid hormones (e.g., estrogen, progesterone) bind to a specific receptor.
c. The hormone-receptor complex binds to DNA, resulting in activation of genes (transcription) that produce enzymes (translation).
d. Steroids act more slowly than peptide hormones because it takes more time to synthesize new proteins than to activate enzymes already present in cells; however, their action lasts longer.
42.2 Hypothalamus and Pituitary Gland
1. The hypothalamus regulates the internal environment through the autonomic system.
a. It controls heartbeat, temperature, water balance, as well as glandular secretions of the pituitary gland.
2. The pituitary gland (hypophysis) is connected to the hypothalamus by a stalklike structure.
a. It is about 1 cm in diameter and lies just below the hypothalamus.
b. It is comprised of two portions: the posterior pituitary and the anterior pituitary.
B. Posterior Pituitary
1. Neurons in the hypothalamus called neurosecretory cells produce antidiuretic hormone (ADH) and oxytocin, which pass through axon endings in the posterior pituitary and are stored until released.
2. Antidiuretic hormone (ADH) promotes reabsorption of water from the collecting ducts in the kidneys.
a. Nerve cells in the hypothalamus determine when the blood is too concentrated; ADH is released and the kidneys respond by reabsorbing water.
b. As the blood becomes dilute, ADH is no longer released; this is a case of negative feedback.
c. Inability to produce ADH causes diabetes insipidus (watery urine), in which the individual produces copious amounts of urine and a resultant loss of ions from the blood.
3. Oxytocin is also made in the hypothalamus and stored in the posterior pituitary.
a. Oxytocin stimulates uterine muscle contraction in response to uterine wall nerve impulses.
b. It also stimulates the release of milk from mammary glands.
c. This positive feedback mechanism increases intensity; such positive feedback does not maintain homeostasis.
d. Oxytocin also may play a role in the propulsion of semen through the male reproductive tract.
C. Anterior Pituitary
1. Stimulation by the hypothalamus controls the release of anterior pituitary hormones through a portal system consisting of two capillary systems connected by a vein.
2. The hypothalamus produces hypothalamic-releasing and hypothalamic-inhibiting hormones which pass to the anterior pituitary by this portal system.
a. Thyroid-releasing hormones released from the hypothalamus act on cells in the anterior pituitary to stimulate the production and secretion of a specific hormone.
b. Thyroid-inhibiting hormones produced in and released from the hypothalamus act on cells in the anterior pituitary to inhibit the production and secretion of a specific hormone.
3. The anterior pituitary produces six different hormones.
a. Three of these anterior pituitary hormones affectother glands.
i. The thyroid-stimulating hormone (TSH) stimulates the thyroid to produce and secrete thyroxin.
ii. Adrenocorticotropic hormone (ACTH) stimulates the adrenal cortex to release cortisol.
iii. Gonadotropic hormones (follicle-stimulating hormone [FSH] and luteinizing hormone [LH]) act on the gonads (ovaries and testes) to secrete sex hormones.
b. The other three hormones do not affect other glands.
i. Prolactin (PRL) is produced in quantity only after childbirth.
1. Prolactin causes the mammary glands to produce milk.
2. It also plays a role in carbohydrate and fat metabolism.
ii. Melanocyte-stimulating hormone (MSH) causes skin color changes in fishes, amphibians, and reptiles with melanophores, special skin cells.
iii. Growth hormone (GH or somatotropic hormone)
1. GH promotes skeletal and muscular growth.
2. GH acts to stimulate the transport of amino acids into cells and to increase the activity of ribosomes.
3. GH promotes fat metabolism rather than glucose metabolism.
4. Too little GH during childhood makes an individual a pituitary dwarf.
5. Too much forms a giant; life expectancy is less because GH affects blood glucose levels and promotes diabetes mellitus.
6. The overproduction of GH in adults results in acromegaly; since long bone growth is no longer possible, only the feet, hands, and face grow.
42.3 Other Endocrine Glands and Hormones
A. Thyroid and Parathyroid Glands
1. The thyroid gland is in the neck and attached to the trachea just below the larynx.
2. The parathyroid glands are embedded in the posterior surface of the thyroid gland.
3. Thyroid Gland
a. The thyroid gland is the largest endocrine gland.
b. The two hormones produced by the many follicles of the thyroid gland both contain iodine.
i. Thyroxine (T4) contains four iodine atoms.
ii. Triiodothyronine (T3) contains three iodine atoms.
c. Lack of iodine causes enlargement of the thyroid (simple goiter).
i. Simple goiter is easily prevented by supplementing iodine intake in salt.
d. Thyroid hormones increase the metabolic rate; there is no one target organ—all organs respond.
e. Cretinism (congenital hypothyroidism) occurs in individuals who have suffered from low thyroid function since birth.
i. Affected individuals are short and stocky and have had hypothyroidism since infancy.
ii. Thyroid treatment helps but unless it is begun in the first two months, mental retardation can occur.
f. Myxedema is hypothyroidism in adults; thyroid hormones can restore normal function.
g. Hyperthyroidism (Graves disease) occurs when the thyroid gland is enlarged or overactive.
i. The eyes protrude because of edema in the eye socket tissue; this is called exophthalmic goiter.
ii. Removal or destruction of some thyroid tissue by surgery or radiation often cures the condition.
h. The thyroid gland also produces calcitonin.
i. Calcitonin lowers the calcium level in the blood and increases deposits in the bone by reducing osteoclasts.
ii. Calcitonin is also necessary for blood clotting.
iii. If blood calcium is lowered to normal, the release of calcitonin is inhibited.
iv. Too low calcium levels stimulate the release of parathyroid hormone (PTH) by the parathyroid glands.
4. Parathyroid Glands
a. Parathyroid glands produce parathyroid hormone (PTH).
i. Under the influence of PTH, the calcium level in blood increases and the phosphate level decreases.
ii. PTH stimulates the absorption of Ca2+ by activating vitamin D, the retention of Ca2+ (and excretion of phosphate) by the kidneys, and demineralization of bone by promoting the activity of osteoclasts.
iii. When the blood calcium level reaches the right level, the parathyroid glands no longer produce PTH.
iv. If PTH is not produced in response to low blood Ca2+, tetany results because the Ca2+ plays an important role in both nerve conduction and muscle contraction.
b. In tetany, the body shakes from continuous muscle contraction due to the increased excitability of nerves that fire spontaneously and without rest; this condition may be due to hypoparathyroidism.
c. In hyperparathyroidism, the abnormally high blood calcium levels can cause the bones to be soft and fragile, and the individual to be prone to kidney stones.
B. Adrenal Glands
1. Two adrenal glands sit atop the kidneys.
2. Each gland consists of two parts: an outer adrenal cortex and an inner adrenal medulla.
3. The cortex and medulla have no physiological connection between them.
4. The hypothalamus exerts control over both portions.
a. Nerve impulses travel via the brain stem to the spinal cord to sympathetic nerve fibers to the medulla.
b. The hypothalamus uses ACTH-releasing hormone to control the anterior pituitary's secretion of ACTH.
5. Adrenal hormones increase during times of physical and emotional stress.
6. Both epinephrine and norepinephrine are produced by the adrenal medulla.
7. Both hormones bring about body changes corresponding to an emergency.
a. The blood glucose level rises and metabolic rate increases.
b. The bronchioles dilate and breathing rate increases.
c. Blood vessels to the digestive tract and skin constrict; those to the skeletal muscles dilate.
d. The cardiac muscle contracts more forcefully and the heart rate increases.
8. The adrenal cortex hormones provide a sustained response to stress.
a. The adrenal cortex secretes two types of hormones: glucocorticoids and mineralocorticoids.
i. Glucocorticoids help to regulate blood glucose levels.
ii. Mineralocorticoids regulate the levels of minerals in the blood.
iii. The adrenal cortex also secretes a small amount of both male and female sex hormones in both sexes.
9. Glucocorticoids: Cortisol is a biologically significant glucocorticoid.
a. Cortisol promotes the breakdown of muscle protein into amino acids taken up by the liver from the blood.
b. Cortisol breaks down fatty acids rather than carbohydrates; cortisol therefore raises blood glucose levels.
c. Cortisol counteracts the inflammatory response; it helps medicate arthritis and bursitis.
10. Mineralocorticoids: Aldosterone is the most important of the mineralocorticoids.
a. The primary target organ is the kidney where it promotes the reabsorption of Na+ and the excretion of K+.
b. Mineralocorticoid secretion is controlled by the renin-angiotensin-aldosterone system
i. Under low blood volume and sodium levels, the kidneys secrete renin.
ii. The enzyme renin converts the plasma protein angiotensinogen to angiotensin I; this becomes angiotensin II by a converting enzyme in the lungs.
iii. Angiotensin II stimulates the adrenal cortex to release aldosterone.
iv. Angiotensin I constricts the arterioles directly; aldosterone causes the kidneys to absorb calcium.
v. When the blood sodium rises, water is reabsorbed as the hypothalamus secretes ADH; blood pressure then increases to normal.
c. Atrial natriuretic hormone (ANH) causes the excretion of sodium.
i. When the atria of the heart are stretched due to increased blood volume, cardiac cells release ANH.
ii. ANH inhibits the secretion of renin by the kidneys and the secretion of aldosterone from the adrenal cortex.
iii. When sodium is excreted, so is water; the blood volume and pressure then return to normal.
11. Malfunction of the Adrenal Cortex
a. Low levels of adrenal cortex hormones (hyposecretion) result in Addison disease.
i. When ACTH is in excess, like MSH, it can lead to the buildup of melanin and a bronzing of the skin.
ii. The lack of cortisol results in low glucose levels; a stressed person has insufficient energy.
iii. The lack of aldosterone drops blood sodium levels; a person then has low blood pressure and dehydration.
iv. Left untreated, Addison disease can be fatal.
b. High levels of adrenal cortex hormones from hypersecretion result in Cushing syndrome.
i. Excess cortisol causes a tendency toward diabetes mellitus.
ii. Muscular protein then decreases and subcutaneous fat forms an obese trunk but normal arms and legs.
C. Pancreas
1. The pancreas lies transversely in the abdomen between the kidneys and near the duodenum.
2. The pancreas is composed of two types of tissue.
a. Exocrine tissue produces and secretes digestive juices into the small intestine by way of ducts.
b. Endocrine tissues called pancreatic islets (islets of Langerhans) produce insulin and glucagon.
3. All body cells utilize glucose; therefore its level must be closely regulated.
4. Insulin is secreted when the blood glucose level is high after eating; insulin has three actions.
a. Insulin stimulates liver, fat, and muscle cells to take up glucose.
b. Insulin stimulates the liver and muscles to store glucose as glycogen.
c. Insulin promotes buildup of fats and proteins and inhibits their use as an energy source.
5. Glucagon is secreted between meals in response to low blood glucose level.
a. Liver and adipose tissue are the main targets.
b. Adipose tissue cells break the fat into glycerol and fatty acids.
c. The liver uses glycerol and fatty acids as substrates for glucose, raising the blood glucose levels.
6. Diabetes Mellitus
a. Diabetes mellitus is a fairly common disease where the body cells do not take up or metabolize sugar.
b. Blood glucose level becomes high enough for the kidneys to excrete glucose; therefore this is detected by a urine test.
c. The liver is not storing glucose as glycogen and cells are not utilizing glucose for energy.
d. Since carbohydrate is not being metabolized, the body breaks down protein and fat for energy.
e. Ketones then build up in blood; the resulting reduced blood volume and acidosis can lead to coma and death.
f. In type 1 (insulin-dependent) diabetes, the pancreas does not produce insulin.
i. A viral infection can cause cytotoxic T cells to destroy pancreatic islets.
ii. This is treated with a daily administration of insulin; an overdose or lack of eating results in hypoglycemia.
iii. The brain also has constant sugar requirements; low blood sugar can result in unconsciousness.
g. An immediate intake of sugar is a simple and effective treatment.
7. Of 18 million diabetics in the U.S., most have type 2 (noninsulin-dependent) diabetes.
a. This form of diabetes usually occurs in obese and inactive individuals of any age.
b. The pancreas does produce insulin but live muscle cells do not respond to it.
c. Initially, this is a result of cells lacking the receptors for insulin.
d. Untreated, type II diabetes can have serious symptoms: blindness, kidney disease, circulatory disorders, strokes, etc.
e. A low fat diet and regular exercise help; oral drugs can make cells more sensitive to insulin or stimulate higher levels of insulin production by the pancreas.
D. Testes and Ovaries
1. The testes located in the scrotum function as gonads and produce androgens (e.g., testosterone).
a. Testosterone is the male sex hormone.
b. It stimulates the development of male secondary sex characteristics: large vocal cords, pubic hair, etc.
c. Testosterone is largely responsible for the sex drive.
d. Anabolic steroids are supplemental testosterone or similar chemicals with serious side effects.
e. Testosterone also affects sweat glands, expression of baldness genes, and other effects.
2. The ovaries, located in the pelvic cavity, produce the female sex hormones estrogens and progesterone.
a. Estrogens secreted at puberty stimulate the maturation of ovaries and other sexual organs.
b. Estrogen is necessary for oocyte development.
c. It is responsible for the development of female secondary sex characteristics: a layer of fat beneath the skin, a larger pelvic girdle, etc.
d. Estrogen and progesterone are required for breast development and the regulation of the uterine cycle.
E. Pineal Gland
1. The pineal gland, located in the brain, produces melatonin, primarily at night.
2. The pineal gland and melatonin help establish circadian rhythms, 24-hour physiological cycles.
3. The pineal gland may also be involved in human sexual development; children in whom a brain tumor has destroyed the pineal gland experience puberty earlier.
F. Thymus Gland
1. The thymus gland is a lobular gland that lies just beneath the sternum in the upper thoracic cavity.
2. It reaches its largest size and is most active during childhood; with age, it shrinks and becomes fatty.
3. Some lymphocytes that originate in the bone marrow pass through the thymus and change into T lymphocytes.
4. The thymus produces thymosins, which aid in the differentiation of T cells and may stimulate immune cells.
G. Other Hormones
1. Leptin is a peptide hormone secreted by adipose tissue.
a. Its main function is its role in the feedback control of appetite; it can suppress appetite.
2. Erythropoietin (EPO) is a peptide hormone produced in the kidneys.
a. It stimulates the production of red blood cells and speeds up their maturation.
3. Local hormones such as prostaglandins are produced by certain cells and act on neighboring cells.
a. Prostaglandins are potent chemical signals which have several known functions.
b. They cause the contraction of uterine muscle, and they mediate the effects of pyrogens (chemicals believed to affect the temperature regulatory center of the brain).
c. Certain prostaglandins reduce gastric secretions, others lower blood pressure.
d. Aspirin reduces temperature and controls pain because of its effect on prostaglandins.
43.1 How Animals Reproduce
• There are two patterns of reproduction.
1. Asexual–only one parent is involved and the offspring are identical to the parent.
2. Sexual–two parents are involved and the offspring are genetically unique.
A. Asexual Reproduction
1. Hydra reproduce asexually by budding; a new individual arises as an outgrowth (bud) of a parent.
2. Obelia undergoes an alternation of generations: an asexual colonial stage and a sexual medusa stage.
3. Flatworms can constrict into two halves, each half becoming a new individual.
4. Fragmentation followed by regeneration is seen among annelids, sponges, and echinoderms.
5. Parthenogenesis is found among some insects, worms, fish, lizards and some other animals; an unfertilized egg develops into a complete individual.
6. In honeybees, the queen can fertilize or not fertilize the eggs, producing diploid female workers (if fertilized) or haploid male drones (if unfertilized).
B. Sexual Reproduction
1. In sexual reproduction, the egg of one parent is fertilized by the sperm of the other; most animals are dioecious (having separate sexes).
2. Hermaphroditic organisms possess both male and female organs.
a. Earthworms undergo cross-fertilization.
b. Tapeworms are capable of self-fertilization.
c. Sequential hermaphroditism, or sex reversal, involves the changing of sex; a male wrass (a reef fish) has a harem but if the male dies, the largest female becomes a male.
3. Gonads are organs specialized to produce gametes.
a. Sponges are an exception since their collar cells give rise to sperm and eggs.
b. Hydras produce only temporary gonads in the fall when sexual reproduction occurs.
c. Animals in other phyla have permanent gonads.
4. There are two types of gonads: testes produce sperm and ovaries produce eggs.
5. Eggs and sperm cells derive from germ cells that specialize early for this development.
6. Other cells in the gonads support and nourish the developing gametes or produce hormones for reproduction.
7. Accessory organs form ducts and storage areas that aid in bringing gametes together.
8. Sexually-reproducing animals have various methods to ensure that the gametes unite.
a. Aquatic animals that practice external fertilization must synchronize egg release.
b. The lunar cycle is one trigger that cues animals by tides.
c. Hundreds of thousands of palolo worms rise to the surface to release eggs during a 2–4 hour period.
9. Copulation is sexual union to facilitate the reception of sperm by a female.
a. The penis is a male copulatory organ typical of terrestrial males; it deposits sperm into the female's vagina.
b. Aquatic animals have other types of copulatory organs or employ other strategies for delivering sperm:
i. Lobsters and crayfish have modified swimmerets.
ii. Cuttlefish and octopuses use an arm.
iii. Sharks have a modified pelvic fin to pass packets of sperm to the female shark.
c. Birds lack a penis or vagina; they transfer sperm from cloaca to cloaca.
