test2.txt

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test2.txt
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  1. Chapter 1
  2. Animals
    Any multicelled heterotroph that ingests other organisms or their tissues, develops through a series of embryonic stages, and is motile during part or all of the life cyle. Most species have epithelial tissues and extracellular matrixes.
  3. Archaea
    Domain of prokaryotic species; one of two lineages that evolved shortly after life originated. Archaeans have many unique molecular and biochemical traits but also share some traits with bacteria and other traits with eukaryotic species.
  4. Artificial selection
    Manipulation of the reproduction of a species as by breeding practices. Only individuals of a captive population that display a valued trait are allowed to reproduce, the goal being to increase the trait�s magnitude and frequency over the generations.
  5. Bacteria
    Domain of prokaryotic species; the first kinds of cells that formed after life originated. Collectively, bacteria are the most metabolically diverse organisms. Most kinds are chemoheterotrophs.
  6. Biosphere
    [Gk. bios, life, + sphaira, globe] All regions of Earth�s waters, crust, and atmosphere in which organisms live.
  7. Cell
    Smallest unit that still displays the properties of life; it has the capacity to survive and reproduce on its own.
  8. Community
    All populations of all species in a habitat.
  9. Consumers
    Type of heterotroph that feeds on the tissues of other organisms as its source of carbon and energy.
  10. Control group
    In experimental tests, a group used as a standard for comparison against one or more experimental groups.
  11. Decomposers
    [L. dis�, to pieces] One of the prokaryotic or fungal heterotrophs that obtains carbon and energy by breaking down wastes or remains of organisms. The collective action of decomposers helps cycle nutrients to producers in ecosystems.
  12. Development
    Of complex multicelled species, a series of stages from formation of gametes, then fertilization, and on through embryonic and adult forms.
  13. DNA
    Deoxyribonucleic acid. Double-stranded nucleic acid twisted into a helical shape; its base sequence encodes the primary hereditary information for all living organisms and many viruses.
  14. Ecosystem
    Array of organisms, together with their environment, interacting by a flow of energy and cycling of materials.
  15. Emergent properties
    With respect to life�s levels of organization, a new property that emerges through interactions of entities at lower levels, none of which displays the property; e.g., living cells that emerge from �lifeless� molecules.
  16. Energy
    A capacity to do work.
  17. Enzymes
    A type of protein that catalyzes (speeds) a chemical reaction. Some RNAs also show catalytic activity.
  18. Eukarya
    Domain of eukaryotic species; all �protists,� plants, fungi, and animals.
  19. Evolution
    [L. evolutio, an unrolling] Genetic change in a line of descent by microevolutionary events (gene mutation, natural selection, genetic drift, and gene flow); basis of large-scale patterns, rates, and trends in the history of life.
  20. Experimental group
    A group of objects or individuals that display or are exposed to the variable under investigation. Test results for this group are compared against the results for a control group.
  21. Experiments
    A test that simplifies observation in nature or the laboratory by manipulating and controlling conditions under which observations are made.
  22. Fungi
    Kingdom of fungi.
  23. Genus, genera
    [L. genus, race or origin] A grouping of species more closely related to one another in morphology, ecology, and history than to others at the same taxonomic level.
  24. Homeostasis
    [Gk. homo, same, + stasis, standing] State in which physical and chemical aspects of internal environment (blood, interstitial fluid) are being maintained within ranges that are tolerable for cell activities.
  25. Hypothesis
    An explanation of a phenomenon, one that has the potential to be proven false by experimental tests.
  26. Inheritance
    Transmission, from parents to offspring, of genes that underlie the traits characteristic of their species.
  27. Mimicry
    A case of one species (the mimic) closely resembling another (its model) in form, behavior, or both.
  28. Models
    Theoretical explanation of any object or event that has not been or cannot be directly observed.
  29. Mutation
    [L. mutatus, a change, + -ion, act, result, or process] Heritable change in DNA�s molecular structure. Original source of new alleles and life�s diversity.
  30. Natural selection
    Microevolutionary process; the outcome of differences in survival and reproduction among individuals of a population that differ in the details of their heritable traits.
  31. Organ systems
    A set of organs that are interacting chemically, physically, or both in a common task.
  32. Organs
    Body structure with definite form and function made of more than one tissue.
  33. Plants
    A multicelled photoautotroph, most with well-developed roots and shoots (e.g., stems, leaves), as well as photosynthetic cells that include starch grains as well as chlorophylls a and b, and polysaccharides such as cellulose, pectin, and lignin in cell walls. The primary producers on land.
  34. Prediction
    A statement, based on a hypothesis, about what you expect to observe in nature; the �if-then process.�
  35. Producers
    An autotrophic organism.
  36. Prokaryotic
    [L. pro, before, + Gk. karyon, kernel] A single-celled organism, often walled, that does not have the organelles characteristic of eukaryotic cells. Only bacteria and archaeans are prokaryotic.
  37. Protists
    Informal name for all structurally simple eukaryotes, which are now being classified as monophyletic groups.
  38. Receptors
    Sensory cell or a specialized ending of one that detects a particular kind of stimulus.
  39. Reproduction
    Any asexual or sexual process by which a parent cell or organism produces offspring.
  40. Scientific theory
    A time-tested, widely accepted intellectual framework used to interpret a broad range of observations and data about some aspect of nature. Tested rigorously but is still open to tests, revision, and tentative acceptance or rejection.
  41. Species
    [L. species, a kind] Of sexually reproducing species, one or more natural populations of individuals that successfully interbreed and are isolated reproductively from other such groups. By a cladistic definition, one or more natural populations of individuals with at least one unique trait derived a common ancestor that occurs in no other groups.
  42. Stimulus
    [L. stimulus, goad] A specific form of energy that activates a sensory receptor able to detect it; e.g., pressure.
  43. Test
    Any standardized or innovative means by which a prediction based on a hypothesis might be disproved; often requires designing and conducting experiments, making observations, or developing models.
  44. Tissues
    Of multicelled organisms, a group of cells and matrixes interacting in the performance of one or more tasks.
  45. Variable
    Of experimental tests, a specific aspect of an object or event of interest that may differ over time and among individuals. A single variable is directly manipulated in an experimental group.
  46. Chapter 2
  47. Acid, acidic
    [L. acidus, sour] Any water-soluble substance that releases hydrogen ions (H+) in water, yielding a pH below 7.0.
  48. Atom
    The smallest unit of an element that still retains the element�s properties.
  49. atomic number
    The number of protons in the nucleus of atoms of a given element.
  50. Base, basic
    Any water-soluble substance that releases hydroxyl ions in water to yield a pH greater than 7.0. Also the nitrogencontaining component of a nucleotide.
  51. Buffer system
    A weak acid and the salt that forms when it dissolves. The two work as a pair to counter slight shifts in pH.
  52. Chemical bonds
    A union between the electron structures of two or more atoms.
  53. cohesion
    A capacity to resist rupturing when placed under tension (stretched).
  54. Compound
    Molecule consisting of two or more elements in proportions that do not vary, as they can in mixtures.
  55. Covalent bond
    [L. con, together, + valere, to be strong] A sharing of one or more electrons between two atoms. In a polar covalent bond, the atoms share electrons unequally; in a nonpolar covalent bond, each atom gets an equal share of the electrons.
  56. Electron
    Negatively charged subatomicparticle. Electrons occupy orbitals around the atomic nucleus.
  57. Element
    Fundamental form of matter that cannot be degraded to a simpler form by ordinary means. All atoms of an element have the same atomic number.
  58. Evaporation
    Process of conversion of a liquid to a gas; requires energy input.
  59. Hydrogen bond
    A weak attraction that has formed between a covalently bonded hydrogen atom and an electronegative atom taking part in another covalent bond.
  60. Hydrogen ion, H+
    Free (or unbound) proton; one hydrogen atom that lost its electron and now bears a positive charge (H+).
  61. Hydrophilic
    [Gk. philos, loving] A polar molecule that dissolves easily in water; e.g., glucose.
  62. Hydrophobic
    [Gk. phobos, dreading] A nonpolar molecule that resists dissolving in water; e.g., oil.
  63. Ion
    Atom having an unequal number ofprotons and electrons; it carries a positive or negative electric charge.
  64. Ionic bond
    Ions interacting through the attraction of their opposite charges.
  65. Isotopes
    One of two or more atoms of the same element (same number of protons) that differ in their number of neutrons.
  66. Mass number
    Sum of protons and neutrons in the nucleus of an element�s atoms.
  67. Mixture
    Two or more types of molecules intermingled in proportions that can and usually do vary.
  68. Molecule
    Two or more covalently bonded atoms of the same or different elements.
  69. Neutron
    Type of subatomic particle in the nucleus of all atoms except hydrogen; has mass but no charge.
  70. Periodic table
    Tabular arrangement of elements in order of their increasing atomic number.
  71. PH scale
    Measure of the H+ concentration of a solution. pH 7 is neutral.
  72. Proton
    Positively charged subatomic particle in the nucleus of all atoms.
  73. Radioactive decay
    Natural, inevitable process by which an atom emits energy as subatomic particles and x-rays as its unstable nucleus spontaneously breaks apart; transforms one element into another in a predictable time span.
  74. Radioisotopes
    Any isotope that has an unstable nucleus.
  75. Salt
    Any compound that releases ions other than H+ and OH- in solution.
  76. Shell model
    Model for how electrons are distributed in an atom; all of the orbitals are shown as a nested series of shells.
  77. Solute
    [L. solvere, to loosen] Any substance dissolved in a solution.
  78. Solvent
    Any fluid (e.g., water) in which one or more substances are dissolved.
  79. Sphere of hydration
    A clustering of water molecules around molecules or ions of a solute by positive and negative interactions.
  80. Temperature
    Measure of molecular motion.
  81. Tracers
    Any substance with a radioisotope attached; researchers can track it after delivering it into a cell, a multicelled body, ecosystem, or some other system.