C. Life History Strategies
1. Many aquatic animals use external fertilization; eggs and sperm join outside the body in the water.
2. Terrestrial animals tend to practice internal fertilization; eggs and sperm join inside the female's body.
3. Both types of animals are usually oviparous; they deposit eggs in the external environment.
4. Insect eggs are produced in ovaries; they mature and increase in size as a result of the accumulation of yolk.
a. Yolk is stored food to be used by the developing embryo.
b. To prevent insect eggs from drying out, their eggshell has several layers of protein or wax.
c. In insects, small holes are left at one end to allow for the entry of sperm.
5. Some insects have a special organ to store sperm so the eggs can be fertilized later.
6. A larval stage is often quite different in appearance and way of life from the adult form.
a. The larva is able to seek its own food to sustain itself until it becomes an adult.
b. Metamorphosis is a major change in form that some animals undergo during development.
c. Incomplete metamorphosis lacks a pupal stage and the nymphs look more like adults.
d. Larval aquatic forms can utilize a different food source than the adults.
e. The bilaterally symmetrical sea star larvae attach to a substrate and become radially symmetrical adults.
f. The free-swimming barnacle larvae metamorphose into sessile adults with calcareous plates.
g. The crayfish lacks a larval stage; eggs hatch into tiny juveniles with the same form as the adults.
7. Reptiles and birds provide their eggs with plentiful yolk; there is no larval stage.
a. Complete development takes place within a shell containing extraembryonic membranes.
b. The chorion is the outermost membrane that lies next to the shell and functions in gas exchange.
c. The amnion forms a water-filled sac around the embryo ensuring that it will not dry out.
d. A yolk sac holds yolk which nourishes the embryo.
e. The allantois holds nitrogen waste products.
f. A shelled egg frees an animal from any need to reproduce in water and also helps it live completely on land.
8. Birds tend their eggs.
a. Newly hatched birds have to be fed before they develop to where they can seek food on their own.
b. Parent bird's reproductive behaviors involve complex hormone and neural regulation.
9. In oysters and sea horses; the eggs remain inside the body until they hatch fully-developed.
10. Garter snakes, water snakes, and pit vipers also retain eggs until they hatch and give birth to live young.
11. Mammals are viviparous, producing living young.
a. The nutrients needed for development are constantly supplied by the mother.
b. Viviparity represents the ultimate in caring for the zygote and the embryo.
c. The evolution of viviparity can be seen in the primitive mammals.
i. The exceptions are the duckbill platypus and the spiny anteater, which are egg-laying mammals.
ii. Marsupials give birth to immature offspring that finish developing within a pouch.
iii. In all other mammals, development occurs in a placenta.
12. The placenta is a complex organ comprised of maternal and embryonic tissues.
a. A placenta exchanges O2, CO2, nutrients, wastes, etc., between the fetal and maternal circulations.
b. Evolution allowed embryos to exchange materials with the mother; this made the shell unnecessary.
43.2 Male Reproductive System
1. Paired testes are suspended in the scrotal sacs of the scrotum.
2. The testes begin development in the abdominal cavity but descend into the scrotal sac during development.
3. If the testes do not descend, without surgery or hormonal therapy, sterility results.
4. The lower temperature of the scrotum is vital to normal sperm production.
5. Sperm produced in the testes mature within the epididymides.
a. These are tightly coiled tubules outside of the testes in which the sperm undergo maturation.
b. The maturation time in the epididymis is required for the sperm to develop the ability to swim to the egg.
6. Once sperm have matured, they are propelled into the vasa deferentia by muscular contractions.
7. Sperm are stored in both the epididymides and the vasa deferentia.
8. When a male is sexually aroused, the sperm enter the urethra, part of which extends through the penis.
9. The penis is a cylindrical copulatory organ used to introduce spermatozoa into the female vagina.
a. Three columns of spongy, erectile tissue extend down the penile shaft.
b. During sexual arousal, nervous reflexes cause an increase in the arterial blood flow to the penis.
c. Increased blood flow fills and distends the erectile tissue, and the penis stiffens and increases in size.
d. These changes cause an erection; failure to achieve an erection is called impotency.
10. Semen (seminal fluid) is thick, whitish fluid that contains sperm and glandular secretions.
a. The seminal fluid is formed by the seminal vesicles, prostate gland, and bulbourethral glands.
b. The seminal vesicles lie at the base of the urinary bladder.
i. Each joins a vas deferens to form an ejaculatory duct that enters the urethra.
ii. They secrete into the ejaculatory duct a thick fluid containing nutrients for use by the sperm.
c. The prostate gland is located just below the urinary bladder and surrounds the upper portion of the urethra.
i. It secretes a milky, slightly alkaline solution that promotes sperm motility and viability.
ii. In older men, the prostate gland may become enlarged and constrict the urethra.
iii. Prostate cancer is also common in older men.
d. The bulbourethral glands are located below the prostate gland and on either side of the urethra; they release mucus secretions that provide lubrication.
11. The urethra also conducts urine from the bladder during urination.
B. Ejaculation
1. Ejaculation results in the expulsion of semen; this is achieved at the peak of sexual arousal.
2. The first phase of ejaculation is emission.
a. Nerve impulses from the spine trigger the epididymides and vasa deferentia to contract.
b. Subsequent motility causes the sperm to enter the ejaculatory duct; seminal vesicles, the prostate gland, and the bulbourethral glands release their secretions.
c. A small amount of secretion from the bulbourethral glands may leak from the end of penis; it functions to clean the urethra of acid but it may contain sperm.
3. The second phase of ejaculation is expulsion.
a. Rhythmical contractions at the base of the penis and within the urethral wall expel the semen in spurts.
b. Rhythmical contractions are a release from myotonia, or muscle tenseness, an important sexual response.
4. An erection lasts for a limited time and the penis generally returns to a flaccid state following ejaculation.
5. A refractory period follows during which stimulation does not bring about an erection.
6. Orgasm is the physiological and psychological sensations that occur at the climax of sexual stimulation.
C. The Testes
1. A longitudinal section shows compartments called lobules, each of which contains one to three seminiferous tubules.
a. Altogether, seminiferous tubules have a combined length of about 250 meters.
b. In a microscopic cross section, tubules show cells undergoing spermatogenesis, a process of meiosis.
c. The sustentacular (Sertoli) cells support, nourish, and regulate spermatogenic cells.
2. Mature sperm (spermatozoa) have three parts.
a. The sperm head contains a nucleus covered by an acrosome.
i. An acrosome is a caplike covering over the anterior end of nucleus; it stores enzymes to penetrate the egg.
ii. A human egg is surrounded by several layers of cells and thick membrane; the enzymes allow the sperm to penetrate.
b. The middle piece contains mitochondria wrapped around microtubules of the flagellum; the mitochondria provide the energy for movement.
c. The tail also contains microtubules as components of a flagellum; its movement propels sperm.
3. The ejaculate of a normal human male contains several hundred million sperm.
4. Fewer than 100 ever reach the vicinity of an egg; and only one sperm normally enters an egg.
D. Hormonal Regulation in Males
1. The hypothalamus has ultimate control of the testes' sexual function through secreting of gonadotropic-releasing hormone (GnRH) that stimulates the pituitary to produce gonadotropic hormones.
2. There are two gonadotropic hormones: follicle-stimulating hormone (FSH) and luteinizing hormone (LH) found in both males and females.
3. In males, FSH stimulates spermatogenesis in the seminiferous tubules.
4. In males, luteinizing hormone (LH) is also called interstitial cell-stimulating hormone (ICSH); it stimulates testosterone secretion by interstitial cells of the testes.
5. The seminiferous tubules also release the hormone inhibin.
6. The hypothalamus-pituitary-testis system are involved in a negative feedback relationship that maintain a fairly constant production of sperm and testosterone.
7. Functions of Testosterone
a. Testosterone is the main sex hormone in males.
b. Testosterone is essential for the development of male secondary sex characteristics that develop at puberty, and for the maturation of sperm.
i. It causes the tallness, longer legs and broader shoulders of males.
ii. Testosterone causes the larynx and vocal cords to enlarge, thus causing a deeper voice.
iii. It is responsible for greater muscle strength of males; some athletes take supplemental anabolic steroids (that are natural or synthetic testosterone).
iv. Testosterone causes males to develop hair on the face, chest, and back.
v. Testosterone is also involved in triggering baldness if baldness genes are present.
43.3 Female Reproductive System
1. The female reproductive system includes: ovaries, oviducts, uterus, and vagina.
2. The ovaries produce a secondary oocyte each month; the ovaries are located in the pelvic cavity.
3. The oviducts (uterine tubes, fallopian tubes) extend from the ovaries to the uterus.
a. The oviducts are not attached to the ovaries.
b. Fingerlike projections called fimbriae sweep over the ovaries and waft in the egg when it erupts.
c. This is the normal site for fertilization; the embryo is slowly moved by ciliary movement toward the uterus.
4. The uterus is a hollow, thick-walled muscular organ the size and shape of an inverted pear.
a. An embryo completes development by embedding itself in uterine lining, the endometrium.
b. The narrow end of the uterus is the cervix.
c. A small opening at the cervix of the uterus leads to the vaginal canal.
5. The vagina is a tube at a 45° angle with the small of the back.
a. Its mucosal lining lies in folds and it can extend, as necessary in childbirth.
b. It receives the penis during copulation and also serves as the birth canal.
6. The external genitalia of women are known collectively as the vulva.
a. The mons pubis, labia minora, and labia majora are to the side of the vaginal and urethral openings.
b. At the front juncture of the labia minora is the clitoris.
i. This is homologous to the penis in males.
ii. The clitoris has a short shaft of erectile tissue and is capped by a pea-shaped glans.
iii. It contains many sensory receptors that allow it to function as a sexually sensitive organ.
c. Orgasm involves the release of neuromuscular tension in the muscles of the genital area, vagina, and uterus.
B. The Ovaries
1. The ovaries alternate in producing one oocyte each month.
2. The ovaries produce both the egg (ovum) and the female sex hormones, estrogens and progesterone, during the ovarian cycle.
3. The Ovarian Cycle
a. In a longitudinal section, an ovary shows many cellular follicles, each containing an oocyte (egg).
b. As a follicle matures during the ovarian cycle, it develops from a primary follicle to a secondary follicle to a vesicular (Graafian) follicle.
c. As oogenesis is occurring, a secondary follicle contains a secondary oocyte pushed to one side of fluid-filled cavity.
d. The vesicular follicle fills with fluid until the follicle wall balloons out on the surface and bursts, releasing a secondary oocyte surrounded by a zona pellucida and follicular cells.
e. Ovulation is the rupture of the vesicular follicle with the discharge of the secondary oocyte into the oviduct.
f. The secondary oocyte completes a second meiotic cell division when fertilization occurs.
g. Meanwhile, the follicle develops into the corpus luteum; if pregnancy does not occur, the corpus luteum begins to degenerate in 10 days.
4. Phases of the Ovarian Cycle
a. The ovarian cycle is under the control of gonadotropic hormones: follicle-stimulating hormone (FSH) and luteinizing hormone (LH).
b. The gonadotropic hormones are not present constantly but are secreted at different rates during the cycle.
c. During the follicular phase, FSH promotes the development of a follicle that secretes estrogens.
d. As the estrogen level in the blood rises, it exerts feedback control over the anterior pituitary secretion of FSH; the follicular phase comes to an end.
e. Estrogen levels in the blood rise, causing the hypothalamus to secret more GnRH; this causes a surge in LH secretion.
f. The LH surge then triggers ovulation.
g. The luteal phase is the second half of the ovarian cycle following ovulation.
i. LH promotes the development of the corpus luteum, which secretes large amounts of progesterone.
ii. As the blood level of progesterone rises, negative feedback to the anterior pituitary's secretion of LH causes the corpus luteum to degenerate.
iii. As the luteal phase ends, menstruation occurs.
C. The Uterine Cycle
1. Estrogens and progesterone affect the endometrium of the uterus to cause a cycle of events known as the uterine cycle.
2. An average 28-day uterine cycle is divided into four sections.
a. During days 1–5, low levels of estrogen and progesterone in the body cause menstruation.
b. During days 6–13, an increased production of estrogens by an ovarian follicle causes the endometrium to thicken and become vascular and glandular (proliferative phase).
c. Ovulation usually occurs on day 14 of the 28-day cycle.
d. Days 15–28 see increased production of progesterone by the corpus luteum that causes the endometrium to double in thickness; uterine glands mature, producing a thick mucoid secretion; this is the secretory phase.
i. The endometrium is now prepared to receive a developing embryo.
ii. If no pregnancy occurs, the progesterone and estrogen levels decline and the corpus luteum degenerates.
iii. With low levels of progesterone, the uterine lining also begins to degenerate.
3. Menstruation is the periodic shedding of tissue and blood from the endometrium; this lining disintegrates and the blood vessels rupture.
a. A flow of blood and tissues passes out through the vagina.
b. The enzyme fibrinolysin prevents the blood from clotting.
c. The first menstrual period, menarche, typically occurs between the ages of 11 and 12.
d. If menarche does not occur by age 16, amenorrhea exists.
i. Primary amenorrhea is usually caused by nonfunctional ovaries or developmental abnormalities.
ii. Secondary amenorrhea may be caused by weight loss or excessive exercise.
e. Menopause, when menstruation ceases because the ovaries are no longer functioning, usually occurs between the ages of 45 and 55.
i. Menopause is not complete until menstruation is absent for a year.
D. Fertilization and Pregnancy
1. If fertilization occurs, the embryo begins development as it travels down the oviduct to the uterus.
2. The embryo becomes embedded in the endometrium several days following fertilization.
3. The placenta develops from both maternal and embryonic tissues.
a. The placenta functions to exchange gases and nutrients between the fetal and maternal circulation.
b. There is normally no mixing of the blood between the maternal and fetal circulations.
4. Initially, the placenta produces human chorionic gonadotropin (HCG) which maintains the corpus luteum.
5. The corpus luteum is maintained by the HCG until the placenta produces its own progesterone and estrogens.
6. The progesterone and estrogens have two effects at this stage.
a. They shut down the anterior pituitary so that no new follicles mature.
b. They maintain the lining of the uterus so the corpus luteum is not needed.
7. There is no menstruation during pregnancy.
E. Estrogen and Progesterone
1. Estrogens maintain the normal development of the related organs and are responsible for the secondary sex characteristics of females.
2. There is less body and facial hair, and more fat beneath the skin provides a more rounded appearance.
3. The pelvic girdle enlarges and the pelvic cavity is larger; therefore, women have wider hips.
4. Both estrogen and progesterone are required for breast development.
F. E.. The Female Breast
1. The female breast contains 15–24 lobules, each with a mammary duct.
2. The mammary duct begins at the nipple and divides into numerous ducts which end in alveoli (blind sacs).
3. The hormone prolactin is needed for lactation (milk production) to begin.
4. Production of prolactin is suppressed by the feedback inhibition that estrogens and progesterone have on the anterior pituitary during pregnancy; therefore, it takes a couple of days after delivery of a baby for milk production to begin.
5. The breasts produce a watery, yellowish white fluid (colostrum) similar to milk but containing more protein and less fat, and it is rich in IgA antibodies providing some immunity to a newborn.
6. Breast cancer is the most common form of cancer in females; women should have regular breast checks and mammograms when recommended.
43.4 Control of Reproduction
A. Birth Control Methods
o The most reliable method of birth control is abstinence; it has the advantage of preventing transmission of a sexually transmitted disease.
2. Contraceptive Injections
a. Contraceptive injections are now being developed—possibilities include vaccination against HCG or sperm.
3. Morning-After Pills
a. These regimens either prevent fertilization or stop a fertilized egg from implanting.
b. Preven is a kit of four synthetic progesterone pills; the medication makes it difficult for the embryo to implant in the endometrium; it is considered 85% effective.
c. Mifepristone, also known as RU-486, causes the loss of an implanted embryo.
i. It blocks the progesterone receptors of the endometrial cells.
ii. Without functioning receptors for progesterone, the uterine lining sloughs off carrying the embryo with it.
iii. Taken in conjunction with a prostaglandin to induce uterine contractions, it is 95% effective.
B. Infertility
o Infertility is the inability of a couple to achieve pregnancy after one year of regular, unprotected intercourse.
1. Causes of Infertility
a. The most frequent causes of male infertility are low sperm count and abnormal sperm; this can be due to a sedentary lifestyle coupled with smoking and alcohol consumption.
b. The major factors involved in female infertility are body weight, blocked oviducts, and endometriosis.
i. Endometriosis is the presence of uterine tissue beyond the uterus.
ii. Blocked oviducts can be due to pelvic inflammatory disease.
iii. If no obstruction is apparent and body weight is normal, females can be given fertility drugs.
iv. Hormone treatments carry the risk of multiple pregnancy.
2. Assisted Reproductive Technologies
a. Artificial Insemination by Donor (AID)
i. A sperm sample is injected into the vagina.
ii. If the husband's sperm count is low, many samples can be combined.
iii. Artificial insemination from a donor is necessary when the husband lacks viable sperm.
iv. Intrauterineinsemination can be coordinated with drugs used to stimulate the ovaries.
v. With artificial insemination, sperm can be sorted into those that are X-bearing (producing a girl) or Y-bearing (producing a girl).
b. In Vitro Fertilization (IVF)
i. In IVF, conception occurs in laboratory glassware.
ii. Ultrasound machines spot maturing follicles and a laparoscope is used to harvest the eggs using a needle.
iii. When sperm and egg are combined in glassware, they can be transferred to the uterus after 2–4 days.
iv. While in glassware, the new embryos can be tested for genetic diseases.
c. Gamete Intrafallopian Transfer (GIFT)
i. A gamete is a sex cell, either a sperm or an egg.
ii. Due to the low success rate of IVF (15–20%), GIFT immediately places the sperm and egg in the oviduct.
iii. A variation is to fertilize the eggs in the laboratory and then place the zygotes in the oviducts.
d. Surrogate Mothers
i. Women can be contracted and paid to have babies; they are then surrogate mothers.
ii. The sperm and/or egg can be contributed by the contracting parents.
e. Intracytoplasmic Sperm Injection (ICSI)
i. A single sperm is injected into an egg.
ii. This is used when a man has severe infertility problems.