  82. Chapter 3
  83. Alcohols, �OH group
    Organic compound having one or more hydroxyl groups that dissolves easily in water; e.g., ethanol.
  84. Amino acid
    A small organic compound with a carboxylic acid group, an amino group, and a characteristic side group (R); monomer of polypeptide chains.
  85. ATP
    Adenosine triphosphate. A type of nucleotide that functions as the main energy carrier between reaction sites in cells. Consists of the base adenine, the five-carbon sugar ribose, and three phosphate groups.
  86. Carbohydrate
    Any molecule of carbon, hydrogen, and oxygen typically in a 1:2:1 ratio. Main kinds are monosaccharides, oligosaccharides, and polysaccharides. They serve as structural materials, energy stores, and transportable energy forms.
  87. Coenzymes
    An organic molecule that is a necessary participant in some enzymatic reactions; helps catalysis by donating or accepting electrons or functional groups; e.g., a vitamin, ATP, NAD+.
  88. Condensation reaction
    Type of chemical reaction in which two molecules become covalently bonded as a larger molecule; water often forms as a by-product.
  89. Denaturation
    Disruption of hydrogen bonds and other interactions holding a molecule in its three-dimensional shape, which thereby changes. Increases in temperature, shifts in pH, and detergents can cause it.
  90. Disaccharide
    [Gk. di, two, + sakcharon, sugar] A carbohydrate composed of two sugar monomers.
  91. DNA
    Deoxyribonucleic acid. Double-stranded nucleic acid twisted into a helical shape; its base sequence encodes the primary hereditary information for all living organisms and many viruses.
  92. Enzymes
    A type of protein that catalyzes (speeds) a chemical reaction. Some RNAs also show catalytic activity.
  93. Fats
    Type of lipid with one, two, or three fatty acid tails attached to a glycerol head.
  94. Fatty acid
    Organic compound having a carboxyl group and a backbone of as many as thirty-six carbon atoms; saturated types have single bonds only; unsaturated types include one or more double covalent bonds.
  95. Functional groups
    An atom or a group of atoms with characteristic properties that is covalently bonded to the carbon backbone of an organic compound.
  96. Glycogen
    Highly branched polysaccharide of glucose monomers; the main storage carbohydrate in animals.
  97. Glycoproteins
    Protein with linear or branched oligosaccharides covalently bonded to it.
  98. Hemoglobin
    [Gk. haima, blood, + L. globus, ball] A hemecontaining protein produced by red blood cells; carries most of the oxygen in blood.
  99. Hydrocarbons
    Organic compound with only hydrogen bonded to its carbon backbone.
  100. Hydrolysis
    [L. hydro, water, + Gk. lysis, loosening] A cleavage reaction; an enzyme splits a molecule, then the components of water (�OH and �H) are attached to the fragments.
  101. Lipids
    One of the nonpolar hydrocarbons; e.g., a fat, oil, wax, sterol, phospholipid, or glycolipid. Cells use as storage forms of energy and building blocks.
  102. Lipoproteins
    A protein complexed with cholesterol, triglycerides, or phospholipids that were absorbed from the small intestine.
  103. Monomers
    Any small molecule that is a repeating subunit in a polymer; e.g., the sugar monomers of starch.
  104. Monosaccharides
    [Gk. monos, alone, single, + sakcharon, sugar] A simple sugar.
  105. Nucleic acids
    Single-stranded or doublestranded molecule of nucleotides joined at phosphate groups; e.g., DNA, RNA.
  106. Nucleotide
    Small organic compound with a five-carbon sugar, a nitrogen-containing base, and a phosphate group. Functions as coenzymes or monomers of nucleic acids.
  107. Oligosaccharide
    Short-chain carbohydrate of two or more covalently bonded sugar monomers; e.g., sucrose and other disaccharides.
  108. Organic compounds
    Any carbon-based molecule that also incorporates atoms of hydrogen and, often, oxygen, nitrogen, and other elements; e.g., fats, proteins.
  109. Peptide bond
    The chemical bond formed between the carboxyl groups and amino groups of neighboring amino acids, constituting the primary linkage of all protein structures.
  110. Phospholipid
    A lipid with a phosphate group in its hydrophilic head. The main constituent of cell membranes.
  111. Polymers
    Large molecule of multiple linked monomers.
  112. Polypeptide chain
    Three or more amino acids linked by peptide bonds.
  113. polysaccharide
    [Gk. polus, many, + sakcharon, sugar] Straight or branched chain of covalently bonded monomers of the same or different kinds of sugars; e.g., cellulose, starch, and glycogen.
  114. proteins
    Organic compound consisting of one or more polypeptide chains. Diverse kinds have structural, functional, and regulatory roles in all organisms.
  115. RNA
    Ribonucleic acid. Any of a class of single-stranded nucleic acids involved in gene transcription and translation; some RNAs show enzyme activity.
  116. sterols
    Any lipid consisting of a rigid backbone of four fused carbon rings.
  117. triglyceride
    A lipid with three fatty acid tails attached to a glycerol backbone.
  118. waxes
    A lipid with long-chain fatty acids attached to an alcohol other than glycerol.
  119. Chapter 4
  120. actin
    Protein monomer of microfilaments that functions in contraction, cell division, and reinforcing or reconfiguring the shape of a cell or its contents.
  121. adhering junctions
    Complex of adhesion proteins that anchors cells to each other and to extracellular matrixes.
  122. Archaea
    Domain of prokaryotic species; one of two lineages that evolved shortly after life originated. Archaeans have many unique molecular and biochemical traits but also share some traits with bacteria and other traits with eukaryotic species.
  123. Bacteria
    Domain of prokaryotic species; the first kinds of cells that formed after life originated. Collectively, bacteria are the most metabolically diverse organisms. Most kinds are chemoheterotrophs.
  124. Basal body
    An organelle that started out as a centriole, the source of a 9+2 array of microtubules in a cilium or flagellum. It remains below the finished array.
  125. Cell
    Smallest unit that still displays the properties of life; it has the capacity to survive and reproduce on its own.
  126. cell cortex
    A dynamic mesh of crosslinked cytoskeletal elements just underneath the plasma membrane and attached to it.
  127. cell junctions
    Of a tissue, any molecular structure that connects adjoining cells physically, chemically, or both at their plasma membranes.
  128. cell theory
    All organisms consist of one or more cells, the cell is the smallest unit of organization still displaying the properties of life, and life�s continuity arises directly from growth and division of single cells.
  129. cell wall
    Of many cells (not animal cells), a semirigid but permeable structure that surrounds the plasma membrane; helps a cell retain its shape and resist rupturing.
  130. central vacuole
    In many mature, living plant cells, an organelle that stores amino acids, sugars, and some wastes; when it enlarges during growth, it forces the cell to enlarge and increase its surface area.
  131. centriole
    A barrel-shaped structure that arises from a centrosome and organizes newly forming microtubules into a 9+2 array inside a cilium or flagellum.
  132. chloroplasts
    Organelle of photosynthesis in plants and algae. Two outer membranes enclose a semifluid interior, the stroma. A third membrane forms a compartment inside that functions in ATP and NADPH formation; sugars form in the stroma.
  133. chromatin
    All of the DNA molecules and associated proteins in a nucleus.
  134. chromosomes
    In eukaryotic cells, a linear DNA double helix with many histones and other proteins attached. See also Bacterial chromosome.
  135. cilium, cilia
    A motile structure with a 9+2 array of microtubules that projects from the plasma membrane of certain eukaryotic cells. Modified cilia, such as those of hair cells, have sensory functions.
  136. cyanobacteria
    A type of single-celled photoautotroph; the first to use a noncyclic pathway of photosynthesis, which slowly enriched the early atmosphere with oxygen.
  137. cytoplasm
    All cell parts, particles, and semifluid substances between the plasma membrane and the nucleus or nucleoid.
  138. cytoskeleton
    In a eukaryotic cell, the dynamic framework of diverse protein filaments that structurally support, organize, and move the cell and internal structures. Prokaryotic cells have a few similar protein filaments.
  139. endomembrane system
    Endoplasmic reticulum, Golgi bodies, and transport vesicles concerned with modification of many new proteins, lipid assembly, and their transport within the cytoplasm or to the plasma membrane for export.
  140. endoplasmic reticulum
    ER. Organelle that extends from the nuclear envelope through cytoplasm. Ribosomes coat the cytoplasmic side of rough ER, which modifes many new polypeptide chains in its lumen. Membrane lipids are assembled, fatty acids are broken down, and some toxins are inactivated in the lumen of smooth ER.
  141. endosymbiosis
    [Endo�, within + symbiosis, living together] An intimate, permanent ecological interaction in which one species lives and reproduces in the other�s body to the benefit of one or both.
  142. flagellum, flagella
    Of many eukaryotic cells, a long, whip-like motile structure with an inner 9+2 array of microtubules. Prokaryotic flagella do not have this array and are not whiplike; they rotate like a propeller.
  143. gap junctions
    Cylindrical arrays of proteins in the plasma membrane of adjoining cells; they pair up as open channels for rapid flows of ions and small molecules.
  144. Golgi bodies
    Organelle of endomembrane system; its enzymes modify many new polypeptide chains, assemble lipids, and package both inside vesicles for secretion or for use inside cell.
  145. intermediate filaments
    Cytoskeletal element that mechanically strengthens some cells.
  146. lignin
    Gluelike polymer deposited in secondary cell walls; makes some plant parts stronger, more waterproof, and less vulnerable to attacks.
  147. lipid bilayer
    Structural basis of all cell membranes; mainly phospholipids arranged tail-to-tail in two layers, with hydrophilic heads of one dissolved in cytoplasmic fluid and heads of the other in extracellular fluid.
  148. lysosome
    Vesicle filled with enzymes that functions in intracellular digestion.