43.5 Sexually Transmitted Diseases
1. Sexually transmitted diseases (STDs) are caused by organisms ranging from viruses to arthropods.
2. Humans cannot develop lasting immunity to any STDs; therefore, prompt medical treatment should be received when exposed to an STD.
3. To prevent STDs, a condom can be used.
4. It is difficult to cure STDs caused by viruses; treatment is available for AIDS and genital herpes.
5. STDs caused by bacteria (e.g., gonorrhea, chlamydia, and syphilis) are treatable with antibiotics.
B. AIDS
1. Acquired immunodeficiency syndrome (AIDS) is caused by the human immunodeficiency virus (HIV).
2. HIV attacks the helper T cells that stimulate the activity of B lymphocytes to produce antibodies.
3. After an HIV infection begins, helper T cells decline in number and a person becomes more susceptible to infections.
4. Symptoms
o AIDS has three stages of infection called category A, B, and C
a. The category A stage may last about a year.
i. An individual is asymptomatic but can pass on the infection.
ii. Immediately after infection but before testing positive, a large number of infectious viruses are in the blood.
iii. After testing positive, a person may remain well for as long as he or she can maintain sufficient helper T cells (above 500 mm3).
b. The category B stage may last six to eight years.
i. Lymph nodes swell.
ii. There is weight loss, night sweats, fatigue, fever, and diarrhea.
iii. Infections such as thrush and herpes reoccur.
c. The category C stage is full-blown AIDS.
i. Nervous disorders and opportunistic diseases (e.g., an unusual type of pneumonia or skin cancer) occur.
ii. Without intensive medical treatment, an AIDS patient usually dies by about 7–9 years after infection.
iii. A recent combination therapy of several drugs allows AIDS patients in the United States to live longer.
6. Transmission
a. AIDS is transmitted by sexual contact with an infected person (vaginal or rectal intercourse and oral/genital contact).
b. Needle sharing among intravenous drug users is a high-risk behavior.
c. Transfusions of blood or clotting factors is now a rare mode of transmission and can be screened.
d. The largest increases in AIDS cases now involve heterosexual contact and intravenous drug use.
e. Women now account for 19% of all newly diagnosed cases of AIDS.
7. Treatment
a. Even though there is no cure for AIDS, a treatment called highly active antiretroviral therapy (HAART) is usually able to stop HIV replication.
i. HAART uses a combination of drugs that interfere with the life cycle of HIV.
1. Entry inhibitors stop HIV from entering a cell.
2. Reverse transcriptase inhibitors (e.g., AZT) interfere with the enzyme reverse transcriptase.
3. Integrase inhibitors prevent HIV from inserting its own genetic material into that of the host.
4. Protease inhibitors prevent protease from processing newly created polypeptides.
C. Genital Warts
0. Genital warts are caused by the human papillomaviruses (HPVs).
1. Many carriers are asymptomatic or they have minimal symptoms.
2. If visible warts are removed, they may recur.
3. HPVs are now associated with cancer of the cervix as well as tumors of the vulva, vagina, anus, and penis.
4. Some researchers believe viruses are involved in 90–95% of all cases of cancer of the cervix.
D. Genital Herpes
0. Genital herpes is caused by the herpes simplex virus.
1. Type 1causes cold sores and fever blisters; type 2 more often causes genital herpes.
2. Individuals infected with this type of virus can be asymptomatic carriers.
3. Symptoms include painful ulcers on the genitals, fever, painful urination, and swollen lymph nodes.
4. Exposure to herpes in the birth canal can cause neurological disorders and even death in a newborn; birth by cesarean section avoids this possibility.
E. Hepatitis
0. Hepatitis A is usually acquired from sewage-contaminated drinking water but is also an STD contracted by oral/anal contact.
1. Hepatitis B is spread in the same manner as AIDS but is more infectious; a vaccine is available.
2. Hepatitis C is called post-transfusion hepatitis, but it can be transmitted through sexual contact.
3. Hepatitis infections infect the liver and can lead to liver failure, liver cancer, and death.
F. Chlamydia
0. Chlamydia is named for the bacterium that causes it: Chlamydia trachomatis.
1. New chlamydial infections have increased faster than any other STD.
2. It also causes cervical ulcerations which increase the risk of acquiring AIDS.
3. It also causes pelvic inflammatory disease (PID).
4. If a baby is exposed at birth, inflammation of the eyes or pneumonia can result.
G. Gonorrhea
0. Gonorrhea is caused by the bacterium Neisseria gonorrhoeae.
1. Male diagnosis is not difficult: typical symptoms include urination pain and a thick, greenish yellow discharge.
2. In males and females, latent infections lead to pelvic inflammatory disease (PID); the vasa deferentia or the oviducts become infected and inflamed.
3. As these tubes heal, they may become partially blocked, resulting in sterility or infertility.
4. If a baby is exposed at birth, an eye infection can lead to blindness; therefore all newborns are given eye drops.
5. Gonorrhea proctitis is an infection of the anus; gonorrhea can infect the mouth, throat, tonsils, heart, and joints.
6. Previously easily cured by antibiotics, nearly 40% of modern strains are now antibiotic resistant.
H. Syphilis
0. Syphilis is caused by the bacterium Treponema pallidum.
1. This disease has three stages typically separated by latent periods.
a. The primary stage involves the appearance of a hard chancre (ulcerated sore).
b. The second stage involves the appearance of a rash all over the body, including the palms and feet.
c. The third stage involves neurological and cardiac disorders.
i. An infected individual may become mentally retarded, blind, walk with a shuffle, or become insane.
ii. Large destructive ulcers (gummas) develop on the skin or within internal organs.
2. Syphilitic bacteria can cross the placenta, causing birth defects or stillbirth.
3. Unlike the other STDs discussed, there is a blood test to diagnose syphilis.
4. Tracing sexual partners is very important in controlling syphilis.
I. Two Other Infections
0. Bacterial vaginosis is caused by Gardnerella vaginalis, Trichomonas vaginalis (a flagellated protozoa), or Candida albicans (a yeast).
1. The protozoan infection causes a frothy, foul-smelling discharge with itching.
2. Trichomoniasis is most often transmitted through sexual intercourse.
3. The Candida yeast infection causes a white, curdy discharge with itching.
4. Candida albicans is a normally-occurring organism in the vagina; yeast infections can result from taking birth-control pill or antibiotics.
44.1 Early Developmental Stages
1. Fertilization requires that sperm and egg interact to form a zygote.
a. A human sperm cell has three parts.
i. The head contains a haploid nucleus covered by a caplike acrosome containing enzymes, allowing the sperm to penetrate the egg.
ii. A middle piece contains ATP-producing mitochondria.
iii. The tail is a flagellum that allows the sperm to swim.
2. The plasma membrane of the egg is surrounded by the zona pellucida.
a. The zona pellucida is surrounded by a few layers of adhering follicular cells, collectively called the corona radiata.
b. These cells nourished the egg when it was in a follicle of the ovary.
3. Fertilization involves the following steps.
a. Several sperm penetrate the corona radiata and several sperm attempt to penetrate the zona pellucida.
b. One sperm enters the egg and their nuclei eventually fuse.
c. After the sperm head binds tightly to the zona pellucida, the acrosome enzymes digest and form a pathway for the sperm through the zona pellucida.
d. The head, middle piece and usually the tail enters the egg.
e. Prevention of polyspermy depends on changes in the egg plasma membrane when the sperm touches the egg and depolarizes the egg plasma membrane; this is called Fast block."
f. Vesicles in the egg called cortical granules secrete enzymes that turn the zona pellucida, forming an impenetrable fertilization membrane; this is called "Slow Block."
g. As soon as plasma membranes of the sperm and egg fuse, the zona pellucida lifts away from the surface of the egg, forming a moat that prevents entrance of any other sperm.
h. The diploid zygote forms when a nuclear envelope surrounds the sperm and egg chromosomes.
B. Embryonic Development
1. Cellular Stages of Development
a. Development is all of the changes that occur during the life cycle of an organism.
b. An organism is an embryo during the first stages of development.
c. After fertilization, a zygote undergoes cleavage, cell division without growth.
d. DNA replication and mitosis occur repeatedly, and the cells get smaller with each division.
e. In the lancelet, the cell divisions are equal in the resulting morula.
f. A cavity called the blastocoel develops forming a hollow ball called the blastula.
2. Tissue Stages of Development
a. The tissue stages of development are early gastrula and late gastrula.
b. The early gastrula stage begins with the invagination of certain cells into the blastocoel to form two of the three primary germ layers.
c. The outer layer of cells becomes ectoderm; ectoderm gives rise to the epidermis of the skin, the epithelial lining of the mouth and rectum, and the nervous system.
d. The inner layer of cells becomes the endoderm that gives rise to the epithelial lining of the digestive tract and the respiratory tract, associated glands of the digestive and respiratory system, and the lining of the urinary bladder; a pore created by invagination is the blastopore.
e. The late gastrula has, in addition to ectoderm and endoderm, a middle layer of cells called the mesoderm.
i. The outpocketings grow and fuse, forming a two layered mesoderm.
ii. The space between them is the coelom that contains the body organs.
iii. The mesoderm gives rise to the skeleton, the dermis of the skin, the skeletal system, the muscular system, the excretory system, the reproductive system (including most epithelial linings), and the outer layers of respiratory and digestive systems.
f. These germ layers then develop into those future organs.
3. Organ Stages of Development
a. The newly formed mesoderm cells along the main axis coalesce to form a dorsal notochord; it persists in lancelets but is replaced in frogs, chicks, and humans by the vertebral column.
b. The nervous system develops from the midline ectoderm located just above the notochord.
i. At first, the cells on the dorsal surface of the embryo thicken, forming the neural plate.
ii. Then neural folds develop on either side of a neural groove which becomes the neural tube when the folds fuse.
iii. At this point the embryo is called a neurula.
iv. Later, the anterior end of the neural tube develops into the brain; the rest becomes the spinalcord.
c. Midline mesoderm cells that did not contribute to the formation of the notochord now become two longitudinal masses of tissue.
i. The two tissue masses become blocked off into somites.
ii. The somites give rise to segmental muscles in all chordates; in vertebrates the somites also form the vertebral bones.
44.2 Developmental Processes
• Development requires growth, differentiation, and morphogenesis.
1. Cellular differentiation occurs when cells become specialized in their structure and function.
2. Morphogenesis produces a change in the shape and form of a body part; this includes both early cell movement and later pattern formation.
A. Cellular Differentiation
1. Each body cells contain a full set of chromosomes; therefore differentiation is not due to parceled out genes.
2. Cells in the adult body are totipotent; each contains all of the instructions to form any specialized cell.
3. Since only muscle cells produce myosin, only red blood cells produce hemoglobin, and only skin cells produce keratin, there must be differential gene expression.
4. Two mechanisms–cytoplasmic segregation and induction–seem especially important.
5. Cytoplasmic Segregation
a. Differentiation begins long before we can recognize specialized cell types.
b. Eggs contain substances called maternal determinants that influence the course of development.
c. Cytoplasmic segregation parcels out the maternal determinants as mitosis occurs and determines how the various cells of morula develop.
d. Early experiments showed the cytoplasm of a frog egg is not uniform in content.
e. After the first cleavage of a frog embryo, only a daughter cell that receives a portion of the gray crescent develops into a complete embryo.
f. Hans Spemann (Nobel Prize in 1935) found particular chemical signals within the gray crescent turn on the genes that control development.
6. Induction and Frog Experiments
a. As development proceeds, differentiation involves signals from neighboring cells.
b. Induction is the ability of one tissue to influence the development of another tissue.
c. Cell migration occurs during gastrulation; one set of cells can influence the migratory path of another set.
d. Spemann showed that the dorsal lip of a blastopore (primary organizer) was necessary for development.
i. The cells closest to the primary organizer become endoderm, those farthest away become ectoderm, and the intermediate cells became mesoderm.
ii. A molecular concentration gradient likely acts as a signal to induce germ layer differentiation.
e. Spemann and Hilde Mangold worked on the dorsal side of the embryo where the notochord and the nervous system develop.
i. The presumptive notochord tissue induces the formation of the nervous system when placed beneath belly ectoderm.
ii. Warren Lewis found that a developing lens induces the optic vesicle to form the optic cup in a frog embryo.
7. Induction in Caenorhabditis elegans
a. Caenorhabditis elegans is a transparent worm, 1mm long and easily raised in Petri dishes or liquid media.
b. It is hermaphroditic and self-fertile; therefore induced recessive mutations appear in the next generation.
c. Its entire genome has been sequenced.
d. Worm development takes only three days and an adult worm contains only 959 cells.
e. The destiny of each cell can be followed in specialization and fate maps drawn.
f. The anchor cell receives the most inducers to become the inner vulva; neighboring cells receive less and become the outer vulva.
g. Work with C. elegans shows that induction requires transcriptional regulation of genes in a particular sequence.
B. Morphogenesis
o Morphogenesis in Drosophila melanogaster (the fruit fly)
2. The first event in successful development is the establishment of the anterior/posterior axes.
3. In Drosophila eggs, there is a greater concentration of bicoid protein at one end.
a. Bicoid means "two-tailed"; a maternal mutation causes the egg to lack a head and it has two tails.
b. By cloning the bicoid gene and using it as a probe, mRNA was found in a gradient from anterior to posterior.
c. Proteins that influence morphogenesis are morphogens.
d. The bicoid gradient switches on the expression of segmentation genes; a gradient has a range of effects.
4. The Segmentation Pattern
a. The bicoid gradient begins a cascade of segmentation genes.
b. Christiane Nusslein-Vollard and Eric Wieschaus won a Nobel Prize for discovering segmentation genes in Drosophila.
c. By exposing flies to mutagens and then mapping mutated genes, they located segmentation abnormalities.
d. The first genes activated are gap genes; if mutated, they cause missing blocks of segments.
e. The pair-rule genes ensure 14 segments; a mutation reduces this to half.
f. Segment-polarity genes cause each segment to have an anterior and posterior half.
g. Morphogen gradients turn on genes because they are transcription factors that regulate which genes are active in which parts of the embryo in what order.
5. Homeotic Genes
a. Homeotic genes control pattern formation in animal morphogenesis.
b. In normal fruit fly development, homeotic genes are activated after the segmentation genes.
c. In the 1940s, Edward Lewis discovered homeotic genes that controlled which segment would bear antenna, legs, wings.
d. Homeotic genes have been found in many organisms; they all contain the same sequence of nucleic acids called a homeobox.
e. Because homeotic genes contain a homeobox in mammals, they are called Hox genes.
f. Each homeobox has a homeodomain, a sequence of sixty amino acids.
g. A homeodomain protein produced by one homeotic gene binds to and turns on the next homeotic gene, and this orderly process determines the overall pattern of the embryo.
h. Homeoboxes are derived from an original nucleic acid sequence that has been conserved because of its importance in regulation of animal development.
6. Apoptosis
a. Apotosis (programmed cell death) is important in morphogenesis.
b. When a cell-death signal is received, an inhibiting protein becomes inactive, allowing a cell-death cascade to proceed.
44.3 Humans Embryonic and Fetal Development
• Development covers events from conception (fertilization followed by implantation) to birth (parturition).
1. The time of birth is calculated by adding 280 days to the start of last menstruation.
2. Only about 5% of babies arrive on the forecasted date due to so many variables.
• Human development is divided into embryonic and fetal development.
1. The embryonic period, during months 1 and 2 of pregnancy, is when the major organs are formed.
2. Fetal development is during months 3–9, during which organ systems are refined.
• Development can also be divided into trimesters.
1. First trimester: embryonic and early fetal development occur.
2. Second trimester: development of organs and organ systems; the fetus is distinctly human at the end of the second trimester.
3. Third trimester: the fetus grows rapidly and the organ systems become functional.
• Extraembryonic membranes
1. Evolution of extraembryonic membranes in reptiles made development on land possible.
a. If an embryo develops in water, the water supplies the oxygen and takes away the wastes.
b. The surrounding water prevents desiccation and provides a protective cushion.
c. For an embryo on land, these functions are performed by extraembryonic membranes.
2. Chick extraembryonic membranes develop from extensions of germ layers, which spread over yolk.
a. The chorion lies next to the shell and carries on gas exchange.
b. The amnion contains protective amniotic fluid that bathes a developing embryo.
c. The allantois collects nitrogenous wastes.
d. A yolk sac surrounds the remaining yolk that provides nourishment.
3. Humans also have these membranes; their function is modified for internal development.
a. The chorion develops into the fetal half of the placenta.
b. A yolk sac is the first site of blood cell formation.
c. Allantoic blood vessels become the umbilical blood vessels.
d. The amnion surrounds the embryo and cushions it with amniotic fluid.
4. Therefore, all chordate animals develop in water, either in bodies of water or within the amniotic fluid.
A. Embryonic Development
1. The First Week
a. Fertilization occurs in the upper third of the oviduct; cleavage begins as the embryo moves down this tube to the uterus.
b. By the time the embryo reaches the uterus on the third day, it is a morula.
c. By the fifth day, the morula is transformed into a blastocyst.
i. A blastocyst is a hollow ball of cells, resulting from cleavage.
ii. The trophoblast is an outer single layer of cells, which later gives rise to the chorion.
iii. The inner cell mass is the mass of cells from which the embryo, and eventually the fetus, will develop.