  149. microfilaments
    The thinnest cytoskeletal element; consists of actin subunits that function in cell contraction, movement, and structural support.
  150. microtubules
    Largest cytoskeletal element; a filament of tubulin subunits. Contributes to cell shape, growth, and motion.
  151. motor proteins
    A type of accessory protein that interacts with microfilaments or with microtubules to move cell structures or the whole cell; e.g., myosin.
  152. myosin
    An ATP-energized motor protein that moves cell components on cytoskeletal tracks. Interacts with actin in sarcomeres to bring about contraction.
  153. nuclear envelope
    A double membrane that is the outer boundary of the nucleus.
  154. nucleoid
    The portion of a prokaryotic cell where DNA is physically organized but not enclosed in a membrane.
  155. nucleolus
    [L. nucleolus, tiny kernel] In an interphase nucleus, a mass of material from which RNA and proteins are assembled into the subunits of ribosomes.
  156. nucleus
    Large organelle with an outer envelope of two pore-ridden lipid bilayers that separates eukaryotic chromosomes from the cytoplasm.
  157. organelles
    One of the membrane-bound compartments that carry out specialized metabolic functions in eukaryotic cells; e.g., a nucleus, mitochondria.
  158. peroxisomes
    Enzyme-filled vesicle that breaks down amino acids, fatty acids, and toxic substances such as ethanol.
  159. phospholipid
    A lipid with a phosphate group in its hydrophilic head. The main constituent of cell membranes.
  160. plasma membrane
    Outer cell membrane; the structural and functional boundary between cytoplasm and extracellular fluid.
  161. plasmodesma, plasmodesmata
    A plant cell junction that connects the cytoplasm of adjoining cells.
  162. primary wall
    The first thin, pliable wall of young plant cells.
  163. prokaryotes
    [L. pro, before, + Gk. karyon, kernel] A single-celled organism, often walled, that does not have the organelles characteristic of eukaryotic cells. Only bacteria and archaeans are prokaryotic.
  164. pseudopods
    A dynamic lobe of membraneenclosed cytoplasm; functions in motility and phagocytosis by amoebas, amoeboid cells, and many white blood cells.
  165. ribosomes
    The site of polypeptide chain synthesis in all cells. An intact ribosome has two subunits of rRNA and proteins.
  166. secondary wall
    A rigid, permeable wall inside the primary wall of many plant cells; forms after the first growing season.
  167. stroma
    The semifluid matrix between the thylakoid membrane system and two outer membranes of a chloroplast where sucrose, starch, cellulose, and other end products of photosynthesis are built.
  168. surface-to-volume ratio
    A relationship in which the volume of an object increases with the cube of the diameter, but the surface area increases with the square.
  169. tight junctions
    An array of many strands of fibrous proteins collectively joining the sides of cells that make up an epithelium; the array prevents solutes from leaking between the cells.
  170. vesicles
    A small, membrane-bound sac in the cytoplasm; different sacs transport or store substances or hold enzymes that digest their contents.
  171. Chapter 5
  172. active transport
    Pumping of a specific solute across a cell membrane against its concentration gradient, through the interior of a transport protein. Requires energy input, as from ATP.
  173. adhesion proteins
    Of multicelled species, a plasma membrane protein that helps cells stick together in tissues and to extracellular matrixes such as basement membrane.
  174. biofilms
    Large microbial populations that anchored themselves to epithelium, rocks, or other surfaces by their own secretions.
  175. bulk flow
    The mass movement of one or more substances in the same direction, most often in response to pressure.
  176. calcium pump
    Active transport protein; pumps calcium ions across a cell membrane against their concentration gradient.
  177. communication proteins
    A membrane protein that helps form an open channel between the cytoplasm of adjoining cells.
  178. concentration gradient
    Difference in the number of molecules or ions of any one substance between two adjoining regions.
  179. diffusion
    Net movement of like ions or molecules from a region where they are most concentrated to an adjoining region where they are less concentrated; they move down their concentration gradient.
  180. electric gradient
    A difference in electric charge between adjoining regions.
  181. endocytosis
    Cell uptake of substances by forming vesicles from patches of plasma membrane. Three modes are receptor-mediated endocytosis, phagocytosis, and the bulk transport of extracellular fluid.
  182. exocytosis
    Fusion of a cytoplasmic vesicle with the plasma membrane; as it becomes part of the membrane, its contents are released to extracellular fluid.
  183. fluid mosaic model
    A cell membrane has a mixed composition (mosaic) of lipids and proteins, the interactions and motions of which impart fluidity to it.
  184. hydrostatic pressure
    Pressure exerted by a volume of fluid against a cell wall, membrane, or some other structure that contains it; also called turgor pressure.
  185. hypertonic solution
    Of two fluids, the one with the higher solute concentration.
  186. hypotonic solution
    Of two fluids, the one with the lower solute concentration.
  187. isotonic solution
    Any fluid having the same solute concentration as another fluid to which it is being compared.
  188. lipid bilayer
    Structural basis of all cell membranes; mainly phospholipids arranged tail-to-tail in two layers, with hydrophilic heads of one dissolved in cytoplasmic fluid and heads of the other in extracellular fluid.
  189. osmosis
    Diffusion of water across a selectively permeable membrane from a region where the water concentration is higher to a region where it is lower.
  190. osmotic pressure
    The amount of pressure which, when applied to a hypertonic fluid, will stop osmosis from occurring across a semipermeable membrane.
  191. passive transport
    Diffusion of a solute across a cell membrane, through the interior of a transport protein.
  192. phagocytosis
    [Gk. phagein, to eat] �Cell eating,� a common endocytic pathway by which various cells engulf food bits, microbes, and cellular debris.
  193. phospholipid
    A lipid with a phosphate group in its hydrophilic head. The main constituent of cell membranes.
  194. pressure gradient
    Difference in pressure between two adjoining regions.
  195. recognition proteins
    One of a class of glycoproteins or glycolipids that project above the plasma membrane and that identify a cell as nonself (foreign) or self (belonging to one�s own body tissue).
  196. selective permeability
    Built-in capacity of a cell membrane to prevent or allow specific substances from crossing it at certain times, in certain amounts.
  197. sodium-potassium pump
    Cotransporter that, when energized, actively transports sodium out of a cell and helps potassium passively diffuse into it at the same time.
  198. tonicity
    Relative solute concentrations of two fluids.
  199. transport proteins
    Membrane protein that passively or actively assists specific ions or molecules into or out of a cell. The solutes move through the protein�s interior.
  200. Chapter 6
  201. activation energy
    Minimum amount of energy required to start a reaction; enzyme action lowers this energy barrier. Reactions differ in the amount required.
  202. active sites
    Chemically stable crevice in an enzyme where substrates bind and a reaction can be catalyzed repeatedly.
  203. ADP
    Adenosine diphosphate. A nucleotide with an adenine base and two phosphate groups.
  204. antioxidants
    Any enzyme or cofactor that helps neutralize free radicals before they damage tissues.
  205. ATP
    Adenosine triphosphate. A type of nucleotide that functions as the main energy carrier between reaction sites in cells. Consists of the base adenine, the five-carbon sugar ribose, and three phosphate groups.
  206. ATP/ADP cycle
    How a cell regenerates its ATP supply. ADP forms when ATP gives up a phosphate group, then ATP forms as ADP binds to inorganic phosphate or a phosphate group split from a molecule.
  207. binding energy
    Energy released as weak bonds form between a substrate, enzyme, and any cofactor.
  208. bioluminescence
    Fluorescent light formed when certain organisms convert chemical bond energy to photon energy.
  209. biosynthetic pathway (anabolic)
    Any metabolic pathway by which one or more organic compounds are synthesized.
  210. chemical energy
    Potential energy in the bonds between atoms in molecules.
  211. chemical equilibrium
    No net change in concentrations of reactants and products in a reversible chemical reaction.
  212. coenzymes
    An organic molecule that is a necessary participant in some enzymatic reactions; helps catalysis by donating or accepting electrons or functional groups; e.g., a vitamin, ATP, NAD+.
  213. cofactors
    A metal ion or a coenzyme that assists an enzyme in catalysis by accepting or donating electrons or functional groups.
  214. degradative pathway (catabolic)
    Any of the stepwise series of metabolic reactions that break down organic compounds.
  215. electron transfer chains
    Array of enzymes and other molecules in a cell membrane that accept and give up electrons in sequence; operation of chain releases the energy of the electrons in small, usable increments.
  216. endergonic reaction
    A chemical reaction that requires a net energy input and converts more stable reactants into less stable products; not spontaneous.
  217. energy
    A capacity to do work.
  218. entropy
    Measure of how much and how far a concentrated form of energy has been dispersed after an energy change.
  219. enzymes
    A type of protein that catalyzes (speeds) a chemical reaction. Some RNAs also show catalytic activity.
  220. exergonic reaction
    Any chemical reaction with a net energy loss.
  221. FAD
    Flavin adenine dinucleotide. A type of nucleotide coenzyme; transfers electrons and H+ from one reaction site to another.
  222. feedback inhibition
    Mechanism by which a change that results from some cellular activity triggers responses that decrease or shut down the activity.
  223. first law of thermodynamics
    Energy cannot be created or destroyed.
  224. free radicals
    Any unbound molecular fragment with an unpaired electron.
  225. induced-fit model
    Explanation of how some enzymes work; their shape changes and fits a bound substrate more closely, and the tension destabilizes substrate bonds so that they can break.
  226. intermediates
    A substance formed between the start and end of a metabolic pathway.
  227. kilocalorie
    1,000 calories of heat energy; amount needed to raise the temperature of 1 kilogram of water by 1�C. Standard unit of measure for food�s caloric content.
  228. kinetic energy
    Energy of motion.
  229. metabolic pathways
    A stepwise sequence of enzyme-mediated reactions.
  230. metabolism
    All the controlled, enzyme-mediated chemical reactions by which cells acquire and use energy as they synthesize, store, degrade, and eliminate substances.
  231. NAD+
    Nicotinamide adenine dinucleotide. A nucleotide coenzyme; after it accepts electrons and H+, abbreviated as NADH.
  232. NADP+
    Nicotinamide adenine dinucleotide phosphate. A phosphorylated nucleotide coenzyme; after it accepts electrons and H+, abbreviated NADPH2.
  233. oxidation-reduction reactions (redox)
    Transfer of electrons between reactant molecules.
  234. phosphorylation
    Enzyme-mediated transfer of a phosphate group to an organic compound.
  235. potential energy
    A object�s capacity to do work owing to its position in space or the arrangement of its parts.
  236. products
    A substance remaining at the end of a reaction.
  237. reactants
    Substance that enters a reaction.
  238. second law of thermodynamics
    Energy tends to flow from concentrated to less concentrated forms.
  239. substrates
    A reactant molecule that is specifically acted upon by an enzyme.
  240. transport proteins
    Membrane protein that passively or actively assists specific ions or molecules into or out of a cell. The solutes move through the protein�s interior.
  241. Chapter 7
  242. absorption spectrum
    Range of wavelengths that a given type of pigment can absorb.
  243. accessory pigment
    A pigment that absorbs and transfers light energy to a photosystem; extends the range of light wavelengths for photosynthesis; e.g., a carotenoid.
  244. anthocyanin
    One of a class of accessory pigments that reflect red to blue light.
  245. ATP synthases
    A type of membrane-bound active transport protein that also catalyzes the formation of ATP.
  246. autotrophs
    [Gk. auto, self, and trophos, feeder] An organism that synthesizes its own food from simple inorganic compounds in its environment with energy captured from the sun or from oxidizing inorganic substances; e.g., a photoautotroph or chemoautotroph.
  247. C3 plants
    Type of plant in which threecarbon PGA is the first stable intermediate to form after carbon fixation.
  248. C4 plants
    Type of plant in which four-carbon oxaloacetate is the first stable intermediate to form after initial carbon fixation; in these plants, carbon is fixed twice, in two different types of photosynthetic cells.
  249. Calvin-Benson cycle
    Cyclic reactions that form sugar and regenerate RuBP in the second stage of photosynthesis. The reactions require carbon (from carbon dioxide). They use energy from ATP and hydrogens and electrons from NADPH, both of which form in the first stage.
  250. CAM plants
    Type of plant that conserves water by opening stomata only at night, when it fixes carbon by repeated turns of the C4 pathway; stands for crassulacean acid metabolism.
  251. carbon fixation
    Process by which any autotrophic cell incorporates carbon atoms into a stable organic compound. Different cells get carbon dioxide from the air or dissolved in water.
  252. carotenoids
    One of a class of accessory pigments in photosynthesis that reflect red, orange, and yellow light. One kind, beta-carotene, is a precursor of vitamin A.
  253. chlorophyll a
    [Gk. chloros, green, + phyllon, leaf] In plants and algae, a pigment that is a receptor for the photon energy required to start photosynthesis; it absorbs mainly violet and red light and reflects or transmits green light.
  254. chlorophyll b
    An accessory pigment that absorbs mainly blue and orange light.
  255. chloroplasts
    Organelle of photosynthesis in plants and algae. Two outer membranes enclose a semifluid interior, the stroma. A third membrane forms a compartment inside that functions in ATP and NADPH formation; sugars form in the stroma.
  256. cuticle
    Of plants, a cover of transparent waxes and cutin on the outer wall of epidermal cells. Of annelids, a thin, flexible coat. Of arthropods, a lightweight exoskeleton hardened with chitin.
  257. cyclic pathway
    Oldest photosynthetic pathway. Photon energy forces electrons out of membrane-bound photosystems to transfer systems, which return them to the photosystems. Electron flow across the membrane sets up H+ gradients that drive ATP formation.
  258. electromagnetic spectrum
    All wavelengths of photon energy from gamma rays less than 10�5 nanometers long to radio waves more than 10 kilometers long.
  259. electron transfer chains
    Array of enzymes and other molecules in a cell membrane that accept and give up electrons in sequence; operation of chain releases the energy of the electrons in small, usable increments.
  260. heterotrophs
    [Gk. heteros, other, + trophos, feeder] Organism that cannot make its own food; feeds on other organisms, their wastes, or their remains.
  261. light-dependent reactions
    First stage of photosynthesis. Pigments trap photon energy, which is transduced to ATP chemical energy. In a noncyclic pathway, a reduced coenzyme, NADPH, also forms.
  262. light-independent reactions
    Second stage of photosynthesis. Involves carbon fixation and cyclic reactions that form sugars and regenerate an organic compound that is the cycle�s entry point. ATP from the first stage delivers energy that drives the reactions. NADPH from the first stage donates electrons and hydrogen building blocks. The carbon and nitrogen come from CO2.
  263. noncyclic pathway
    [L. non, not, + Gk. kylos, circle] The light-dependent reactions of photosynthesis that produce both ATP and NADPH; its oxygen by-product is the basis of Earth�s oxygen-rich atmosphere.
  264. PGA
    Phosphoglycerate. During glycolysis, the intermediate that results after ATP has formed by substrate-level phosphorylation; also the first stable intermediate of the Calvin�Benson cycle of photosynthesis.
  265. PGAL
    Phosphoglyceraldehyde. During glycolysis, the intermediate that gives up electrons and hydrogen to form NADH. During turns of the Calvin�Benson cycle, two PGALs form one sugar; rearrangements of ten others regenerate a compound that is the entry point for the cycle.
  266. photolysis
    [Gk. photos, light, + -lysis, breaking apart] Reactions that split water molecules, which release electrons for the noncyclic pathway of photosynthesis; oxygen is a by-product.
  267. photons
    Unit of electromagnetic energy; has wave-like and particle-like properties.
  268. photosynthesis
    The process by which photoautotrophs capture sunlight energy and use it in the formation of ATP and NADPH, then in the formation of sugars from carbon dioxide and water. ATP gives up energy that drives the sugar-building reactions, and NADPH donates electrons and hydrogen building blocks.
  269. photosystems
    In photosynthetic cells, a cluster of membrane-bound pigments and other molecules; it converts light energy to chemical energy.
  270. phycobilins
    One of a class of accessory pigments in cyanobacteria and red algae that reflects red to blue light.
  271. pigments
    Any light-absorbing molecule.
  272. reaction center
    At a photosystem�s center, a special pair of chlorophyll a molecules; the center loses electrons on absorption of photon energy, thereby initiating the lightdependent reactions of photosynthesis.
  273. rubisco
    RuBP carboxylase. Carbon-fixing enzyme of the C3 photosynthesis pathway.
  274. RuBP
    Ribulose bisphosphate. A five-carbon organic compound; the entry point for the Calvin�Benson cycle, which regenerates it.
  275. stomata
    A gap between two plumped guard cells that lets water vapor and gases diffuse across the epidermis of a leaf or primary stem; diffusion stops when the cells lose water and collapse.
  276. stroma
    The semifluid matrix between the thylakoid membrane system and two outer membranes of a chloroplast where sucrose, starch, cellulose, and other end products of photosynthesis are built.
  277. thylakoid membrane
    A chloroplast�s inner membrane system, often folded as flattened sacs, that forms a continuous compartment in the stroma. In the first stage of photosynthesis, pigments and enzymes in the membrane function in the formation of ATP and NADPH.
  278. wavelength
    The distance between the crests of two successive wavelike forms of energy in motion.
  279. xanthophylls
    One of a class of accessory pigments in photosynthesis that reflects yellow to orange light.
  280. Chapter 8
  281. acetyl-CoA
    Coenzyme A bound to a twocarbon fragment from pyruvate, which it transfers to oxaloacetate for Krebs cycle.
  282. aerobic respiration
    [Gk. aer, air, + bios, life] Oxygen-requiring pathway of ATP formation in mitochondria: from glycolysis, to Krebs cycle and electron transport phosphorylation. Typical net energy yield: 36 ATP per glucose molecule.
  283. alcoholic fermentation
    An anaerobic ATPforming pathway using pyruvate and NADH from glycolysis. NADH transfers electrons to an intermediate, acetaldehyde, forming ethanol. Occurs in cytoplasm only. The net yield is 2 ATP from glycolysis; the steps remaining only regenerate NAD+.
  284. anaerobic electron transfers
    [Gk. an, without, + aer, air] Of some bacteria and archaeans, ATP formation by way of a flow of electrons through transfer chains in the plasma membrane to a final electron acceptor that is not oxygen.
  285. ATP
    Adenosine triphosphate. A type of nucleotide that functions as the main energy carrier between reaction sites in cells. Consists of the base adenine, the five-carbon sugar ribose, and three phosphate groups.
  286. ATP synthase
    A type of membrane-bound active transport protein that also catalyzes the formation of ATP.
  287. coenzymes
    An organic molecule that is a necessary participant in some enzymatic reactions; helps catalysis by donating or accepting electrons or functional groups; e.g., a vitamin, ATP, NAD+.
  288. electron transfer phosphorylation
    Final stage of aerobic respiration; electron flow through electron transfer chains in inner mitochondrial membrane sets up H+ concentration and electric gradients that drives ATP formation. Oxygen accepts electrons at the end of the chain.
  289. FAD, FADH2
    Flavin adenine dinucleotide. A type of nucleotide coenzyme; transfers electrons and H+ from one reaction site to another.
  290. glycolysis
    Breakdown of glucose or another organic compound to two pyruvates. First stage of aerobic respiration, fermentation, or anaerobic electron transfer. Oxygen has no role in glycolysis, which takes place in the cytoplasm of all cells. Two NADH form. Net yield: 2 ATP per glucose molecule.
  291. Krebs cycle
    The second stage of aerobic respiration in which many coenzymes form as pyruvate from glycolysis is fully broken down to CO2 and H2O. Two ATP also form. Occurs only in mitochondria.