2. The Second Week
a. At end of the first week, an embryo begins the process of implantation in the wall of the uterus.
b. The trophoblast secretes enzymes to digest away some of the tissue and blood vessels of the uterine wall.
c. The trophoblast begins to secrete human chorionic gonadotropin causing the corpus luteum to be maintained.
d. As the week progresses, the inner cell mass detaches itself from the trophoblast, and two more extraembryonic membranes form: the yolk sac and amnion.
e. The yolk sac forms below the embryonic disk; with no nutritive function in humans, it is the site of blood cell formation.
f. As in chick development, a human amnion and its cavity are where the embryo (and then the fetus) develop.
g. In humans, amniotic fluid insulates against any thermal changes; it also cushions and protects the fetus from trauma.
h. Gastrulation occurs during this week resulting in the inner cell mass flattening into an embryonic disk.
i. The embryonic disk is composed of two cell layers: the ectoderm above and the endoderm below.
ii. Once an embryonic disk elongates to form the primitive streak (similar to that found in birds), a third germ layer, the mesoderm, forms by invagination of the cells along the streak.
i. The trophoblast is reinforced by mesoderm and becomes the chorion.
3. The Third Week
a. The nervous system is the first organ system to become visually evident.
i. It appears as a thickening along the entire dorsal length of the embryo; invagination occurs as the neural folds appear.
ii. When the neural folds meet at the midline, the neural tube is formed.
iii. After the notochord is replaced by the vertebral column, the nerve cord is called the spinal cord.
b. The development of the heart begins in the third week and continues into the fourth.
i. The right and left heart tubes fuse; the heart begins pumping blood, although the chambers are not fully formed.
ii. The veins enter this largely tubular heart posteriorly, and the arteries exit anteriorly.
iii. Later the heart twists so that all of the major vessels are located anteriorly.
4. The Fourth and Fifth Weeks
a. A bridge of mesoderm (the body stalk) connects the caudal (tail) end of the embryo with the chorion, which has projections called chorionic villi.
b. The fourth extra embryonic membrane (the allantois) is contained in this stalk; its blood vessels become the umbilical blood vessels.
c. The head and tail then lift up, and the body stalk moves anteriorly by constriction.
d. Once this process is complete, the umbilical cord is fully formed.
e. Limb buds appear from which the arms and legs will later develop.
f. The head enlarges and the sense organs become more prominent.
g. Rudiments of the eyes, ears, and nose are evident.
5. The Sixth Through Eighth Weeks
1. The developing human becomes more humanlike in appearance.
2. As the brain develops, the head achieves its normal relationship with the body as a neck region develops.
3. The nervous system is developed well enough to permit reflex actions (e.g., the startle response to touch).
4. At the eighth week, the embryo is about 38 mm long and weighs no more than an aspirin tablet; all organs are established.
B. The Structure and Function of the Placenta
1. Providing gas, nutrient and waste exchange, the placenta begins formation once the embryo is fully implanted.
2. Chorionic villi are treelike extensions of the chorion.
. Chorionic villi project into the maternal tissues.
a. Later, the chorionic villi disappear in all areas except where the placenta develops.
3. By the tenth week, the placenta is fully formed and has already begun to produce progesterone and estrogen.
. Due to the negative feedback control by the hypothalamus and anterior pituitary, no new follicles mature.
a. They maintain the lining of uterus and there is no menstruation during pregnancy.
4. The chorionic villi are surrounded by maternal blood sinuses; the maternal and fetal blood do not mix.
5. Exchange of molecules between the fetal and maternal blood takes place across the walls of the chorionic villi.
6. CO2 and wastes move across from the fetus; O2 and nutrients flow from the maternal side.
7. The umbilical cord stretches between the placenta and the fetus.
8. Umbilical arteries transport CO2 and other waste molecules to the placenta for disposal; the umbilical vein transports O2 and nutrient molecules from the placenta to the rest of the fetal circulatory system.
9. Harmful chemicals can cross the placenta.
. This is of particular concern during the embryonic period, when various structures are first forming.
a. Each organ has a sensitive period during which a substance can alter the normal development.
b. A pregnant woman who takes thalidomide tranquilizer between days 27 and 40 is likely to have an infant born with deformed limbs; after this time period, the infant is born normal.
C. Fetal Development and Birth
1. Fetal development (months 3–9) involves an extreme increase in size; the weight multiplies 600 times.
2. The genitalia appear in the third month and gender can be identified anatomically.
3. A fetus soon acquires hair, eyebrows, eyelashes, and nails.
4. Fine, downy hair (lanugo) covers the limbs and trunk; it later disappears.
5. The skin grows so fast that it wrinkles; a waxy vernix caseosa protects the skin from the watery amniotic fluid.
6. A fetus at first only flexes its limbs and nods its head; later it moves its limbs vigorously; a mother feels movements from the fourth month onward.
7. After 16 weeks, a fetal heartbeat is heard through a stethoscope.
8. A fetus born at 24 weeks may survive; the lungs are still immature and often cannot capture O2 adequately.
D. Stages of Birth
1. When the fetal brain matures, the hypothalamus causes the pituitary to stimulate the adrenal cortex so that androgens are released.
2. The placenta uses androgens as precursors for estrogens that stimulate the production of prostaglandin and oxytocin.
3. The hormones estrogen, prostaglandin, and oxytocin all cause the uterus to contract and expel the fetus.
4. The process of birth (parturition) has three stages: dilation of the cervix, birth of the baby, and expulsion of the placenta.

45.1 Nature Versus Nurture: Genetic Influences
• Behavior is any action that can be observed and described.
A. Experiments with Lovebirds, Snakes, and Snails
1. Several species of lovebirds differ in the way the build nests.
2. Hypothesis: if the behavior for obtaining and carrying nesting material is inherited, then hybrids might show intermediate behavior.
a. When two species of lovebirds were mated, the hybrid birds had difficulty carrying nesting material.
b. These studies supported the hypothesis that behavior has a genetic basis.
3. Experiments with the garter snake (Thamnophis elegans) were conducted to determine if food preference have a genetic basis.
a. Inland populations are more aquatic and feed on frogs and fish; they refused to feed on slugs.
b. Coastal populations are more terrestrial and feed on slugs.
c. The hybrid newborn garter snakes have an intermediate acceptance of slugs.
d. Work with smell receptors and tongue flicks showed that physiological differences underlie the behavior.
4. Behavior of the marine snail Aplysia shows endocrine involvement in behavior.
a. Following copulation, the slug extrudes long strings of eggs and uses its head movements to attach the eggs to rocks.
b. Scientists isolated an egg-laying hormone (ELH) that causes the animal to lay eggs even if it has not mated.
c. ELH is a small protein of 36 amino acids that excites the reproductive tract and causes egg expulsion.
d. Recombinant DNA studies isolated the ELH gene and showed it controls the egg-laying behavior.
B. Experiments with Humans
1. Twin studies in the humans have been used to probe the nature-versus-nurture question.
2. Fraternal twins in same family are often different; identical twins reared apart are often similar.
3. The studies with twins support the hypothesis that at least certain types of behavior are primarily influenced by nature.
45.2 Nature versus Nurture: Environmental Influences
1. Fixed action patterns (FAPs) were believed to be behaviors that were always performed the same way, and they were elicited by a sign stimulus.
2. Many behaviors formerly thought to be fixed action patterns are found to have developed after practice.
3. Learning is defined as a durable change in behavior brought about by experience.
B. Learning in Birds
1. Laughing gull chicks beg food from parents by pecking at the parents' beaks; however, their accuracy improves with practice.
2. The chicks first peck at any beak model; later they only peck at models resembling the parents.
3. This interaction between chicks and parents appears to be a FAP.
4. Imprinting
a. Imprinting, another form of learning, involves a sensitive period.
i. Chicks, ducklings, and goslings follow the first moving object they see after hatching (usually their mother).
ii. A sensitive period is the only period during which a particular behavior such as imprinting, develops.
5. Song Learning
a. Song learning in birds involves a sensitive period when an animal is primed to learn; songs heard outside this period have no effect.
b. When provided with an adult tutor, birds learned other species' songs, indicating that social interactions assist in learning.
C. Associative Learning
1. A change in behavior that involves an association between two events is termed associative learning.
2. Classical Conditioning
a. In classical conditioning, the paired simultaneous presentation of two different types of stimuli causes an animal to form an association between them.
b. This suggests that an organism can be trained (conditioned) to associate any response with any stimulus.
c. Unconditioned responses are those that occur naturally; conditioned responses are those that are learned.
3. Operant conditioning
a. In operant conditioning, a stimulus-response connection is strengthened.
b. This resulted from reinforcing a particular behavior.
c. F. Skinner was famous for his studies in operant conditioning, always rewarding animals for the proper response.
D. Other Means of Learning
1. Animals also learn through insight, observation, and habituation.
2. Insight learning occurs when an animal suddenly solves a problem without any prior experience with the problem.
3. Many animals learn through observation and imitation.
4. Deer grazing on the side of a busy highway, oblivious to traffic, is an example of habituation.
45.3 Adaptive Mating Behavior
1. Sexual selection refers to adaptive changes in males and females that lead to an increased ability to secure a mate.
a. In males, this may result in an increased ability to compete with other males for a mate.
b. Females may select a mate with the best fitness (ability to produce surviving offspring), thereby increasing her own fitness.
B. Female Choice
1. Two hypothesis regarding a female's choice of a mate are:
a. The good genes hypothesis contends that females choose mates based on traits for improving the survival of offspring.
b. The runaway hypothesis states that females chose mates on the basis of traits that attract them to females; the trait can then become exaggerated until it is a handicap.
2. The Raggiana Bird of Paradise is dimorphic (males and females differ in size and other traits)
a. The ornateness of the male is a factor in selection by a female.
b. More feathery Raggiana are parasite-free, and their selection by females would increase the chance for survival.
C. Male Competition
1. A cost-benefit analysis can be applied to determine if the benefit of access to mating is worth the cost of competition among males.
2. Baboons have a dominance hierarchy.
a. A dominance hierarchy is a ranking within a group where the higher ranking individuals acquire more resources.
b. Dominance is determined by confrontation where one animal gives way to the other.
c. Baboons are dimorphic: males are larger and have large canine teeth; they decide when the troop moves, and they defend it.
d. Females mate with dominant males when ovulation is near; the dominant males then protect all young.
e. The drawbacks to being large and in danger are outweighed by the chance of fathering young.
f. The subordinate males have less chance to mate but they do have avenues to have some offspring.
3. Red deer stake out a territory, an area that is defended against competitors.
a. Territoriality involves the type of behavior needed to defend a particular territory.
b. A stag competes for females that form a harem that mates only with him.
c. A stag remains at peak fighting ability for only a short time; one stag can only father about two dozen offspring.
D. Mating in Humans
1. Human Males Compete
a. Humans, like many other mammals, are dimorphic.
b. Males are larger and more aggressive, perhaps due to past sexual selection by females.
2. Females Choose
a. Male mating success correlates best with income—wealthy males attracted mates better than nonwealthy males.
b. Apparently, females prefer to mate with a male who is wealthy and has a successful career—this will ensure that the children will live to reproduce.
3. Men Also Have a Choice
a. Men prefer women who will present them with children: health, age, "figure," faithfulness are all factors in a male's choice of a mate.
45.4 Sociobiology and Animal Behavior
1. Sociobiology applies the principles of evolutionary biology to the study of social behavior in animals.
2. It is based on a reproductive cost-benefit analysis of the value of living in a society.
3. There are both benefits and costs to living in a social group.
4. Only if the benefits, in terms of reproductive success, outweigh the disadvantages will societies evolve.
a. Advantages to living in a social group might include help to avoid predators, to raise young, and to find food.
i. A group of impalas has more eyes to see approaching predators, etc.
ii. Many fish moving rapidly in a school can distract a predator.
iii. The trumpet manucode (a bird) pair bonds for life; both sexes are needed to raise the young.
iv. Weaver birds form giant colonies to protect them from predators.
v. Primate members signal the group when they find a bountiful fruit tree.
vi. Lions working together can capture larger prey, such as a zebra or buffalo.
b. There are also disadvantages to living in a social group.
i. Disagreements occur between members over the best feeding places and resting sites.
ii. Among red deer, subordinate females are at a disadvantage in producing more prolific sons.
iii. Primate grooming may be necessary to keep them healthy since parasites spread easily in groups.
B. Altruism Versus Self-Interest
1. Altruism is defined as behavior that has the potential to decrease the lifetime reproductive success of the altruist, while benefitting the reproductive success of another member of the society.
2. Fitness, judged by reproductive success, may explain altruistic behavior.
3. Genes can be passed directly from parents to offspring.
4. Genes are also passed by relatives; an individual helping a relative survive and reproduce passes on the individual's genes.
5. Direct selection is adaptation to the environment due to the reproductive success of an individual.
6. Indirect selection, also called ken selection, involves adaptation to the environment due to the reproductive success of an individual's relatives.
7. Thus, the inclusive fitness of an individual includes personal reproductive success plus that of its close relatives.
8. In social insects, altruism is extreme and is explained on the basis that it helps reproducing siblings survive.
a. Only the queen among an army ant colony reproduces; the three castes of ants have given up reproducing.
b. In social bees and wasps, the queen is diploid but the drone is haploid; therefore sterile female workers are 75% related to their nest mates but would be only 50% related to their own offspring if they reproduced; thus, it is an advantage to care for the queen and her offspring.
9. Male chimpanzees in Africa did not interfere with each other's matings because they shared genes.
10. Reciprocal Altruism
a. In some species, bird offspring from one clutch of eggs may stay at the nest to help parents rear and feed the next batch of offspring.
b. For Florida scrub jays, the number of fledglings produced by an adult pair doubled when they had helpers.
c. Mammal offspring also help their parents; African jackals raise 1.4 pups alone; with helpers they raise 3.6.
d. The above are examples of reciprocal altruism: an animal helps or cooperates with another animal with no immediate benefit, but the animal that was helped will repay the debt at a later time.
45.5 Animal Communication
1. Some animals are largely solitary and join with a member of the opposite sex only to reproduce.
2. Others pair, bond, and cooperate in raising offspring.
3. Society members are organized in a cooperative manner extending beyond sexual or parental behavior.
B. Communicative Behavior
1. Communication is an action by a sender that influences the behavior of a receiver.
2. When the sender and receiver are members of the same species, signals will benefit both the sender and the receiver.
3. Chemical Communication
a. These signals are chemicals (e.g., pheromones, urine, and feces) and have the advantage of working both night and day.
b. A pheromone is a chemical released to cause a predictable reaction of another member of the same species.
c. Female moths attract males with tail gland pheromones; cats mark territory with urine, etc., and antelope mark twigs with eye gland secretions.
4. Auditory Communication
a. Auditory (sound) communication has advantages.
i. It is faster than chemical communication.
ii. It is effective both night and day.
iii. It can be modified by loudness, pattern, duration, and repetition.
b. Male crickets have calls for reproduction.
c. Birds have various songs for distress, courting and marking territories.
d. Whale songs have six basic themes for sexual and group identification.
e. Only humans can produce many different sounds and assemble them in many different ways.
f. Nonhuman primates are limited to about 40 distinct vocalizations with limited meaning.
g. Chimpanzees using artificial language cannot advance beyond the level of a 2-year-old child.
h. Chimps appear incapable of using language to reason or of using grammar.
5. Visual Communication
a. Visual signals are most often used by species that are active during the day.
b. Contests between males make use of threat postures and may prevent fighting.
c. Defense and courtship displays are exaggerated and always performed in the same way so the meaning is clear.
6. Tactile Communication
a. Tactile communication occurs when one animal touches another.
b. Gull chicks peck at the parent's beak in order to induce the parent to feed them.
c. A male leopard nuzzles the female's neck to calm her and to stimulate her willingness to mate.
d. Honeybees use tactile communication to impart information about the environment.


46.1 Scope of Ecology
1. Ecology is the study of the interactions of organisms with other organisms and with the physical environment.
2. Ecology studies how environmental factors determine the distribution and abundance of populations.
3. Ecology and evolution are related because ecological interactions are natural selection pressures that have long-term effects.
4. A habitat is the place where an organism exists.
5. A population is a group of the same species occupying a certain area.
6. A community consists of all populations at one locale (e.g., a coral reef population).
7. An ecosystem contains the community organisms and abiotic factors (e.g., energy flow, chemical cycling).
8. The biosphere is the layer on the earth where living organisms can live.
9. Modern ecology is both descriptive and predictive, with applications to wildlife management, agriculture, and many other problems.
46.2 Demographics of Populations
• Demography is the statistical study of a population, e.g., its density, distribution, rate of growth.
A. Density and Distribution
1. The population density is the number of individuals per unit area.
2. The population distribution is the pattern of dispersal of individuals across an area.
3. Resources are nonliving and living components of an environment that support living organisms.
4. Limiting factors are those environmental aspects that particularly determine where an organism lives.
a. Such factors include oxygen supply, light availability, space, temperature, and precipitation amounts.
5. Distribution can be due to biotic (living) factors.
a. Biotic factors can be illustrated by red kangaroos that are limited to inland Australia by the grasses that grow there.
6. Clumped, random, and uniform are terms used to describe patterns of distribution.
a. In certain cases, the pattern of distribution can change as the organisms under consideration mature; thus, distribution patterns are not necessarily constant.