  292. lactate fermentation
    One of the anaerobic pathways of ATP formation. NADH from glycolysis donates hydrogen and electrons to pyruvate, converting it to three-carbon lactate, and regenerating NAD+. The net energy yield is 2 ATP (from glycolysis).
  293. mitochondrion
    Double-membraned organelle of ATP formation; only site of the second and third stages of aerobic respiration.
  294. NAD+, NADH
    Nicotinamide adenine dinucleotide. A nucleotide coenzyme; after it accepts electrons and H+, abbreviated as NADH.
  295. oxaloacetate
    A fourcarbon compound with roles in metabolism; e.g., the point of entry into the Krebs cycle.
  296. PGAL
    Phosphoglyceraldehyde. During glycolysis, the intermediate that gives up electrons and hydrogen to form NADH. During turns of the Calvin�Benson cycle, two PGALs form one sugar; rearrangements of ten others regenerate a compound that is the entry point for the cycle.
  297. pyruvate
    Three-carbon compound that forms as an end product of glycolysis.
  298. substrate-level phosphorylation
    Direct, enzyme-mediated transfer of a phosphate group from a substrate to another molecule.
  299. Chapter 9
  300. anaphase, mitosis
    Nuclear division stage. Sister chromatids of each chromosome are separated from each other and move to opposite spindle poles.
  301. bipolar mitotic spindle
    Of eukaryotic cells, a dynamic array of microtubules that moves chromosomes with respect to its two poles during mitosis or meiosis.
  302. cancers
    A malignant neoplasm; a mass of abnormally dividing cells that can leave their home tissue and invade and form new masses in other parts of the body.
  303. cell cycle
    Of eukaryotic cells, a series of events from the time a cell forms until it reproduces. A cycle consists of interphase, mitosis, and cytoplasmic division.
  304. cell plate formation
    The mechanism of cytoplasmic division in plant cells. After nuclear division, vesicles derived from Golgi bodies deposit the material for a cross-wall that cuts through the cytoplasm and connects to the parent cell wall.
  305. centrioles
    A barrel-shaped structure that arises from a centrosome and organizes newly forming microtubules into a 9+2 array inside a cilium or flagellum.
  306. centromere
    Of a eukaryotic chromosome, a constricted region having binding sites (kinetochores) for spindle microtubules.
  307. centrosome
    Dense mass of material in the cytoplasm of eukaryotic cells from which microtubules start to grow.
  308. chromosome
    In eukaryotic cells, a linear DNA double helix with many histones and other proteins attached. See also Bacterial chromosome.
  309. chromosome number
    The sum of all of the chromosomes in cells of a given type.
  310. contractile ring mechanism
    Mechanism of cytoplasmic division of animal cells. Just beneath the plasma membrane, a thin band of contractile filaments around the cell midsection contracts and pinches the cytoplasm in two.
  311. cytokinesis
    [Gk. kinesis, motion] Cytoplasmic division.
  312. cytoplasmic division
    Cytokinesis. After nuclear division, a splitting of the parent cell cytoplasm that completes formation of daughter cells.
  313. diploid number
    Of many sexually reproducing species, having two chromosomes of each type, or pairs of homologues, in somatic cells.
  314. germ cells
    Animal cell set aside for sexual reproduction; gives rise to gametes.
  315. growth factors
    A protein that stimulates increases in size; e.g., by inducing mitosis.
  316. HeLa cells
    Cancer cell of a lineage used in research laboratories around the world.
  317. histones
    Type of structural protein that helps organize and condense eukaryotic chromosomes and control access to genes during interphase.
  318. interphase
    In a eukaryotic cell cycle, the interval between mitotic divisions when a cell grows in mass, roughly doubles the number of its cytoplasmic components, and replicates its DNA.
  319. kinases
    Type of enzyme that transfers a phosphate-group to an organic molecule.
  320. kinetochores
    A mass of protein and DNA in the centromere to which microtubules of the spindle attach.
  321. meiosis
    [Gk. meioun, to diminish] A nuclear division process that halves the parental chromosome number, to a haploid (n) number. Prerequisite to the formation of gametes and sexual spores.
  322. metaphase
    Of meiosis I, stage when all pairs of homologues are positioned at the equator of a bipolar spindle. Of mitosis or meiosis II, the stage when all duplicated chromosomes are positioned at the equator.
  323. metastasis
    Abnormal migration of cancer cells that break away from home tissues and may start colonies in other tissues.
  324. microtubules
    Largest cytoskeletal element; a filament of tubulin subunits. Contributes to cell shape, growth, and motion.
  325. mitosis
    [Gk. mitos, thread] Type of nuclear division that maintains the parental chromosome number. The basis of growth in size, tissue repair, and often asexual reproduction for eukaryotes.
  326. neoplasms
    Mass of cells (tumor) that lost control over the cell cycle.
  327. nucleosome
    Small stretch of eukaryotic DNA wound twice around a spool of proteins called histones.
  328. prophase, mitosis
    All of the duplicated chromosomes in a cell condense and get attached to a newly forming spindle.
  329. sister chromatids
    One of the two attached members of a duplicated eukaryotic chromosome.
  330. somatic cells
    [Gk. soma�, body] Any body cell that is not a germ cell.
  331. telophase
    Of meiosis I, a stage when one member of each pair of homologous chromosomes has arrived at a spindle pole. Of mitosis and of meiosis II, the stage when chromosomes typically decondense into threadlike structures and two daughter nuclei form.
  332. tumor
    Tissue mass of cells dividing at an abnormally high rate. Benign tumor cells stay in their home tissue; malignant ones metastasize, or slip away and invade other places in the body, where they may start new tumors. See also neoplasm.
  333. Chapter 10
  334. allele
    One of two or more molecular forms of a gene at a given locus; alleles arise by mutation and encode slightly different versions of the same trait.
  335. asexual reproduction
    Any reproductive mode by which offspring arise from one parent and inherit that parent�s genes only; e.g., prokaryotic fission, transverse fission, budding, vegetative propagation.
  336. centromere
    Of a eukaryotic chromosome, a constricted region having binding sites (kinetochores) for spindle microtubules.
  337. chromosome number
    The sum of all of the chromosomes in cells of a given type.
  338. clones
    A genetically identical copy of DNA, a cell, or a multicelled organism.
  339. crossing over
    At prophase I of meiosis, reciprocal exchange of segments between two nonsister chromatids of a pair of homologous chromosomes. Puts novel combinations of alleles in gametes.
  340. diploid number
    Of many sexually reproducing species, having two chromosomes of each type, or pairs of homologues, in somatic cells.
  341. egg
    Mature female gamete, or ovum.
  342. fertilization
    Fusion of a sperm nucleus and an egg nucleus, the result being a single-celled zygote.
  343. gametes
    Haploid cell formed by meiotic cell division of a reproductive cell; required for sexual reproduction.
  344. gametophyte
    [Gk. phyton, plant] A haploid multicelled body in which haploid gametes form during the life cycle of plants and some algae.
  345. genes
    Unit of heritable information in DNA, transmissable from parents to offspring.
  346. genetic recombination
    Outcome of any process that puts new genetic information in a DNA molecule; e.g., by crossing over.
  347. germ cells
    Animal cell set aside for sexual reproduction; gives rise to gametes.
  348. haploid number
    The sum of all chromosomes in cells with one of each type of chromosome characteristic of the species; e.g., in a gamete.
  349. homologous chromosomes
    [Gk. homologia, correspondence] One of a pair of chromosomes in body cells of diploid organisms; except for a pairing of nonidentical sex chromosomes, a pair has the same size, shape, and gene sequence.
  350. meiosis
    [Gk. meioun, to diminish] A nuclear division process that halves the parental chromosome number, to a haploid (n) number. Prerequisite to the formation of gametes and sexual spores.
  351. oocyte
    A type of immature egg.
  352. ovum
    Mature secondary oocyte.
  353. polar body
    One of four cells that form by meiotic cell division of an oocyte but that does not become the ovum.
  354. sexual reproduction
    Production of genetically variable offspring by meiosis, gamete formation, and fertilization.
  355. sister chromatids
    One of the two attached members of a duplicated eukaryotic chromosome.
  356. sperm
    Mature male gamete.
  357. spores
    A structure of one or a few cells, often walled or coated, that protects and/or disperses a new sexual or asexual generation. Many bacteria as well as apicomplexans, fungi, and plants form spores.
  358. sporophytes
    [Gk. phyton, plant] A sporeproducing vegetative body of a plant or multicelled alga that grows by mitotic cell divisions from a zygote.
  359. Chapter 11
  360. ABO blood typing
    Method of identifying which self-recognition proteins of types A and B are at the surface of an individual�s red blood cells; the absence of either type is designated O.
  361. alleles
    One of two or more molecular forms of a gene at a given locus; alleles arise by mutation and encode slightly different versions of the same trait.
  362. codominance
    A condition in which a pair of nonidentical alleles that influence two different phenotypes are expressed at the same time in heterozygotes.
  363. continuous variation
    Of individuals of a population, a range of small differences in the phenotypic expression of a trait.
  364. dihybrid cross
    Type of experiment that starts with a cross between two truebreeding, homozygous parents that differ in two traits governed by alleles of two genes. The actual experiment is a cross between two of their F1 offspring that are identically heterozygous for alleles of the two genes; e.g., AaBb x AaBb.
  365. dominant allele
    Of diploid cells, an allele that masks the phenotypic effect of any recessive allele paired with it.
  366. epistasis
    An interaction among products of two or more gene pairs that influence the same trait.
  367. genes
    Unit of heritable information in DNA, transmissable from parents to offspring.
  368. genotype
    Genetic makeup of an individual; a single gene pair or the sum total of an individual�s genes.
  369. homozygous dominant
    Having a pair of dominant alleles at a gene locus on homologous chromosomes; e.g., AA.
  370. homozygous recessive
    Having a pair of recessive alleles at a gene locus on homologous chromosomes; e.g., Aa.
  371. hybrids
    Individual having a nonidentical pair of alleles for a trait being studied.
  372. incomplete dominance
    Condition in which one allele of a pair is not fully dominant; the heterozygous phenotype is somewhere between both homozygous phenotypes.
  373. linkage group
    All genes on a chromosome.
  374. melanin
    • A brownish-black pigment.
    • monohybrid cross
    • An experiment that starts with a cross between two truebreeding, homozygous parents that differ in a trait governed by alleles of one gene. The experiment is a cross between two F1 offspring that are identically heterozygous for the two genes; e.g., Aa x Aa.
    • multiple allele system
    • Three or more slightly different molecular forms of a gene that persists among the individuals of a population.
    • mutation
    • [L. mutatus, a change, + -ion, act, result, or process] Heritable change in DNA�s molecular structure. Original source of new alleles and life�s diversity.
    • P, F1, F2
    • The first and second generation offspring of experimental crosses.
    • phenotype
    • [Gk. phainein, to show + typos, image] Observable trait or traits of an individual.
    • pleiotropy
    • A case of alleles at a single gene locus having positive or negative impact on two or more traits.
    • polygenic inheritance
    • Inheritance of multiple genes that affect the same trait.
    • probability
    • The odds that each outcome of an event will occur is proportional to the total number of ways in which that outcome can be reached.
    • Punnett-square method
    • A simple way to predict the probable outcomes of a genetic cross by constructing and filling in a diagram of all possible combinations of genotypes, phenotypes, or both.
    • recessive allele
    • Allele whose expression in heterozygotes is fully or partially masked by expression of a dominant partner allele. It is fully expressed only in homozygous recessives.
    • testcross
    • A cross that might reveal the (unknown) genotype of an individual showing dominance for a trait; the individual is crossed with a known homozygous recessive individual.
    • true-breeding lineage
    • A group consisting of parents and their offspring in which only one version of a trait persists over time.
  375. Chapter 12
  376. alleles
    One of two or more molecular forms of a gene at a given locus; alleles arise by mutation and encode slightly different versions of the same trait.
  377. aneuploidy
    A type of chromosome abnormality in which body (somatic) cells have one extra or one less chromosome relative to the parental chromosome number.
  378. autosomes
    Of a sexually reproducing species, any chromosome of a type that is the same in both males and females.
  379. crossing over
    At prophase I of meiosis, reciprocal exchange of segments between two nonsister chromatids of a pair of homologous chromosomes. Puts novel combinations of alleles in gametes.
  380. deletion
    Loss of a chromosome segment; often leads to genetic disorders. Also the loss of one or more nucleotide bases from a DNA molecule.
  381. duplication
    Base sequence in DNA that has been repeated two or more times.
  382. embryo
    Of animals, a new individual that forms by cleavage, gastrulation, and other early stages of development. Of plants, a young sporophyte until germination.
  383. fetus
    In mammalian development, the stage after all major organ systems have formed until time of birth.
  384. gene
    Unit of heritable information in DNA, transmissable from parents to offspring.
  385. genetic abnormality
    A less common or rare version of a heritable trait.
  386. genetic disorder
    An inherited condition causing mild to severe medical problems.
  387. genetic recombination
    Outcome of any process that puts new genetic information in a DNA molecule; e.g., by crossing over.
  388. inversion
    A chromosomal alteration; part of the DNA sequence gets oriented in the reverse direction, with no molecular loss.
  389. karyotype
    Preparation of an individual�s metaphase chromosomes arranged by length, centromere location, and shape.
  390. linkage groups
    All genes on a chromosome.
  391. nondisjunction
    Failure of sister chromatids or homologous chromosomes to move apart in meiosis or mitosis. Daughter cells get too many or too few chromosomes.
  392. pedigree
    Chart of connections among individuals related by descent.
  393. polyploidy
    A case of somatic cells having three or more of each type of chromosome characteristic of the species.
  394. reciprocal crosses
    A paired cross that may identify the role of parental sex on the inheritance of a trait. In the second cross, a trait characteristic of each sex is reversed compared to the original cross.
  395. sex chromosomes
    One of two kinds of homologous chromosomes that, in certain combinations, dictate the gender of the new individual. Also has genes unrelated to sexual traits.
  396. testosterone
    A sex hormone necessary for the development and functioning of the male reproductive system of vertebrates.
  397. translocation
    Attachment of a piece of a broken chromosome to another chromosome. Also, a mechanism by which organic compounds are conducted in phloem.
  398. trisomy 21
    Having one extra chromosome in somatic cells; e.g., trisomy 21 (2n + 1).
  399. wild-type allele
    Of a given gene locus, the allele that occurs normally or with the greatest frequency among individuals of a population.
  400. X chromosome
    A type of sex chromosome that influences sex determination; e.g., XX mammalian embryo becomes female; an XY pairing causes it to develop into a male.
  401. X-linked inheritance
    Recessive condition in which the responsible, mutated gene is on the X chromosome.
  402. Y chromosome
    Distinctive chromosome in males or females of many species (not both); e.g., human males XY, females, XX.
  403. Chapter 13
  404. adenine
    One of four nitrogen-containing bases in nucleotide monomers of DNA or RNA; also refers to a nucleotide having an adenine base component.
  405. bacteriophages
    One of a class of viruses that infects bacteria.
  406. clone
    A genetically identical copy of DNA, a cell, or a multicelled organism.
  407. cytosine
    One of the four nitrogen-containing bases in nucleotide monomers of DNA or RNA; also applies to a nucleotide that contains a cysteine base.
  408. DNA
    Deoxyribonucleic acid. Double-stranded nucleic acid twisted into a helical shape; its base sequence encodes the primary hereditary information for all living organisms and many viruses.
  409. DNA ligases
    Type of enzyme that catalyzes the sealing of short stretches of DNA into a continuous strand during replication; also seals strand breaks.
  410. DNA polymerases
    Type of enzyme that catalyzes the addition of free nucleotides to new DNA strands during replication; also proofreads and corrects mismatches.
  411. DNA proofreading mechanisms
    Any enzyme-mediated process that fixes DNA replication errors or strand breaks.
  412. DNA replication
    Process by which a cell duplicates its DNA molecules before it divides into daughter cells.
  413. guanine
    One of four nitrogen-containing bases in nucleotide monomers of DNA or RNA; also may refer to a nucleotide that contains a guanine base.
  414. helicases
    Type of enzyme that catalyzes breaking of hydrogen bonds during DNA replication so the two strands of double helix can unwind from each other.
  415. nucleotides
    Small organic compound with a five-carbon sugar, a nitrogen-containing base, and a phosphate group. Functions as coenzymes or monomers of nucleic acids.
  416. purines
    A nucleotide base with a double ring structure; e.g., adenine or guanine.
  417. pyrimidines
    A nucleotide base with a single ring structure; e.g., cytosine, thymine, uracil.
  418. repair enzymes
    Type of enzymes that repairs nucleotide mismatches in a DNA strand.
  419. semiconservative replication
    [Gk. semi�, half, + L. conservare, to keep] Mechanism by which a DNA molecule is duplicated. The double helix unzips along its length, exposed bases of each strand are a template upon which a new strand is assembled, then each conserved strand and its new partner wind up in a double helix. Two double helixes, each with a parental strand and new strand of DNA, result.
  420. stem cells
    Self-perpetuating, undifferentiated animal cell. A portion of its daughter cells becomes specialized; e.g., red blood cells from stem cells in bone marrow.
  421. telomeres
    A cap of repetitive DNA sequence on the end of a chromosome. Each nuclear division, enzymes digest a bit of it; cells stop dividing when only a nubbin remains.
  422. thymine
    One of four nitrogencontaining bases in nucleotide monomers of DNA; also applies to a nucleotide with a thymine base component.
  423. x-ray diffraction images
    Film image of xrays scattered by a crystalline sample; the resulting pattern of streaks and dots can be used to calculate the spacing between the atoms in the crystal lattice.
  424. Chapter 14
  425. alternative splicing
    Event by which the same gene can specify two or more slightly different proteins. All exons in a pre-mRNA transcript of the gene are retained or some are removed and the rest spliced in various combinations for the mature transcript.
  426. amino acids
    A small organic compound with a carboxylic acid group, an amino group, and a characteristic side group (R); monomer of polypeptide chains.
  427. anticodon
    Series of three nucleotide bases in tRNA that can base-pair with mRNA codons.
  428. base sequence
    Linear order of nucleotides that compose a DNA or RNA strand.
  429. base-pair substitution
    Mutation in which one nucleotide is wrongly substituted for another during DNA replication.
  430. deletions
    Loss of a chromosome segment; often leads to genetic disorders. Also the loss of one or more nucleotide bases from a DNA molecule.
  431. exons
    A base sequence in eukaryotic DNA that is part or all of a protein-encoding gene; may or may not be excised from a pre-mRNA during transcript processing.
  432. gene mutations
    Small-scale change in the nucleotide sequence of a gene; can result in an altered protein product.
  433. genetic code
    Correspondence between triplets of nucleotides in DNA and mRNA, and specific sequences of amino acids in a polypeptide chain; near-universal language of protein synthesis; mitochondria and a few species have a few variant code words.
  434. insertions
    A mutation by which one or more bases are introduced into a DNA strand. Also a movable attachment of muscle to bone.
  435. introns
    One of the noncoding sequences in eukaryotic genes; it is excised from the pre-mRNA transcripts before translation.
  436. ionizing radiation
    Form of radiation with enough energy to eject electrons from atoms.
  437. messenger RNA
    mRNA. A single strand of ribonucleotides transcribed from DNA; the only type of RNA that carries proteinbuilding information to ribosomes.