7. Other factors, such as territoriality, seed dispersal, etc., can influence distribution patterns.
B. Population Growth
1. The rate of natural increase (r) is dependent on the number of individuals born every year and the number of individuals that die every year. (It is assumed that immigration and emigration are equal and are thus not considered.)
2. The highest possible rate of natural increase for a population when resources are unlimited is called its biotic potential, and it depends on the following:
a. Usual number of offspring per reproduction
b. Chances of survival until age of reproduction
c. How often each individual reproduces
d. Age at which reproduction begins
3. Mortality Patterns
a. A cohort is all members of a population born at the same time.
b. Survivorship is the probability of newborn individuals of a cohort surviving to particular ages.
c. A survivorship curve, obtained by plotting the number of individuals surviving at each age, is characteristic of each species.
i. In the Type I survivorship curve, most individuals live out their life span and die of old age (e.g., humans in well-developed countries).
ii. In the Type II survivorship curve, individuals die at a constant rate across their lifespan (e.g., birds, rodents, and perennial plants).
iii. In the Type III survivorship curve, most individuals die early in life (e.g., fishes, invertebrates, and plants).
4. Age Distribution
a. A population contains three age groups: prereproductive, reproductive, and postreproductive.
b. Because populations differ according to what proportion of the population falls into each group, three different age structure diagrams are possible; an age structure diagram is a representation of the number of individuals in each age group in a population
i. A pyramid-shape indicates the population has high birthrates; the population is undergoing exponential growth.
ii. A bell-shape indicates that prereproductive and reproductive age groups are more nearly equal, with the postreproductive group being smallest due to mortality; this is characteristic of stable populations.
iii. An urn-shaped diagram indicates the postreproductive group is largest and the prereproductive group is smallest, a result of the birthrate falling below the death rate; this is characteristic of declining populations.
iv. Information obtained from these graphs is used to determine past and future history of a population.
46.3 Population Growth Models
1. There are two patterns of population growth.
a. In discrete breeding, organisms reproduce once and cease to grow as adults; they expend energy in reproduction and then die.
b. In continuous breeding, organisms reproduce throughout their lifetime, which invests energy in their future survival.
2. Some organisms do not exactly fit these two patterns.
a. Aphids can switch between sexual and asexual reproduction according to the season.
b. Annual plants can reproduce both by seeds and by vegetative extensions.
B. Exponential Growth
1. The J-shaped growth curve depicts exponential growth, and it has two phases.
a. In the lag phase, growth is slow because the population is small.
b. In the exponential growth phase, growth is accelerating.
2. A mathematical equation can be used to calculate the exponential growth and size for any population that has discrete generations.
3. With exponential growth, the number of individuals added each generation increases as the total number of females increases.
4. For exponential growth to continue unchecked, each member of the population has to have unlimited resources.
5. As the population increases in size so do the effects of competition between members, predation, parasites, and disease.
C. Logistic Growth
1. When growth encounters environmental resistance, populations experience logistic growth, and the growth curve has a characteristic sigmoidal or S-shaped curve.
2. In addition to the lag phase and exponential growth phase, there is a deceleration phase where the rate of population growth slows, and a stable equilibrium phase with little if any growth, because births equal deaths.
3. This curve is called "logistic" because the exponential portion of the curve would plot as a straight line as log of N.
4. A mathematical equation calculates logistic growth.
5. Environmental resistance results in the deceleration phase and the stable equilibrium phase; population is at the carrying capacity of the environment.
6. Carrying Capacity
a. The carrying capacity (K) is the maximum number of individuals of a species that can be supported by the environment.
b. The closer population size gets to the carrying capacity, the greater is the environmental resistance.
c. When N is small, a large portion of the carrying capacity has not been utilized, but as N approaches K, population growth slows because (K - N) / K is nearing zero.
d. For example, over-fishing drives a population into the lag phase.
e. It is best to maintain desirable populations in the exponential phase of the logistic growth curve.
f. Farmers can reduce the carrying capacity for a pest by alternating rows of different crops.
46.4 Regulation of Population Size
1. The J-shaped and S-shaped growth curve models do not always predict real populations.
a. In the winter moth, many eggs did not survive the winter and exponential growth did not occur.
b. The growth curve of a reindeer herd introduced to St. Paul Island in Alaska overshot its carrying capacity and crashed.
B. Factors That Regulate Population Growth
1. Some populations are considered to be regulated primarily by density-independent factors; these are also considered abiotic factors.
a. The number of organisms present does not affect the influence of the factor.
b. The damage to a population from, for example, a flash flood does not change with or depend on the number of organisms present.
c. Thus, density-independent factors show no correlation with the size of the population.
2. Populations regulated by density-dependent factors are affected by the number of organisms present.
a. Predation, parasitism, and competition are considered density-dependent; the more these organisms crowd together, the more damaging are the food shortages, the parasites, and the predators.
b. Thus, density-dependent factors have some effect relative to the size of the population.
3. Other considerations
a. Intrinsic factors (e.g., anatomy, physiology, behavior) can also influence population size: territoriality, recruitment, immigration and emigration.
b. Populations can sometimes show extreme fluctuations in size and growth rates in spite of extrinsic and intrinsic regulating mechanisms—such wild fluctuations with no recurring pattern are termed chaos.
46.5 Life History Patterns
A. The logistic population model predicts two main life history patterns.
1. r-Selection
a. Species that underwent selection to maximize their rate of natural increase are categorized as r-selected.
b. These populations are often opportunistic species, and tend to be colonizers.
c. Their strategy for continued existence is based on individuals having the following traits:
i. small size,
ii. short life span,
iii. mature fast,
iv. produce many offspring, and
v. engage in little care of offspring.
d. Such organisms rely on rapid dispersal to new unoccupied environments.
2. K-Selection
a. Species that hold their populations fairly constant near the carrying capacity are called K-selected.
b. Such populations are equilibrium species; they are strong competitors, tend to be specialists rather than colonizers, and may become extinct when their evolved way of life is disrupted (e.g., the grizzly bear, Florida panther, etc.).
c. Their overall strategy for continued existence is based on having the following traits:
i. large size,
ii. long life span,
iii. slow to mature,
iv. produce few offspring, and
v. expend considerable energy in care.
3. Most populations cannot be characterized as either r- or K-strategists; they have intermediate characteristics.
46.6 Human Population Growth
1. The human population is now in an exponential part of a J-shaped growth curve.
2. World population increases the equivalent of one medium-sized city (216,000) per day and 79 million per year.
3. The doubling time is the length of time for a population size to double, now 53 years.
4. Zero population growth is when the birthrate equals the death rate and the population size remains steady.
5. The world population may level off at 8, 10.5 or 14.2 billion, depending on the decline in net reproductive rate.
B. More-Developed Versus Less-Developed Countries
1. The more developed countries underwent a demographic transition from 1950–1975; their growth rate is now low.
a. The more developed countries (MDCs) (e.g., Europe, North America, Japan, etc.) have low population growth and people enjoy a good standard of living.
b. Less developed countries (LDCs) (e.g., countries in Africa, Asia, Latin America) are those in which population growth is expanding rapidly and the majority of people live in poverty.
c. LDC growth rate peaked at 2.5% between 1960–1965; it has been declining slowly to about 1.6%.
d. Demographic transition is a decline in death rate followed by declining birthrate; it results in slower growth, about 0.1%.
2. The less developed countries (LDCs) are now undergoing demographic transition.
3. Most of the explosive growth will occur in Africa, Asia and Latin America unless
a. family planning or birth control are strengthened,
b. the desire for more children is reduced, and
c. the onset of childbearing is delayed.
C. Age Distributions
1. Age structure diagrams divide populations into: dependency, reproductive, and postreproductive.
2. Replacement reproduction, or each couple having just two children, will still cause population growth to continue due to the age structure of the population.
3. Mere replacement does not produce zero population growth (no increase in population) because more women enter reproductive years than leave them.
4. The LDCs have a higher growth rate because of a youthful age structure and more women entering reproductive ages than leaving.
5. The MDCs have a low growth rate because of a stabilized age structure.
D. Population Growth and Environmental Impact
1. Environmental Impact (E.I.)
a. Both the growing populations of LDCs and the high consumption of MDCs stress the environment.
b. An average American family, in terms of consumption and waste production, is equal to thirty people in India.
c. MDCs account for one-fourth the world population but provide 90% of the hazardous waste production.
d. Resource consumption affects the cycling of chemicals and contributes to pollution and extinction of species.
e. Conservation biology seeks sustainable practices to prevent mass extinction of species.


47.1 Concept of the Community
• A community is a group of populations that interact with one another in the same environment.
1. Communities vary in size and may have boundaries that are difficult to determine.
2. A fallen log supports a community but a passing bird can eat one of its members.
3. A forest may appear distinct but it gradually fades into the surrounding areas.
A. Community Composition and Diversity
1. The composition of a community is a listing of the species within a community; it does not reveal the relative abundance of organisms.
2. Diversity of a community includes not only a listing of the species in the community, but also the abundance of each species.
a. The greater the diversity, the greater the number and the more even the distribution of the species.
B. Models Regarding Composition and Diversity
1. The number of species in a community increases as we move from the poles to the equator.
2. The individualisticmodel states that each population is there because of its adaptations.
a. A species range is based on its tolerance for abiotic factors including light, water, salinity, etc.
b. Species will have independent distributions; boundaries between communities will not be distinct.
3. The interactivemodel of community structure proposes:
a. A community is simply a higher level of organization beginning from cell to tissue to organism;
b. Just as cells are adapted to each other, a community had species adapted to each other;
c. A community remains stable because of homeostatic mechanisms;
d. The same species will recur in communities whose boundaries are distinct from one another.
4. Modern ecological data supports the individualistic model.
a. F. H. Talbot and co-workers built artificial reefs and set them in a uniform tropical lagoon.
b. Of the 42 species that colonized the reefs, there was only a 32% similarity reef-to-reef.
c. From month to month, 20–40% of the species changed.
d. The reef species composition appeared to depend on chance migrations.
e. Certain animals occurred near their food source and this determined their range.
f. Most likely, community structure depends on both abiotic and biotic factors.
C. Island Biogeography
1. Robert MacArthur and E. O. Wilson developed the general theory of islandbiogeography.
2. Nearby islands have more species because immigration is easier.
3. Larger islands have more species because a large island has more resources.
4. "Islands" can also include patches of forest surrounded by cropland, housing developments, etc.
a. The spatial heterogeneity model describes the patchiness of an environment.
b. The greater the number of habitat patches, the greater the diversity.
5. Stratification is an increase in vertical living spaces; a tree canopy provides a high-rise habitat and vertical patchiness.
6. An equilibrium point is reached when the rate of species immigration matches the rate of species extinction.
7. An equilibrium point can be dynamic with many species arriving and departing, or steady unless disturbed.
47.2 Structure of the Community
1. Interactions include: competition for resources, predator-prey interaction, and parasite-host interactions.
2. Competition between two species for limited resources negatively affects the population size of both species.
3. Predation and parasitism increase the predator population at the expense of the prey and host populations.
4. In parasitism, one species is benefitted, the other is harmed.
5. In commensalism, one species is benefitted, the other is neither benefitted nor harmed.
6. In mutualism, both species benefit.
B. Habitat and Ecological Niche
1. A habitat is where an organism lives and reproduces in the environment.
2. The ecological niche is the role an organism plays in its community, including its habitat and its interactions with other organisms.
a. The fundamentalniche is the range of conditions under which it can survive and reproduce.
b. The realizedniche is the set of conditions under which it exists in nature.
3. Generalist species (e.g., raccoons, roaches, humans) have a broad range of niches.
a. They have a survival advantage when environmental conditions are apt to change.
4. Specialist species (e.g., pandas, spotted owls, freshwater dolphins) have a narrow range of niches.
a. They have a survival advantage in stable environments.
C. Competition Between Populations
1. Interspecific competition occurs when different species utilize a resource (e.g., light, nutrient) that is in limited supply.
2. If the resource is not in limited supply, there is no competition.
3. Lotka and Volterra (1920s) developed a formula: competition favors one species and can eliminate the other.
4. Gause grew two species of Paramecium in one test tube; only one survived if they were grown together.
5. Competitive exclusion principle: no two species can indefinitely occupy the same niche at the same time.
6. Over time, either one population replaces the other or the two species evolve to occupy different niches.
7. If it appears two species occupy the same niche, there must be slight differences; Gause found two species of paramecium coexisted if one fed on bacteria at the bottom of the tube and the other fed on suspended bacteria.
8. Resource partitioning occurs when species shift niches; they no longer directly compete.
a. Three species of Galapagos Island finches have three sizes of beaks for small, medium, and large seeds.
b. When species live on separate islands, their beak sizes are intermediate; when they live together, their beak sizes are diversified; this is character displacement.
c. Five species of warblers in the same tree spent time in different tree zones to avoid competition; they had different niches.
d. Swallows, swifts, and martins fly in mixed flocks eating aerial insects but have different nesting sites, etc.
e. The above examples are deduced from already completed partitioning.
f. Joseph Connell studied the competition occurring in barnacles that consistently shift to match shoreline tidal zones.
i. By removing the larger Balanus barnacles from the lower zone, the smaller barnacles easily moved in.
ii. The smaller barnacle is more resistant to drying out; but the larger one can overgrow it.
D. Predator-Prey Interactions
1. Predation occurs when one organism (predator) feeds on another (prey).
2. In a broad sense, it includes not only single predator-prey kills, but also filter feeding whales that strain krill, parasitic ticks that suck blood, and even herbivorous deer that eat leaves.
3. Predator-Prey Population Dynamics
a. Some predators reduce the densities of their prey.
i. When Gause reared the protozoans Paramecium caudatum and Didinium nasutum together in culture, Didinium ate all the Paramecium and then died of starvation.
ii. When prickly-pear cactus was introduced to Australia from South America, it spread wildly without competition on the desert; a natural predator moth from South America was introduced and the cactus and moth populations plummeted dramatically.
b. Natural predator-prey relationships allow persistent populations of both predator and prey populations, though both may fluctuate over time.
i. Often a graph of predator-prey population densities shows regular peaks and valleys with the predator population lagging slightly behind the prey; two reasons are possible.
1. The biotic potential of the predator may be great enough to overconsume the prey; the prey population declines and the predator population then follows.
2. Or the biotic potential of the prey is unable to keep pace and the prey population overshoots the carrying capacity and suffers a crash.
4. The Classic Case of the Snowshoe Hare and the Canadian Lynx
a. Careful records of pelts of both animals for over a century have demonstrated regular fluctuations.
b. To test whether the lynx or hare food supply was causing the cycling, three experiments were done.
1. A hare population was given a constant supply of food and predators were excluded; the cycling ceased.
2. The hare populations were given a constant food supply but predators were not excluded; the cycling continued.
3. Predators were then excluded but no extra food was added; the cycling continued.
c. The interpretation of these results is that both a hare-food cycle and a predator-hare cycle combine to produce the overall effect.
d. The grouse population also cycle, perhaps because the lynx switches to grouse when the hare populations decline; thus predators and prey do not normally exist as simple two-species systems.
E. Prey Defenses and Other Interactions
1. Prey have evolved a variety of antipredator defenses.
2. Plant adaptations for discouraging predation include:
a. sharp spines,
b. tough leathery leaves,
c. poisonous chemicals in their tissues, and
d. chemicals that act as hormone analogues to interfere with insect larval development.
3. Animals have defenses that include:
a. camouflage for concealment; this also requires behavior (stillness),
b. cryptic coloration to blend into the surroundings,
c. fright of the predator,
d. warning coloration,
e. vigilance and association with other prey for better warning.
F. Mimicry
1. Mimicry occurs if one species (the mimic) resembles another species (the model) possessing an antipredator defense.
2. Batesian mimicry, named for Henry Bates, is a form of mimicry in which one species that lacks defense mimics another that has successful defenses.
3. Mullerian mimicry, named for Fritz Mhller, is where several different species with the protective defenses mimic one another (e.g., stinging insects all share same black and yellow color bands).
G. Symbiotic Relationships
1. Symbiosis is a close relationship between members of two populations.
2. Parasitism
a. Parasitism is similar to predation; the parasite derives nourishment from the host.
b. Viruses are always parasitic; parasites occur in all kingdoms of life.
c. Endoparasites are small and live inside the host.
d. Ectoparasites are larger and remain attached to the body of hosts by specialized organs or appendages.
e. Many parasites have two hosts.
. The primary host is the main source of nutrition.
i. The secondary host may serve to transport (vector) the parasite to other hosts.
f. Parasites are specific and require certain species as hosts.
g. Lyme Disease
. The bacterium Borrelia burgdorfei causes arthritis-like symptoms in humans.
i. The bacterium primarily lives in white-tailed deer mice.
ii. The larval deer ticks of Ixodes dammini or I. ricinus feed on deer mice and can transfer the bacteria to humans.
3. Commensalism
a. In commensalism, one species benefits and the other is neither harmed nor benefitted.
b. It is difficult to determine true commensalism because it is difficult to ensure that the host is not harmed.
c. Barnacles secure a home by attaching themselves to the backs of whales and the shells of horseshoe crabs.
d. Remora fish attach themselves to the bellies of sharks, securing a free ride and the remains of the shark's meals.
e. Epiphytes (Spanish moss) grow in the branches of trees to receive light but take no nourishment from the tree.
f. Clownfish live within the tentacles of sea anemones for protection.
g. Some relationships are so loose that it is difficult to know if they are true commensalism.
. Cattle egrets feed near cattle because the egrets flush insects as they graze.
i. Baboons and antelopes forage together for added protection.
4. Mutualism
a. In mutualism, both species benefit.
b. Mutualism can be found among organisms in all kingdoms of life.
c. Examples include the following:
. Bacteria in the human intestinal tract are provided with some of our food but also provide us with vitamins.
i. Termites can only feed on wood because their gut contains the protozoa that digest cellulose.
ii. Mycorrhizae are symbiotic associations between the roots of fungal hyphae and plants.
iii. Flowers and insect pollinators represent a shift from insects eating pollen to eating nectar.
iv. Lichens are made of algae that produce food and fungi that preserve water, although the algae can survive alone.
d. Classic Example of the Ant and the Acacia Tree
. In tropical America, the bullhorn acacia provides a home for ants in its hollow thorns.
i. The acacia also provides ants with food from its nectaries, and protein in nodules called Beltian bodies.
ii. In return, the ant protects the plant from herbivores and other plants that might shade it.
iii. When ants on an experimental tree were killed with insecticide, the tree also died.
e. Tree-Ant-Caterpillar Complex
. Trees in the genus Croton also have nectaries that feed ants.
i. Ants have a mutualistic relationship with Thisbe caterpillars that feed on Croton saplings.
ii. Thisbe caterpillars offer nourishment to ants, keeping them nearby.
iii. The caterpillar releases the same chemical that causes the ants to defend an ant colony.
iv. The result is that the caterpillars are protected while feeding on the trees.
f. Cleaning Symbiosis
. Crustacea, fish, and birds act as cleaners to a variety of vertebrate clients; this is called cleaning symbiosis.
i. Large fish in coral reefs line up at cleaning stations and wait their turn to be cleaned by small fish.
ii. The possibility of feeding on host tissues as well as on ectoparasites complicates this case of mutualism.
47.3 Community Development
• Communities change over both short and long intervals of time due to continental drift, glaciation, etc.
A. Ecological Succession
1. Ecological succession is a change involving a series of species replacements in a community following a disturbance.
2. Primarysuccession begins in a habitat lacking soil; this might occur following a volcanic eruption.
3. Secondary succession begins when soil is already present but it has been disturbed and returns to a natural state, as in an abandoned cornfield.
a. In the first years, wild grasses and other pioneer species (plants that are invaders of disturbed areas) invade.
b. Soon sedges and shrubs invade.
c. Later, there is a mixture of shrubs and trees.
4. In 1916, Clements proposed the climax-pattern model of succession: that succession leads to a climax community that is characteristic for an area.
a. A climax community has a community composition that depends on climate.
i. Dry climates eventually produce deserts.
ii. Wet climates proceed to forests.
iii. Intermediate moisture will result in grasslands, shrubs, etc.
iv. Soils will also influence the developing community.
b. Each stage facilitates the occurrence of the next stage (called the facilitation model).
i. Shrubs cannot grow on dunes until the dune grass has developed the soil.
ii. Therefore the grass-shrub-forest must occur sequentially.
5. The inhibition model challenged Clements's view of succession.
a. Colonists hold onto their space and inhibit the growth of other plants until the colonists die.
b. Death releases resources that allow different, longer-lived species to invade.
6. The tolerance model provides yet another view of succession.
a. Sheer chance may determine which seeds arrive first; in this case, the successional stages may merely reflect the maturation time.
b. Trees merely take more time to develop; however, both facilitation and inhibition of growth may be taking place.
7. All models are probably involved and succession may not often reach the same final potential natural community.
47.4 Community Biodiversity
1. Community stability is seen in three ways: persistence through time, resistance to change, and recovery once a disturbance occurred.
2. A forest may remain unchanged year after year; that is persistence.
3. A deciduous forest resists change by regrowing its leaves after an insect infestation.
4. A chaparral community is resilient to fire and quickly recovers to its normal state.
B. The Intermediate Disturbance Hypothesis
1. The intermediate disturbance hypothesis states a moderate level of disturbance yields the highest community diversity.
2. Fire, wind, severe weather, and water erosion are abiotic and external factors that cause such disturbances.
3. If disturbances affect one type of patch and not another, the effect of patchiness is to provide overall stability.
4. If widespread disturbances occur frequently, diversity is limited and a community will be dominated by rapid growth and short life span (r-strategist) colonizers.
5. When disturbances are less widespread and infrequent, species with slow growth rates and long life spans will (K-strategists) dominate.
6. Therefore, too much disturbance, or not enough, may threaten the diversity of tropical rain forests and coral reefs.
7. Archeological remains show that the Maya cultivated huge areas from 300 to 900 ad; the civilization collapsed, and 1,200 years later the community composition still remains different from a local tropical rainforest.
C. Predation, Competition, and Biodiversity
1. Predation by a particular species can reduce competition and increase diversity.
a. Robert Payne removed the starfish Pisaster from test areas along the coast of North America.
b. In the control area, there was no change in numbers of species.
c. In the removal area, the mussel Mytilus increased in number and excluded other invertebrates and algae from attachment sites.
2. Such predators that regulate competition and maintain diversity are called keystone species; thus, keystone species are organisms that play a greater role in maintaining the function and diversity of an ecosystem than would be predicted by their abundance.
3. The elephant may be a keystone species in the savannah where it feeds on shrubs and trees and causes the woodlands to become grassy savannas.
D. Island Biogeography and Biodiversity
1. Predation has changed the Barro Colorado Island.
a. This island was formed in Panama from damming a river in the 1910s.
b. Island biogeography predicts fewer species can survive on islands; the jaguar, puma and ocelot are now gone.
c. As a result, the medium-sized coatimundi increased in numbers; it is a predator of bird eggs.
d. Thus, the numbers of bird species is less on the island than is expected for its size.
2. Therefore preserves must be larger in order to conserve the k-strategists, especially top predators.
E. Exotic Species and Biodiversity
1. Introduction of exotic species is devastating if they are not held in check by predators and competitors.
2. African honeybees introduced into Brazil displaced domestic honeybees.
3. The brown tree snake introduced to Guam has devastated the bird population.
4. The red fox was introduced in Australia to prey on the introduced European rabbit; it was successful but has also reduced the populations of native small mammals.
48.1 The Nature of Ecosystems
• The Earth
1. The hydrosphere is the zone of water that covers over three-quarters of the earth.
a. The oceans take up a great amount of heat and then release it slowly to the atmosphere.
b. The ability of water to absorb and release great quantities of heat keeps our earth's climate within a livable range.
2. The atmosphere is the gaseous layer near the earth.
a. The atmosphere is concentrated in the lowest 10 kilometers, but it extends thinly out to 1,000 km.
b. The major atmospheric gases are nitrogen, oxygen and carbon dioxide.
c. Carbon dioxide is necessary for photosynthesis.
d. Oxygen is necessary for cellular respiration, and oxygen makes up the protective ozone (O3) in the upper atmosphere.
3. The lithosphere is the rocky substratum that extends from the surface about 100 kilometers deep.
a. The weathering of rocks supplies minerals to plants and eventually forms soil.
b. Soil contains decayed organic material (humus) that recycles nutrients to plants.
4. The biosphere is the thin layer where life is possible between the outer atmosphere and the lithosphere.
5. Ecosystems are characterized by one-way flow of energy through the biotic community and a cycling of materials from the abiotic environment to the biotic community and back.
A. Biotic Components of an Ecosystem
1. Living things are organized in an ecosystem by how they secure their food: autotrophs or heterotrophs.
2. Autotrophs
a. Autotrophs capture energy (e.g., sunlight) and use it, along with inorganic nutrients, to produce organic compounds; therefore they are also called producers.
b. Photosynthetic organisms possess chlorophyll and carry on photosynthesis.
i. Algae are the main producers in freshwater and marine environments.
ii. Green plants are the main land photosynthesizers.
c. Chemoautotrophs are bacteria that obtain energy from the oxidation of inorganic compounds such as ammonia, nitrites, and sulfides; they synthesize carbohydrates and are found in cave communities and ocean depths.
3. Heterotrophs
a. Heterotrophs need a source of preformed organic nutrients and consume tissues of other organisms; they are called consumers.
b. Herbivores are animals that feed directly on green plants.
c. Carnivores are animals that eat other animals.
d. Omnivores can feed upon a variety of organisms, including plants and animals; humans are omnivores.
4. Decomposers are nonphotosynthetic bacteria and fungi that extract energy from dead matter, including animal wastes in the soil, and make nutrients available.
5. Some animals (e.g., earthworms) feed on detritus¾the decomposing products of organisms—these organisms are called detritivores.
B. Energy Flow and Chemical Cycling
1. All ecosystems are dependent upon solar energy flow and finite pools of nutrients.
2. Most ecosystems cannot exist without a continual supply of solar energy.
3. Energy flow in an ecosystem is a consequence of two fundamental laws of thermodynamics:
a. The first law of thermodynamics states energy can neither be created nor destroyed; it can only be changed from one form of energy to another.
b. The second law of thermodynamics states when energy is transformed from one form to another, there is always some loss of energy from the system, usually as low grade heat.
48.2 Energy Flow
• Food Webs
1. The complex trophic (feeding) relationships that exist in nature are called food webs.
2. A grazing food web begins with leaves, stems and seeds eaten by herbivores and omnivores.
3. A detrital food web begins with detritus, followed by decomposers (including bacteria and fungi).
4. Detrital food chains are connected to a grazing food chain when consumers of a grazing food chain feed on the decomposers of the detrital food chain.
5. In some ecosystems, more energy may move through the detrital food web than moves through the detritus food web.
A. Trophic Levels
1. A food chain represents a single path sequence of organisms that form links.
2. A trophic level is a feeding level of one or more populations in a food web; those organisms in an ecosystem that are the same number of food chain steps from the energy input into the system:
a. first trophic level—primary producers,
b. second trophic level—all the primary consumers,
c. third trophic level—all the secondary consumers, etc.
B. Ecological Pyramids
1. About 10% of the energy at a particular trophic level is incorporated into the next trophic level.
a. Thus, 1,000 kg (or kcal in an energy pyramid) of plant material converts to 100 kg of herbivore tissue, which converts to 10 kg of first carnivores, which can support 1 kg of second level carnivores.
b. This rapid loss of energy is the reason food chains have from three to four links, rarely five.
c. This rapid loss of energy is also the reason there are few large carnivores.
2. An ecological pyramid shows this trophic structure of an ecosystem as a graph representing biomass, organism number, or energy content of each trophic level in a food web.
3. The base of the pyramid represents the producer trophic level, and from there the consumer trophic level is stacked, with the apex representing the highest consumer trophic level.
4. A pyramid of numbers is based on the number of organisms in each trophic level.
5. A pyramid of biomass is based on the weight (biomass) of organisms at each trophic level at one time; this eliminates size of the organisms as a factor.
a. Usually a large mass of plants supports a medium mass of herbivores and a small mass of carnivores.
b. However, at one point in time at seashores, herbivores can have greater biomass feeding on algae that reproduce fast but are eaten, producing an inverted pyramid; over long time periods, the biomass is a normal pyramid.
6. One problem is where to fit in the decomposers; a large portion of energy becomes detritus in many ecosystems.
48.3 Global Biogeochemical Cycles
1. All organisms require a variety of organic and inorganic nutrients.
2. Biogeochemical cycles are the pathways by which chemicals circulate through the biotic and abiotic components of an ecosystem.
3. A reservoir is that portion of the earth that acts as a storehouse for the element.
4. An exchange pool is the portion of the environment from which producers take chemicals, such as the atmosphere or soil.
5. The biotic community is the pathway through which chemicals move through food chains.
6. Some cycles are primarily gaseous cycles (carbon and nitrogen); others are sedimentary cycles, (phosphorus).
B. The Water Cycle
1. A transfer rate is defined as the amount of a substance that moves from one component of the environment to another within a specified period of time.
2. In the water or hydrologic cycle, freshwater evaporates and condenses on the earth.
3. The evaporation of water from the oceans leaves behind salts; during condensation, a gas is exchanged into a liquid—vaporized fresh water rises into the atmosphere and returns to Earth in the form of precipitation.
4. Precipitation that percolates into the earth forms a water table at the surface of the groundwater.
5. An aquifer is an underground storage of fresh water in porous rock trapped by impervious rock.
6. Freshwater makes up about 3% of the world's supply of water and is considered a renewable resource.
7. However, freshwater becomes unavailable when consumption exceeds supply or is so polluted that it is not usable; when water withdrawal from aquifers exceeds replenishment, it is called "groundwater mining."
C. The Carbon Cycle
1. Both terrestrial and aquatic organisms exchange carbon dioxide with the atmosphere—this is called the carbon cycle.
2. On land, photosynthesis removes CO2 from the atmosphere; respiration then returns CO2 to the atmosphere.
3. CO2 from the air combines with water to produce bicarbonate (HCO3), which is a source of carbon for aquatic producers, primarily protists.
4. Similarly, when aquatic organisms respire, the CO2 they release combines with water to form bicarbonate ions (HCO3-).
5. The reservoir for the carbon cycle is largely composed of organic matter, calcium carbonate in shells, and limestone, as well as fossil fuels.
6. Carbon Dioxide and Global Warming
a. The transfer rates between photosynthesis and respiration (including decay) are about even.
b. Because we burn fossil fuels and forests, there is now more CO2 entering the atmosphere than is removed.
c. In 1850, atmospheric carbon dioxide was about 280 ppm; today it is about 350 ppm.
d. CO2, nitrous oxide, and methane are greenhouse gases that contribute to the rise in Earth's temperature, a phenomenon called global warming.
e. The above gases and water vapor increase the greenhouse effect that holds heat next to the Earth.
f. The increased heat may cause more clouds that in turn increase global warming.
g. Computer models cannot incorporate all variables; predictions are for 1.5–4.5°C increase by 2100.
h. Possible results may include glaciers melting, sea levels rising, a redistribution of dry and wet regions, and an increase in species extinctions.
D. The Nitrogen Cycle
1. Nitrogen gas (N2) is 78% of the atmosphere, yet nitrogen deficiency can limit plant growth.
2. In the nitrogen cycle, plants cannot incorporate N2 into organic compounds and they therefore depend on the various types of bacteria to make nitrogen available to them.
3. Nitrogen fixation occurs when N2 is converted to a form that plants can use.
a. Other nitrogen-fixing bacteria, living in nodules on the roots of legumes, make reduced nitrogen and organic compounds available to a host plant.
b. Some cyanobacteria in water and the free-living bacteria in soil are able to reduce N2 to ammonium (NH4+ ).
c. Plants take up both NH4+ and nitrate (NO3-) from the soil.
d. After plants take up NO3-, it is enzymatically reduced to NH4+ that is then used to synthesize amino and nucleic acids.
4. Nitrification is the production of nitrates (NO3-).
a. Nitrogen gas is converted to NO3-in the atmosphere when cosmic radiation, meteor trails, and lightning provide the high energy for nitrogen to react with oxygen.
b. Nitrifying bacteria convert NH4+ to NO3-.
c. Ammonium in the soil is converted to NO3-by nitrifying bacteria in the soil in a two-step process that does not depend on nitrogen gas.
i. First, nitrite-producing bacteria convert NH4+ to nitrite (NO3-).
ii. Then, nitrate-producing bacteria convert NO2-to NO3-.
5. Denitrification is conversion of NO3-to nitrous oxide (N2O) and N2.
a. There are denitrifying bacteria in both aquatic and terrestrial ecosystems.
b. Denitrification counterbalances nitrogen fixation, but not completely; more nitrogen fixation occurs.
6. Nitrogen and Air Pollution
a. Production of fertilizers and burning of fossil fuels adds three times the nitrogen oxides to the atmosphere as normal.
b. Acid deposition occurs when nitrogen oxides and sulfur oxides combine with water vapor in the atmosphere.
c. Photochemical smog results when nitrogen oxides and hydrocarbons react in the presence of sunlight; smog contains ozone (O3) and peroxyacetylnitrate (PAN) and causes respiratory problems.
d. Air pollutants, that might otherwise escape, are trapped near the ground by thermal inversions where cold air is trapped near the ground by warm air above.
E. The Phosphorus Cycle
1. In the phosphorus cycle, weathering makes phosphate ions (PO4 and HPO42-) available to plants that take up phosphate from the soil.
2. Some of this phosphate runs off into aquatic ecosystems where algae incorporate it into organic molecules before it is entrapped in sediments.
3. Phosphate that is not taken up by algae is incorporated into sediments in the oceans.
4. Sediment phosphate only becomes available when geological upheaval exposes sedimentary rocks.
5. Phosphate taken up by producers is incorporated into a variety of organic compounds.
6. Animals eat producers and incorporate some of the phosphate into phospholipids, ATP, and nucleotides of DNA; however what is in teeth, bones, and shells does not decay for long periods.
7. Decay of organisms and decomposition of animal wastes eventually makes phosphate ions available again.
8. Available phosphate is generally taken up quickly; it is usually the limiting nutrient in most ecosystems.
9. Phosphorus and Water Pollution
a. Humans boost the supply of phosphate by mining phosphate ores for fertilizers, detergents, etc.
b. Run-off of animal wastes from livestock feedlots and commercial fertilizers from cropland as well as discharge of untreated and treated municipal sewage can all add excess phosphate to nearby waters.
c. Eutrophication is the name of this over-enrichment that leads to algal blooms; when the algae die off, decomposers use up all of the oxygen and this can cause a massive fish kill.
d. Biological magnification is the concentration of non-degraded chemicals as they move up the food chain; DDT is a classic example.
e. Oil spills add over 5 million metric tons of oil a year to oceans.
f. Human activities including pollution and fishing have exploited ocean resources to the brink of extinction.