  438. mutation rate
    Of a given gene locus, the probability that a spontaneous mutation will happen in a specified interval.
  439. nonionizing radiation
    Form of radiation that carries enough energy to boost electrons to higher energy levels but not enough to eject them from an atom.
  440. polysomes
    A series of ribosomes that are all translating the same mRNA molecule at the same time.
  441. promoter
    Short stretch of DNA to which RNA polymerase binds. Transcription then begins at the gene closest to the promoter.
  442. ribosomal RNA
    rRNA. A class of RNA that becomes complexed with proteins to form ribosomes; some catalyze assembly of polypeptide chains.
  443. ribosomes
    The site of polypeptide chain synthesis in all cells. An intact ribosome has two subunits of rRNA and proteins.
  444. RNA polymerase
    Enzyme that catalyzes transcription of DNA into RNA.
  445. transcription
    [L. trans, across, + scribere, to write] First stage of protein synthesis. An RNA strand is assembled from nucleotides using a gene region in DNA as a template.
  446. transfer RNA
    tRNA. One of a class of small RNA molecules that delivers amino acids to a ribosome. Its anticodon pairs with an mRNA codon during translation.
  447. translation
    Second stage of protein synthesis. At ribosomes, information encoded in an mRNA transcript guides the synthesis of a new polypeptide chain from amino acids.
  448. transposons
    Transposable element. A stretch of DNA that jumps spontaneously and randomly to a different location in the genome and may mutate a gene.
  449. uracil
    One of four nitrogencontaining bases in nucleotide monomers of RNA; also applies to a nucleotide with a uracil base component. Like thymine, uracil can base-pair with adenine.
  450. Chapter 15
  451. ABC model
    Model for the genetic basis of flower formation; products of three master genes (A, B, C) control the development of sepals, petals, and stamens and carpels from meristematic tissue.
  452. acetylation
    The attachment of an acetyl group (CH3CO) to an organic compound.
  453. Barr body
    Of the two X chromosomes in the somatic cells of female mammals, the one that has been condensed.
  454. cell differentiation
    In developing embryos of multicelled organisms, the process by which different cell lineages selectively express a different fraction of their genome and thereby become specialized in their composition, structure, and function.
  455. dosage compensation
    A gene control mechanism in female mammals in which most genes on one of two X chromosomes in somatic cells are inactivated; ensures that X chromosome genes are expressed at the same levels as in males (XY).
  456. enhancers
    A small sequence in DNA that binds transcription-regulating molecules; enhances transcription rates.
  457. gene controls
    A molecular mechanism that governs if, when, or how a specific gene is transcribed or translated.
  458. homeotic genes
    One of a class of master genes; helps determine identity of body parts during embryonic development.
  459. hormones
    [Gk. hormon, stir up] Signaling molecule secreted by one cell that can alter activities of any cell with receptors for it.
  460. introns
    One of the noncoding sequences in eukaryotic genes; it is excised from the pre-mRNA transcripts before translation.
  461. knockout experiments
    An experiment in which a living organism is engineered so that one of its genes does not function.
  462. methylation
    Attachment of a methyl group to an organic compound; also a common gene control mechanism.
  463. mosaic tissue effect
    In female mammals, an outcome of random X chromosome inactivation; different patches of tissue are expressing different X-linked alleles.
  464. negative control
    Control mechanism by which one or more regulatory proteins slow down a cell activity.
  465. operator
    Part of an operon; a DNA binding site for a regulatory protein.
  466. operon
    Group of bacterial genes together with a promoter�operator DNA sequence that controls their transcription.
  467. pattern formation
    In animal embryonic development, the sculpting of specialized tissues and organs from clumps of cells in the proper places, in the proper order by way of embryonic induction.
  468. polytene chromosomes
    Of some insects, a chromosome consisting of many parallel copies of the same DNA molecule.
  469. positive control
    Use of regulatory proteins to promote gene expression.
  470. promoter
    Short stretch of DNA to which RNA polymerase binds. Transcription then begins at the gene closest to the promoter.
  471. promoters
    Short stretch of DNA to which RNA polymerase binds. Transcription then begins at the gene closest to the promoter.
  472. regulatory proteins
    Part of mechanisms that control transcription, translation, and gene products by interacting with DNA, RNA, new polypeptide chains, or proteins such as enzymes.
  473. repressor
    Type of protein that can block transcription of a prokaryotic gene by binding to an operator.
  474. selective gene expression
    Outcome of controls over which gene products a cell makes or activates in a specified interval. Basis of cell differentiation.
  475. X chromosome inactivation
    In a female mammalian embryo, the programmed painting of special RNAs over most of one of the two X chromosomes, which cuts off access to the majority of its genes. Which X chromosome gets painted in each cell is a random event, so tissues of adult female mammals are a mosaic of traits. See also Dosage compensation.
  476. Chapter 16
  477. automated DNA sequencing
    Extremely rapid, robotic method of identifying the nucleotide sequence of a region of DNA. Gel electrophoresis and laser detection of fluorescent tracers are part of method.
  478. cDNA
    DNA synthesized from an mRNA transcript through the use of the enzyme reverse transcriptase.
  479. cloning vector
    Any DNA molecule that can accept foreign DNA and that can be replicated inside a host cell.
  480. DNA chips
    Microarray of thousands of gene sequences that represents a large subset of a genome; stamped onto a glass plate and used to study gene expression.
  481. DNA clone
    Fragment of DNA inserted into a vector such as a plasmid and introduced into a host organism; used to make many copies of a particular segment of DNA.
  482. DNA fingerprint
    A way to distinguish one individual from all others based on unique differences in parts of their DNA; fragments cut from an individual�s DNA (RFLPs) have a unique pattern of sizes.
  483. DNA ligase
    Type of enzyme that catalyzes the sealing of short stretches of DNA into a continuous strand during replication; also seals strand breaks.
  484. gel electrophoresis
    Method of separating DNA molecules according to length, or protein molecules according to size and charge. The molecules move apart while migrating through a gel matrix in response to a weak electric current.
  485. gene library
    Collection of host cells that contain different cloned DNA fragments representing all or most of a genome.
  486. genetic engineering
    Manipulation of an organism�s DNA, usually to alter at least one aspect of phenotype.
  487. genome
    All DNA in a haploid number of chromosomes for a species.
  488. genomics
    The study of genes and gene function in humans and other organisms.
  489. human gene therapy
    The transfer of one or more normal or modified genes into a person to correct a genetic defect, or to boost resistance to a disease.
  490. nucleic acid hybridization
    Any basepairing between DNA or RNA strands from different sources.
  491. PCR
    Polymerase chain reaction. A method to rapidly copy DNA fragments.
  492. plasmids
    A small, circular bacterial DNA molecule having a few genes; replicated independently of the bacterial chromosome.
  493. primer
    Short nucleotide sequence that researchers design as an initiation site for synthesis of a DNA strand on a DNA or RNA template.
  494. probe
    Short nucleotide sequence that has been labeled with a tracer; designed to hybridize with part of a gene or mRNA.
  495. recombinant DNA technology
    Techniques by which DNA molecules from different species can be cut into fragments, spliced together into cloning vectors, and then amplified to useful quantities.
  496. restriction enzymes
    One of hundreds of proteins that recognize and cut specific base sequences in double-stranded DNA.
  497. reverse transcriptase
    A viral enzyme that catalyzes the assembly of free nucleotides into a strand of DNA on an RNA template.
  498. seed banks
    A storage facility where genes of diverse plant lineages are preserved.
  499. tandem repeats
    One of many copies of short base sequences positioned one after another on a chromosome; used in DNA fingerprinting.
  500. xenotransplantation
    Surgical transfer of an organ from one species to another.
  501. Chapter 17
  502. analogous structures
    [Gk. analogos, similar to one another] Dissimilar body parts that have become similar in structure, function, or both in lineages that are not closely related but were subjected to similar pressures.
  503. Archean eon
    Eon extending from the time that life originated, 3.8 billion years ago, to 2.5 billion years ago.
  504. Cenozoic
    The modern geologic era, from 65 million years ago to the present.
  505. comparative morphology
    [Gk. morph, form] Scientific study of comparable external body parts of embryonic stages and adult forms of major lineages.
  506. evolution
    [L. evolutio, an unrolling] Genetic change in a line of descent by microevolutionary events (gene mutation, natural selection, genetic drift, and gene flow); basis of large-scale patterns, rates, and trends in the history of life.
  507. fossilization
    How fossils form over time. An organism or evidence of it gets buried in sediments or volcanic ash; water slowly infiltrates the remains, and metal ions and other inorganic compounds dissolved in it replace the minerals in bones and other hardened tissues.
  508. fossils
    Recognizable, physical evidence of an organism that lived in the distant past.
  509. geologic time scale
    Time scale for Earth�s history; major subdivisions correspond to mass extinctions. Dates are now absolute as a result of radiometrically dating.
  510. half-life
    The unvarying time it takes for half of a quantity of any radioisotope to decay into a more stable form.
  511. inheritance of acquired characteristics
    Transmission, from parents to offspring, of genes that underlie the traits characteristic of their species.
  512. lineage
    Line of descent.
  513. macroevolution
    Large-scale patterns, rates of change, and trends among lineages.
  514. Mesozoic
    Era of spectacular expansion in the range of global diversity; lasted from 240 million to 65 million years ago.
  515. molecular clock
    Model used to calculate the time of origin of one lineage relative to others; assumes that a group of genes accumulates mutations at a constant rate, measurable as a series of predictable ticks back through time. The last tick stops close to the time the lineage originated.
  516. morphological convergence
    A pattern of macroevolution. In response to similar environmental pressures, body parts of evolutionarily distant lineages slowly evolve in similar ways and end up being alike in function, appearance, or both.
  517. morphological divergence
    Pattern of macroevolution. One or more body parts of genetically diverging lineages undergo structural and functional changes from the parts in the common ancestor.