49.1 Climate and the Biosphere
1. Climate is the prevailing weather conditions in a region over time.
2. Climate is primarily dictated by temperature and rainfall which is influenced by two factors:
a. variations in solar radiation due to the tilt of the spherical Earth, and
b. other effects such as topography and whether a body of water is nearby.
B. Effect of Solar Radiation
1. The Earth is a sphere; the sun's rays are more direct near the equator and spread out near the poles.
2. The tropics are therefore warmer than temperate areas.
3. The tilt of the Earth's axis as it rotates about the sun causes one pole to be more directly exposed to sunlight.
4. Cold air is heavy and sinks; hot air is lighter and rises.
a. Therefore if the Earth were standing still, equatorial air would rise and move toward the poles.
b. This would replace heavy polar air that sinks and flows toward the equator, now a low pressure area.
c. In a world that stood still, this would produce high winds moving toward the poles and surface winds moving toward the equator.
5. The Earth's Rotation Has an Effect
a. The wet equatorial air loses its moisture as it rises and cools near the equator.
b. By the time it moves 30° to the north, the air descends, reheats and is dry; this is a zone of deserts.
c. Because of the Earth's rotation, from the equator to 30° north and south, surface winds blow from east-southeast in the Southern Hemisphere and from the east-northeast in the Northern Hemisphere making east coasts wet.
d. Between 30° and 60° north and south, strong winds called the prevailing westerlies blow from west to east.
e. The west coasts of continents in these latitudes are wet as is the Pacific Northwest.
f. Weaker polar easterlies blow from east to west between 60° north or south and the respective poles.
g. The Earth's rotation, continents, and oceans alter the three circulation cells between the equator and poles.
C. Other Effects
1. Topography is the physical features or "lay" of the land.
2. Mountains cause rain and rain shadows.
a. Air blowing up over a mountain range rises and cools; the windward side therefore receives more rainfall.
b. The leeward side of the mountain range receives dry air; it is in a rain shadow.
c. The Hawaiian Islands experience over 750 cm of rain on the windward side but only average 50 cm in the rain shadow.
d. The western side of the Sierra Nevada Mountains is lush; the eastern side is a semidesert.
3. Coastal Breezes
a. Since the land heats up and cools down faster than oceans, it causes a daily pattern.
b. In the day, land heats up and warm air rises; then cool sea breezes blow inland to replace the rising air.
c. At night, the land cools first and the cold air sinks and blows out to sea.
4. Monsoon Climates
a. The India and south Asia climate generates wet ocean winds for almost half the year.
b. The land heats more rapidly than the waters of the Indian Ocean during spring.
c. The difference in temperature causes a gigantic circulation of air with warm air rising and cooler air continuously coming in from the ocean to replace it.
d. As the warm air rises, it loses its moisture and the monsoon season begins.
5. The "lake effect"
a. Winter Arctic winds blowing across the Great Lakes become warm and moisture laden.
b. When these winds rise and lose their moisture, a large amount of snow falls.
49.2 Terrestrial Ecosystems
A. Biome Distribution
1. The biosphere is divided into large biogeographic units called biomes.
2. A biome has a particular mix of plants and animals adapted to live under certain environmental conditions.
3. The average temperature and rainfall influences where the different biomes are found on the surface of the Earth.
4. Climate, and mainly solar radiation and topography, is the principle determinant of the distribution of biomes.
5. A latitude temperature gradient is also seen when we consider altitude; the rain forest–deciduous forest–coniferous forest–tundra sequences are also seen when ascending a mountain.
a. The mountain coniferous forest is a montane coniferous forest.
b. The tundra near the peak is an alpine tundra.
B. Tundra
1. The Arctic tundra encircles the Earth south of the ice-covered polar seas in the Northern Hemisphere.
2. Arctic tundra covers 20% of the Earth's land surface; it is cold and dark much of the year.
3. The tundra receives about 20 cm of rainfall annually; this would constitute a desert but the melting snow provides water during summer and very little evaporates.
4. Only the topmost layer of Earth thaws; the permafrost beneath is always frozen.
5. Trees are not found in the tundra because
a. the growing season is too short,
b. their roots cannot penetrate the permafrost, and
c. trees cannot become anchored in the boggy soil of summer.
6. In the summer, the ground is covered with sedges and short-grasses with patches of lichens and mosses.
7. Dwarf woody shrubs flower and seed quickly while there is sunlight for photosynthesis.
8. Only a few animals adapted to cold live in the tundra year-round (e.g., lemming, ptarmigan, and musk-ox).
9. During the summer, the tundra contains many insects, birds, and migratory animals (e.g., shore birds, waterfowl, caribou, reindeer, and wolves).
C. Coniferous Forests
1. Conifer forests are found in three locations: taiga, montane coniferous forests, and temperate coniferous forests.
2. Taiga is coniferous forest extending across northern Eurasia and North America.
3. Near a mountain top is a similar conifer forest called a montane coniferous forest.
4. On the Pacific Coast from Canada down to California is part of the temperate rain forest.
5. Conifer forests contain great stands of spruce, fir, hemlock, and pine; these trees have thick protective leaves or needles and bark.
6. The needlelike leaves can withstand the heavy weight of snow.
7. There is a limited understory of plants; the floor is covered by low-lying mosses and lichens beneath the layer of needles.
8. Birds harvest the seeds of conifers; bears, deer, moose, beaver and muskrat live around the cool lakes and streams.
9. Major carnivores include wolves, wolverines, and mountain lions.
10. The temperate rain forest along the Pacific Coast has the largest trees in existence, some as old as 800 years.
D. Temperate Deciduous Forests
1. Temperate deciduous forests are found south of the taiga in eastern North America, eastern Asia, and much of Europe.
2. Climate in these areas is moderate with a relatively high annual rainfall (75–150 cm).
3. The seasons are well-defined with a growing season that ranges between 140 and 300 days.
4. The trees of a deciduous forest (e.g., oak, beech, and maple) have broad leaves which they lose in the fall and grow again in the spring.
5. Enough sunlight penetrates the canopy to support a well-developed understory composed of shrubs, a layer of herbaceous plants, and a ground cover of mosses and ferns.
6. Stratification beneath the canopy provides a variety of habitats for insects and birds.
7. Deciduous forest contains many rodents that provide food for bobcats, wolves, and foxes.
8. Deciduous forest also contains deer and black bears.
9. Compared to the taiga, the winters are milder and allow many amphibians and reptiles to survive.
10. Minerals are washed into the ground and eventually brought back up by deep roots of trees.
E. Tropical Forests
1. Tropical rain forests are found in South America, Africa, and the Indo-Malayan region near the equator.
2. The climate is warm (20°–25° C) and rainfall is plentiful with a minimum of 190 cm per year.
3. This is probably the richest biome, both in number of species and in their abundance.
4. A tropical rain forest has a complex structure, with many levels of life.
5. Although there is animal life on the ground (e.g., pacas, agoutis, peccaries, and armadillos), most of the animals live in the trees.
6. Insects are abundant in tropical rain forests; the majority have not been identified.
7. Termites are critical in the decomposition of woody plant material.
8. Various birds tend to be brightly colored.
9. Amphibians and reptiles are represented by many species of frogs, snakes, and lizards.
10. Lemurs, sloths and monkeys feed on fruits.
11. The largest carnivores are cats (e.g., jaguars in South America and leopards in Africa and Asia).
12. Epiphytes are air plants that grow on other plants.
a. They have roots of their own to absorb moisture and minerals leached from the canopy.
b. Others catch rain and debris in hollows of overlapping leaf bases.
c. Common epiphytes are related to pineapples, orchids and ferns.
13. Tropical forests in India, Southeast Asia, West Africa, West Indies, and Central and South America are seasonal.
a. They have deciduous trees that shed leaves in the dry season; layers of undergrowth are below.
b. Certain of these forests contain elephants, tigers and hippopotami.
14. A year-long growing season and high temperatures mean productivity is high.
15. But the warm, moist climate that supports high productivity also promotes rapid recycling of litter.
16. The soil is called laterite and the iron and aluminum oxides give it a red color and a brick texture when it bakes in the hot sun.
17. Consequently the soil is relatively poor because the nutrients are rapidly cycled into the biomass; this makes a poor agricultural soil.
F. Shrublands
1. Shrubland is dominated by shrubs with small but thick evergreen leaves coated with a thick, waxy cuticle, and with thick underground stems that survive dry summers and frequent fires.
2. Shrubland is found more along the coasts in South America, western Australia, central Chile, and around the Mediterranean Sea..
3. The dense shrubland in California, where the summers are hot and very dry, is chaparral.
a. This Mediterranean-type shrubland lacks an understory and ground litter and is highly flammable.
b. Seeds of many species require heat and the scarring action of fire to induce germination.
4. West of the Rocky Mountains is a cold desert region dominated by sagebrush and dependent birds.
G. Grasslands
1. Grasslands occur where rainfall is greater than 25 cm but is insufficient to support trees.
2. In temperate areas with rainfall between 10 and 30 inches a year, grassland is the climax community; it is too wet for desert and too dry for forests.
3. Natural grasslands once covered over 40% of the Earth's land surface.
4. Most grasslands now grow crops, especially wheat and corn.
5. Grasses generally grow in different seasons; therefore some grassland animals migrate and ground squirrels hibernate when there is little grass.
6. The temperate grasslands include the Russian steppes, South American pampas, and North American prairies.
7. Tall-grass prairie occurs where moisture is not sufficient to support trees.
8. Short-grass prairie survives on less moisture and is between a tall-grass prairie and desert.
9. Animal life includes mice, prairie dogs, and rabbits and the animals that feed on them: hawks, snakes, badgers, coyotes, and foxes.
10. Prairies once contained large herds of buffalo and pronghorn antelope.
11. Savannas are tropical grasslands that contain some trees.
a. The savanna occurs in regions where a relatively cool dry season is followed by a hot, rainy one.
b. The savanna contains the greatest variety and numbers of herbivores (e.g., antelopes, zebras, wildebeests, water buffalo, rhinoceroses, elephants, and giraffes).
c. Any plant litter not consumed by grazers is attacked by termites and other decomposers.
d. Termites also build towering nests and tend fungal gardens.
e. The savanna supports a large population of carnivores (e.g., lions, cheetahs, hyenas, and leopards).
H. Deserts
1. Deserts usually occur at latitudes about 30o both north and south of the equator.
2. Deserts have an annual rainfall of less than 25 cm because incoming descending winds lack moisture.
3. Lacking cloud cover, the desert days are hot and the nights are cold.
4. The Sahara and a few other deserts are nearly devoid of vegetation.
5. Most have a variety of plants, all adapted to heat and scarcity of water (e.g., succulents).
6. Animal life includes many insects, reptiles such as lizards and snakes, running birds (e.g., roadrunner), rodents (e.g., kangaroo rat), and a few larger birds and mammals such as hawks and coyotes.
49.3 Aquatic Communities
1. Aquatic ecosystems are classified as freshwater (inland) or saltwater (marine).
2. Wetlands near the sea have mixed fresh and saltwater and are brackish.
3. Seawater evaporates and then precipitates and flows through lakes and ponds, streams and rivers, and groundwater.
a. The top of the saturation zone defines the water table.
b. Groundwater sometimes occurs in underground layers called aquifers.
4. Wetlands are areas that are wet for at least part of the year; they are generally classified by their vegetation.
5. Marshes are wetlands that are frequently or continually inundated by water; they are characterized by the presence of rushes, reeds, and other grasses.
6. Bogs are wetlands that a characterized by acidic waters, peat deposits, and sphagnum moss; they receive most of their water from precipitation and are nutrient-poor.
7. Wandering streams are often channelized into straight channels; this eliminates storage for flood control.
8. The elimination of wetlands removes unique habitat for fish, waterfowl and other wildlife.
9. Wetlands also filter toxic wastes and use excess nutrients.
B. Lakes
1. Lakes are freshwater bodies classified by their nutrient status.
a. Oligotrophic (nutrient-poor) lakes have low organic matter and therefore low productivity.
b. Eutrophic (nutrient-rich) lakes are highly productive from natural nutrients or agricultural runoff.
c. Eutrophication occurs when added nutrients change an oligotrophic lake to eutrophic; this process is called eutrophication.
2. In the temperate zone, deep lakes are stratified in summer and winter.
a. Epilimnion is the surface layer warmed from solar radiation; it soon becomes nutrient-poor but photosynthesis keeps oxygen levels high.
b. At the thermocline, there is an abrupt drop in temperature.
c. The hypolimnion is the lower cold region; it becomes depleted in oxygen but is nutrient rich from detritus falling from above.
d. The less dense epilimnion floats on the heavier cold hypolimnion; this prevents mixing.
3. Fall and Spring Overturns
a. In the fall, the upper epilimnion waters become cooler than the hypolimnion.
b. This causes the surface water to sink and deep water to rise.
c. This fall overturn continues until the temperature is uniform.
d. In the winter, ice forms on top because ice is lighter; this provides an insulating cover and organisms can live through a harsh winter in this moderate water.
e. In spring, the ice melts and the cooler water on top sinks below the warmer water on the bottom.
f. After the spring overturn, water returns to a more uniform temperature and sun warms the surface.
g. Fish and other aquatic life are adapted to the strata and seasonal changes; for instance, cold water fish move deeper in the summer.
C. Life Zones
1. Plankton includes freshwater and marine microscopic organisms that freely drift in fresh or saltwater.
2. Phytoplankton are the photosynthetic plankton, including algae.
3. Zooplankton are animals that feed on phytoplankton.
4. The littoral zone is shallow and closest to shore; plants root in this zone and harbor some animals.
5. The limnetic zone is the open sunlit layer of body of a lake; it contains plankton, a few insect larvae, and fish.
6. The profundal zone is that portion of a lake below any significant sunlight penetration; it contains zooplankton and fishes that feed on the debris that falls from above.
7. The benthic zone is at the soil-water interface with the bottom-dwelling organisms; it includes worms, mollusks, and crustaceans.
D. Coastal Ecosystems
1. An estuary is a partially enclosed body of water at the end of a river where the fresh water and sea water mix.
a. Not many organisms are tolerant of this mix of fresh river water and salty tidal water.
b. For organisms suited to the rapid changes in salinity, estuaries provide abundant nutrients.
c. Estuaries are a nutrient trap since nutrients are
i. delivered by the river,
ii. brought in from the sea by tides, and
iii. released from decaying vegetation.
d. Estuaries are a nursery estimated as spawning and rearing over half of all marine fishes.
2. Seashores are constantly bombarded by tidal seas.
a. The littoral zone is between high and low tide and is covered and uncovered daily.
b. The upper littoral is covered by barnacles.
c. The midportion harbors brown algae that may overlie barnacles.
d. The lower portion has oysters and mussels attached to rock by byssal threads; various snails hide in crevices or seaweed.
e. Below the littoral zone, seaweeds are the main photosynthesizers and are anchored to rocks by holdfasts.
f. Sandy beaches have no anchor holds; therefore permanent beach organisms are burrowing or tube-living.
E. Oceans
1. Moisture that evaporates into the air carries the heat used to evaporate it with it.
2. Water is warm at the equator and cold at the poles due to the distribution of the sun's rays.
3. Air takes on the temperature of the water below and warm air moves from the equator toward the poles.
4. Therefore, the oceans make winds blow.
5. Oceans hold heat or remain cool longer than landmasses.
6. Winds generate ocean currents due to friction at the ocean surface.
7. Since ocean currents are bounded by land, they move in a circular path, counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.
8. Ocean currents take heat from the equator to the polar regions.
a. The Gulf Stream brings warmer tropical Caribbean water to the east coast of North America and to upper western Europe.
b. Without the Gulf Stream, Great Britain would be as cold as Greenland.
c. A major Atlantic Ocean current warms the eastern coast of South America.
d. The Humboldt Current in the Pacific flows toward the equator off the west coast of South America.
9. Upwellings occur when cold nutrient-rich water rises to supplant warm nutrient-poor water.
a. The Humboldt Current brings rich nutrients north; this supports rich marine life and the fisheries of Peru and northern Chile.
b. Seabirds deposit droppings (guano) on land where it is a major source of phosphorus mining.
c. When the Humboldt Current is not as cool as usual, upwelling does not occur, stagnation results, fisheries decline, and climate patterns change globally; this is called an El-Nino-Southern Oscillation.
F. The pelagic division includes the neritic and oceanic provinces.
1. The neritic province lies over the continental shelf.
a. This contains a greater concentration of organisms than are in the oceanic province.
b. It is a more productive part of the ocean because of the concentration of sunlight and nutrients.
c. It provides the base of the food web leading to commercially valuable fishes (e.g., herring, cod, and flounder).
d. Coral reefs are areas of biological abundance found in shallow, warm waters just below the surface; there is much concern about future survival of coral reefs since they are vulnerable to environmental changes, temperature shifts, salinity, and light availability.
2. The oceanic province lies over the continental slope and the abyssal plane.
a. The epipelagic zone extends from the surface to the maximum depth that photosynthesis significantly occurs.
i. It does not have a high concentration of phytoplankton because it lacks nutrients.
ii. However, the numbers of producers in this zone still support a large assembly of zooplankton, which support large numbers of other marine organisms, when the entire ocean is considered.
iii. The epipelagic animals include mackerels, tunas, and sharks.
b. The mesopelagic zone extends below maximum depth at which photosynthesis significantly occurs.
i. This zone is dominated by carnivores adapted to the absence of light (e.g., luminescent shrimps, squids, and fishes).
ii. Organisms here tend to be translucent or red colored.
c. The bathypelagic zone is in absolute darkness except for an occasional flash of bioluminescent light.
i. Animals here are carnivores and scavengers.
ii. This level supports a variety of very strange carnivores.
G. Benthic Division
1. The benthic division includes all organisms that live on or in the soil of the ocean floor, including the continental shelf, continental slope, and the abyssal plain.
2. The sublittoral zone is located on the continental shelf up to the low tide mark on the coast.
a. It supports a mixed food web with seaweeds and filter-feeding organisms as the first trophic level.
b. The seaweeds comprise the first trophic level for a grazing food web; the detritivores (e.g., clams and worms) comprise the first trophic level for a detrital food web.
c. Starfishes, lobsters, crabs, brittle stars, and some bottom-dwelling fish occupy the upper trophic levels.
3. The bathyal zone is located on the continental slope and extends through mesopelagic and bathypelagic depths.
a. It contains a detrital food web with detritivores (e.g., clams and worms) as the first trophic level.
b. Again, starfishes, crabs, brittle stars, and some bottom-dwelling fish occupy the upper trophic levels.
4. The abyssal zone is located on and immediately above the abyssal plane.
a. This is a region of extreme cold and intense pressure.
b. It contains a detrital food web in which the detritivores (e.g., sponges, worms, tube worms, sea cucumbers, sea lilies, and sea urchins) comprise the first trophic level.
c. Starfishes, crabs, brittle stars, and some bottom-dwelling fish occupy the upper trophic levels.
5. Hydrothermal vents are areas where seawater percolates through cracks.
a. The water is heated to about 350°C.
b. This causes sulfate to react with water to form hydrogen sulfide (H2S).
c. Chemosynthetic bacteria obtain energy by oxidizing hydrogen sulfide.
d. These communities are not based on light energy but support huge tube worms and clams.
50.1 Conservation Biology and Biodiversity
1. Conservation biology is a new discipline studying aspects of biodiversity in order to conserve natural resources.
2. Conservation biology involves both scientific concepts and their application to practical problems.
3. It supports four ethical principles.
a. Biodiversity is desirable for both the biosphere and for humans.
b. Extinctions due to human actions are undesirable.
c. The complex interactions in ecosystems support biodiversity and are therefore desirable.
d. Biodiversity from evolutionary change has value by itself regardless of any practical benefit.
4. Estimates vary but at least 10–20% of all species now living will most likely become extinct in 20–50 years.
5. Bioinformatics, the collecting and analyzing of biological information, is used in the study of conservation biology.
B. Biodiversity
1. Biodiversity is the variety of life on Earth; it is generally described as the number of species of the various groups of organisms.
2. Most estimates place the number of species living on earth as between 10 and 50 million species; most are yet to be found and described.
3. A threatened species is one that is likely to become an endangered species (in danger of immediate extinction) in the foreseeable future
4. Biodiversity also includes genetic diversity, community diversity, and landscape diversity.
5. Genetic diversity helps maintain reproductive vitality and assists adaptation.
a. The 1846 potato blight in Ireland was due to too little genetic diversity.
b. 1922 saw a similar Soviet wheat failure.
c. Florida had an outbreak of citrus canker in 1984 made worse by limited genetic variation.
d. Such limited genetic variation creates the risk of extinction in natural populations.
6. Community diversity refers to the variation in species composition in a community.
a. Different communities have different species; therefore different communities add to species diversity.
b. Attempts to save just one species are shortsighted when the community itself is threatened.
c. Disrupting a community can threaten many species.
7. Landscape diversity incorporates a number of interacting ecosystems: plains, mountains, rivers, etc. within one landscape.
a. Fragmented landscapes reduce reproductive capacity, food availability, and affect seasonal behavior.
8. Distribution of Diversity
a. Biodiversity is not evenly distributed; saving some areas saves more species than saving others.
b. Biodiversity is highest in the tropics and declines toward the poles on land, in fresh water, and in the ocean.
c. Biodiversity hotspots contain unusually large concentrations of species; hotspots cover only about 1.4% of Earth's land area but contain 44% of the higher plant species and 35% of the terrestrial vertebrate species.
d. Madagascar, the Cape of South Africa, and the Great Barrier Reef of Australia are all biodiversity hotspots.
e. Biodiversity frontiers such as the rain forest canopies and the deep sea benthos have more species than formerly suspected.
50.2 Value of Biodiversity
A. Direct Value
1. Medicinal Value
a. Most U.S. prescription drugs were originally derived from living organisms.
b. The rosy periwinkle from Madagascar provides chemicals that treat the two cancers: leukemia and Hodgkin disease.
c. Based on the past rate of drug discovery, there are perhaps 328 more drugs likely to be found in tropical rain forests with $147 billion value.
d. Fungi and soil bacteria have provided penicillin, tetracycline, and streptomycin.
e. The nine-banded armadillo is the only other animal to contract human leprosy; this allowed research to find a cure.
f. The blood of horseshoe crabs contains limulus amoebocyte lysate that keeps pacemakers free of bacteria.
2. Agricultural Value
a. Wheat, corn and rice crops are uniform; when devastated by pests, the wild strains they came from are a source of genes for disease resistance.
b. When rice crops were devastated by a virus in Africa, it was necessary to locate a wild rice plant that was resistant and breed the gene into the high yield rice.
c. Biological pest controls are economically important replacements when pests are pesticide-resistant.
d. Most flowering plants are pollinated by animals (e.g., bees, wasps, butterflies, birds, bats, etc.).
e. The honeybee is a multi-faceted example.
i. Domesticated bees pollinate over $10 billion worth of food crops annually.
ii. Tracheal mites have wiped out more than 20% of commercial honeybees in the U.S.
iii. Any hope for a resistant bee depends on wild bees; wild pollinators provide a $4.1 to $6.7 billion service each year.
3. Consumptive Use Value
a. The cultivation of crops and domestication of animals (farming) have been successful enterprises.
b. However, fishing of wild species has not yet been replaced by aquaculture.
c. Harvesting wild fruits, vegetables, skins, fibers, beeswax, seaweed, and hunting meat are important to many peoples.
d. Calculations show that the timber harvested from the natural environment in the Peruvian Amazon is of less value than harvest of tree fruits and rubber production.
B. Indirect Value
1. Biogeochemical Cycles
a. Biodiversity contributes to the water, carbon, nitrogen, and phosphorus cycles of our ecosystem.
b. We depend upon normal cycles to provide fresh water, remove carbon dioxide, etc.
c. Technology cannot artificially create these cycles in place of the ecosystem.
2. Waste Disposal
a. Decomposers break down organic matter and other wastes into nutrients used by producers.
b. Decomposition in nature is more economical and complete than sewage treatment.
c. Biological communities purify water and break down pollutants; Canada estimates this wetland value at $50,000 per hectare.
3. Provision of Fresh Water
a. Most terrestrial organisms, including humans, need freshwater ecosystems.
b. Desalination plants cost four to eight times the average cost of water taken from the water cycle.
c. Forests are "sponges" that hold and release water over time; the value of marshland outside of Boston, Massachusetts, is estimated at $72,000 per hectare based on its ability to reduce floods.
4. Prevention of Soil Erosion
a. Intact ecosystems naturally retain soil and prevent erosion.
b. Deforestation results in silt that fills reservoirs and denudes hillsides; a dam in Pakistan is filling much faster due to silt.
c. Silt from deforestation also smothers mangrove and coastal ecosystems and ruins fisheries.
5. Regulation of Climate
a. Trees provide both shade and natural "air conditioning."
b. Globally, tropical rain forests act as a sink for carbon dioxide; when trees are burned the CO2 is released back into the atmosphere.
c. CO2 is a greenhouse gas and contributes to global warming; not all life may be able to adjust to the climate change.
6. Ecotourism
a. In the U.S., 100 million people spend a total of $4 billion a year on fees, travel, lodging, and food in order to enjoy natural environments.
b. Activities include sport fishing, boating, hiking, birdwatching, whale watching, etc.
C. Biodiversity and Natural Ecosystems
1. Massive changes in biodiversity impact ecosystems and the ability to provide the above values.
2. Research indicates that high diversity improves the efficiency of ecosystems.
a. Minnesota grassland plots with more species had lower inorganic soil nitrogen.
b. California plots with more diversity had greater overall resource usage.
c. Net primary productivity increased as diversity increased at all trophic levels.
d. Computer modeling predicts 30% more photosynthesis with nine different tree species rather than one single species.
3. Additional research may determine the effects of environmental change, invasion, pathogens, and fragmentation.
50.3 Causes of Extinction
1. 1,880 threatened and endangered species were examined for the cause of their status.
a. Habitat loss was involved in 85% of cases.
b. An alien (exotic or introduced) species was involved in nearly 50%.
c. Pollution was a factor in 24%.
d. Overexploitation occurred in 17%.
e. Disease was involved in 3%.
2. As an example, the decline in macaws is the result of timber and mining activities, and hunting for food and pet trade.
B. Habitat Loss
1. Rain forest destruction follows a pattern.
a. A highway is constructed into the forest interior.
b. Small towns, industry and roads then branch off into the forest.
c. Settlers, often subsidized, burn the trees and raise crops on the three-year supply of nutrients.
d. After the land degrades, the farmers must move to another portion of forest.
2. Coastal degradation is due to high human populations that live along the shore.
3. Already 60% of coral reefs are destroyed or near destruction; all coral reefs could disappear in 40 years.
4. 45% of Indonesia's mangrove forests have already been destroyed.
5. Wetlands, estuaries and seagrass beds are being rapidly destroyed.
C. Alien Species
1. Alien species, or exotics, are introduced accidently or deliberately into new ecosystems.
2. Natural ecosystems have evolved with their native organisms in balance.
3. Migration out of ecosystems is blocked by barriers; however, humans circumvent these barriers by various means.
a. Colonization of European pioneers brought the dandelion as a salad green and they introduced pigs to North America.
b. Horticulture and agriculture have resulted in some exotic plants like kudzu (a vine from Japan) escaping into the countryside.
c. Accidental transport due to global trade carried the European zebra mussels in ballast water to the U.S. where it now squeezes out native mussels.
4. Alien species disrupt food webs; an opossum shrimp introduced into Montana lakes led to less food for eagles and grizzly bears.
5. Exotics on Islands
a. Islands are especially susceptible to disruption by introduction of exotics.
1. When myrtle trees from the Canary Islands were introduced to Hawaii, symbiotic bacteria gave them a competitive edge over native trees.
2. The brown tree snake introduced to Guam has reduced ten bird species to the point of extinction.
3. On the Galapagos Islands, black rats have reduced the giant tortoise; goats and feral pigs have harmed cactus and converted forest to grassland.
4. Mongooses introduced into Hawaii to control rats have preyed on native birds.
D. Pollution
1. Pollution is any environmental change that adversely affects the lives and health of living things.
2. Pollution is directly the third main cause of extinctions and can lead to disease, the fifth main cause of extinction.
3. Acid Deposition
a. Automobile exhaust and sulfur dioxide from power plants form acids when combined with water vapor in the air.
b. These acids return to Earth as acid rain/snow or dry deposition.
c. Although sulfur and nitrogen oxides are emitted in one locale, deposition occurs elsewhere across boundaries.
d. Acid deposition causes trees to weaken, kills small invertebrates and decomposers, and kills the life in northern lakes.
4. Eutrophication
a. Lakes are stressed by excess nutrients from sewage treatment and agricultural runoff.
b. Algae grow in abundance and then die off; the bacterial decomposers then use up all of the oxygen and kill the fish.
5. Ozone Depletion
a. The ozone shield is a layer of ozone (O3) in the stratosphere, some 50 km above Earth's surface.
b. It absorbs most of the harmful ultraviolet (UV) radiation, preventing it from reaching Earth's surface.
c. The cause of ozone depletion traces to chlorine (Cl- ) atoms that come from the breakdown of CFCs.
d. Freon is a common CFC that was used in refrigerators and air conditioners.
e. Ozone shield depletion will lead to depression of the human immune system, impaired crop and tree growth, and death of plankton.
6. Organic Chemicals
a. A wide variety of organic chemicals are produced and enter the environment.
b. Nonylphenols are used in plastics, spermicides, cosmetics, etc.; such chemicals mimic the effects of hormones.
c. Salmon switch development between fresh and salt water but this chemical prevents adaptation in 20–30% of young fish.
7. Global Warming
a. Global warming refers to an expected increase in the average temperature in the 21st Century.
b. Greenhouse gases (named for their ability to trap heat like greenhouse glass) contribute to warming:
i. carbon dioxide (CO2) is produced by burning fossil fuels; and
ii. methane (CH4) is produced by animal guts, oil and gas wells, and flooded rice paddies.
c. Data collected worldwide show a rise in greenhouse gases.
d. Computer models predict rising average temperatures.
. The global climate appears to have risen since the industrial revolution.
i. Some computer models predict a rise of from 1.5o C to 4.5o C by 2060.
e. As oceans warm, temperatures in polar regions would likely rise to a greater degree than other areas.
f. Glaciers would melt and sea levels would rise; a one meter rise would inundate 20–50% of coastal wetlands.
g. Regions of suitable climate for species would shift rapidly, probably faster than plants could migrate.
h. Coral reefs would suffer from high temperature driving off algae and the higher water "drowning" them.
E. Overexploitation
1. Overexploitation occurs when removal of individuals from the wild population drastically reduces their numbers.
2. Rarity causes a positive feedback cycle: the fewer specimens left, the more valuable they are.
3. There are many cases of overexploited organisms.
a. "Rustlers" dig up rare cacti to sell to gardeners.
b. Parakeets and macaws are sold to pet stores.
c. Tropical fish are harvested using dynamite and cyanide that kill many more.
d. Siberian tigers are hunted for hides.
e. Rhinoceros horn is ground up as medicine.
f. Elephant tusk ivory is used for jewelry.
4. Fish stocks are being depleted by overfishing.
a. The U.N. Food and Agricultural Organization considers 11 of 15 major oceanic fishing areas "overexploited."
b. Purse-seine fishing surrounds tuna.
c. Huge trawling nets capture bottom-dwelling fish; this has been called the marine equivalent of clear-cutting trees.
5. Overfishing perch and herring caused a decline in seals and sea lions; orca killer whales had to switch to eating sea otters; sea otters ate the sea urchins that fed on kelp and without sea otters, the urchins decimated the kelp beds.
50.4 Conservation Techniques
A. Habitat Preservation
1. Biodiversity hotspots merit preservation first.
2. In Madagascar tropical rain forests, 93% of primates, 99% of frog species, and over 80% of plant species are endemic (unique and native).
3. When keystone species, species that influence the viability of a community although their numbers might not be high, are lost, their extinction leads to other extinctions.
a. Bats pollinate and disperse seeds of tropical trees; loss of the bats leads to loss of the trees.
b. Grizzly bears disperse berry seeds in their dung, keep prey populations healthy, and turn over soil.
4. A flagship species is one treasured for its beauty, cuteness, etc. Examples are pandas, dolphins, tigers, etc.
5. Metapopulations
a. Metapopulations are subdivided into several small, isolated populations due to habitat fragmentation.
b. A source population lives in a favorable habitat and has a higher birth than death rate.
c. Sink populations have death rates that equal or exceed birth rates.
d. When trying to save a species, it is best to prevent it from moving into a sink habitat.
6. Landscape Dynamics
a. Organisms like grizzly bears utilize many ecosystems; saving just one system would not save the species.
b. Saving diverse ecosystems connected by corridors involves national forests, refuges and private land.
c. Landscape protection for one species helps protect others; the grizzly range overlaps 40% of Montana's vascular plants of special concern.
d. The Edge Effect
i. The edge of a habitat is different from the interior; the smaller the patch, the more edge produced.
ii. Forest edges are brighter, warmer, drier, windier and have more vines.
iii. Forest nesting songbirds have less success at the edge; cowbirds are nest parasites at the edge.
7. Computer Analyses
a. Gap analysis uses computers to locate where the biodiversity is high outside of preserves.
i. When species maps are superimposed on land-use maps, areas in need of preservation are exposed.
b. Population viability analysis helps determine how much habitat a species needs to survive.
i. Adequate size protects a population from chance swings in birth and death rates.
ii. A red-cockaded woodpecker population of 1,323 is needed to provide a breeding population of 500.
iii. Analysis of grizzly populations predicted a population of 70–90 bears were needed; more were necessary because only a few males bred.
iv. The Florida panther population is inbred; eight Texas cougars were introduced to bolster genetic diversity.
B. Habitat Restoration
1. Restoration ecology seeks scientific ways to return ecosystems to their former state.
2. Restoration involves three principles.
a. Restoration should begin immediately before the remaining fragments are lost.
b. Techniques that mimic natural processes should be used (i.e., controlled burning, biological pest control, etc.).
c. The goal is sustainable development where the resulting ecosystem should be able to maintain itself.
3. The Everglades
a. The Everglades is a natural wet sawgrass prairie with cypress or hardwood islands.
b. Early settlers drained the land to the south and established a dike around the feeder lake; water was also channeled to prevent flooding.
c. The water supplied by natural cycles of wet and dry seasons has been replaced by discharges from conservation lakes timed for public convenience.
d. The resulting abnormal water supply has devastated the Everglades ecology; bird populations are dramatically reduced, etc.
e. Restoration involves providing a natural seasonal flow of water to the Everglades.
f. Sustainable development involves switching agriculture to sugarcane and rice and establishing an extended buffer zone with interconnected marshes.