  518. natural selection
    Microevolutionary process; the outcome of differences in survival and reproduction among individuals of a population that differ in the details of their heritable traits.
  519. nucleic acid hybridization
    Any basepairing between DNA or RNA strands from different sources.
  520. Paleozoic
    Era from 544 million to 248 million years ago; Cambrian through Permian.
  521. Pangea
    Paleozoic supercontinent; the first land plants and animals evolved on it.
  522. plate tectonics theory
    Theory that great slabs or plates of Earth�s outer layer float on a hot, semi-molten mantle. All plates are moving slowly and have rafted continents to new positions over time.
  523. Proterozoic
    Era between 2.5 million to 544 million years ago. An oxygen-rich early atmosphere formed, sparking the Cambrian explosion of biodiversity.
  524. radiometric dating
    Method of measuring proportions of a radioisotope in a mineral trapped long ago in newly formed rock and a daughter isotope that formed from it by radioactive decay in the same rock. Used to assign absolute dates to fossil-containing rocks and to the geologic time scale.
  525. species
    [L. species, a kind] Of sexually reproducing species, one or more natural populations of individuals that successfully interbreed and are isolated reproductively from other such groups. By a cladistic definition, one or more natural populations of individuals with at least one unique trait derived a common ancestor that occurs in no other groups.
  526. stratification
    Stacks of sedimentary rock layers, built up by deposition of silt and other materials over time.
  527. theory of uniformity
    Theory that Earth�s surface has changed in slow, uniformly repetitive ways except for expected annual catastrophes, such as big floods. Changed Darwin�s view of evolution; has since been discredited by plate tectonics theory.
  528. Chapter 18
  529. adaptation
    [L. adaptare, to fit] Any long-term, heritable aspect of form, function, or behavior that improves an individual�s chances of surviving and reproducing; outcome of natural selection and other microevolutionary processes.
  530. allele frequencies
    Abundance of one allele relative to others at a gene locus among individuals of a population.
  531. alleles
    One of two or more molecular forms of a gene at a given locus; alleles arise by mutation and encode slightly different versions of the same trait.
  532. balanced polymorphism
    An outcome of natural selection against homozygotes, so that two or more alleles for a trait are being maintained in the population.
  533. bottleneck
    Severe reduction in the size of a population, brought about by intense selection pressure or a natural calamity.
  534. dimorphism
    Persistence of two forms of the same trait in a population.
  535. directional selection
    Mode of natural selection by which forms at one end of a range of phenotypic variation are favored.
  536. disruptive selection
    Mode of natural selection that favors different forms of a trait at both ends of a range of variation; intermediate forms are selected against.
  537. emigration
    Permanent move of one or more individuals out of a population.
  538. evolution
    [L. evolutio, an unrolling] Genetic change in a line of descent by microevolutionary events (gene mutation, natural selection, genetic drift, and gene flow); basis of large-scale patterns, rates, and trends in the history of life.
  539. fixation
    Of a population, the loss of all alleles but one at a gene locus; all individuals have become homozygous for the allele.
  540. founder effect
    A form of bottlenecking. By chance, a few individuals that establish a new population differ in allele frequencies relative to the original population.
  541. gene flow
    Microevolutionary process; alleles enter and leave a population by immigration and emigration. Counters mutation, natural selection, and genetic drift, hence reproductive isolation.
  542. gene mutation
    Small-scale change in the nucleotide sequence of a gene; can result in an altered protein product.
  543. genetic drift
    Change in allele frequencies over generations due to chance alone. Most pronounced effects in small populations.
  544. genetic equilibrium
    In theory, a state in which a population is not evolving with respect to a specified gene locus. Compare Hardy�Weinberg rule.
  545. Hardy�Weinberg equilibrium equation
    Theoretical baseline for tracking changes in allele frequencies over the generations. Frequencies do not change as long as there is no mutation, the population is infinitely large and isolated from other populations, and all individuals are reproducing equally and randomly.
  546. immigration
    One or more individuals move and take up residence in another population of its species.
  547. inbreeding
    Nonrandom mating among very close relatives that share many identical alleles; may fix harmful alleles.
  548. lethal mutation
    Mutation having drastic effects on phenotype; usually causes death.
  549. microevolution
    Of a population, a smallscale change in allele frequencies resulting from mutation, genetic drift, gene flow, natural selection, or a combination of them.
  550. neutral mutation
    A mutation with no effect on phenotype; natural selection thus cannot change its frequency in a population.
  551. polymorphism
    [Gk. polus, many,+morphe, form] Persistence of two or more qualitatively different forms of a trait, or morphs, in a population.
  552. population
    All individuals of the same species living in a specified area.
  553. probability
    The odds that each outcome of an event will occur is proportional to the total number of ways in which that outcome can be reached.
  554. sexual dimorphism
    A notable difference between female and male phenotypes of a population.
  555. sexual selection
    A category of natural selection; an outcome of differences in success at attracting mates and reproducing among individuals of a population.
  556. stabilizing selection
    Mode of natural selection; intermediate phenotypes are favored over extremes at both ends of the range of variation.
  557. Chapter 19
  558. adaptive radiation
    A macroevolutionary pattern. A burst of genetic divergences from a lineage that gives rise to many species, each able to use a novel resource or to move into a new, or newly vacated, habitat.
  559. adaptive zones
    A set of different niches that become be filled by a group of species.
  560. allopatric speciation
    [Gk. allos, different, + L. patria, native land] Speciation model. A physical barrier arises and separates populations or subpopulations of a species, ends gene flow, and so favors divergences that result in new species.
  561. anagenesis
    A major pattern of speciation. Directional changes in allele frequencies and morphology are confined within a single lineage, and in time a new type differs so much from the ancestral type that it is classified as a separate species.
  562. biological species concept
    Definition of a sexually reproducing species as one or more populations of individuals that interbreed under natural conditions, produce fertile offspring, and are reproductively isolated from other such populations.
  563. clade
    [Gk. klados-, branch] All species that share a unique trait, being descended from an ancestral species in which the trait first evolved.
  564. cladogenesis
    One speciation pattern. A lineage branches when one or more of its populations or subpopulations become reproductively isolated, and then genetic divergences result in new species.
  565. cladograms
    Evolutionary tree diagram that depicts relative relatedness among groups. Each branch is monopyletic; it includes only an ancestral species in which a unique trait first evolved and all of its descendants.
  566. derived trait
    A novel feature shared only by descendants of an ancestral species in which it originated.
  567. evolutionary trees
    A treelike diagram in which each branch point represents a divergence from a shared ancestor; each branch is a separate line of descent.
  568. extinction
    Irrevocable loss of a species.
  569. gene flow
    Microevolutionary process; alleles enter and leave a population by immigration and emigration. Counters mutation, natural selection, and genetic drift, hence reproductive isolation.
  570. genetic divergence
    An accumulation of differences in the gene pools of two or more populations or subpopulations of a species after gene flow stops entirely; mutation, natural selection, and genetic drift operate independently in each one.
  571. genus, genera
    [L. genus, race or origin] A grouping of species more closely related to one another in morphology, ecology, and history than to others at the same taxonomic level.
  572. gradual model of speciation
    Addresses the rate of speciation and cites fossil evidence that morphological changes accumulate slowly over great time spans.
  573. higher taxa
    One of ever more inclusive groupings of species; e.g., family, order, class, phylum, kingdom.
  574. key innovation
    A chance modification in some body structure or function that gives a species the opportunity to exploit the environment more efficiently or in a novel way; e.g., modifications of the forelimbs of amniotes into diverse legs and wings during radiations into adaptive zones.
  575. macroevolution
    Large-scale patterns, rates of change, and trends among lineages.
  576. mass extinctions
    Catastrophic event or phase in geologic time when families or other major groups are lost.
  577. monophyletic group
    A set of species that share a derived trait, a novel feature that evolved in one species and is present only in its descendants; all of the evolutionary branchings from a single stem.
  578. niche
    [L. nidas, nest] Sum total of all activities and relationships in which individuals of a species engage as they secure and use the resources required to survive and reproduce.
  579. parapatric speciation
    A speciation model. Populations in contact along a common border evolve into new species; hybrids that form in the contact zone are less fit than individuals on either side of it and thereby act as a reproductive isolating mechanism.
  580. polyploidy
    A case of somatic cells having three or more of each type of chromosome characteristic of the species.
  581. punctuation model of speciation
    Addresses the rate of speciation; cites fossil evidence that morphological changes required for reproductive isolation evolve in a relatively brief time span, within the tens to hundreds of thousands of years when two or more populations are diverging from each other.
  582. reproductive isolating mechanisms
    Any heritable feature of body form, function, or behavior that prevents interbreeding between two or more populations; sets the stage for genetic divergences.
  583. speciation
    One of the macroevolutionary processes; formation of daughter species from a population or subpopulation of a parent species; the routes vary in their details and duration.
  584. species
    [L. species, a kind] Of sexually reproducing species, one or more natural populations of individuals that successfully interbreed and are isolated reproductively from other such groups. By a cladistic definition, one or more natural populations of individuals with at least one unique trait derived a common ancestor that occurs in no other groups.
  585. sympatric speciation
    [Gk. sym, together, + patria, native land] A speciation model. Occurs inside the home range of a species in the absence of a physical barrier; e.g., by way of polyploidy in flowering plants.
  586. taxon
    A set of organisms of a given type.
  587. taxonomy
    Field of biology that identifies, names, and classifies species.
  588. three-domain system
    A classification system that groups all organisms into domains Bacteria, Archaea, and Eukarya.
  589. Chapter 20
  590. autotrophs
    [Gk. auto, self, and trophos, feeder] An organism that synthesizes its own food from simple inorganic compounds in its environment with energy captured from the sun or from oxidizing inorganic substances; e.g., a photoautotroph or chemoautotroph.
  591. chemoautotrophs
    Any prokaryotic cell that makes its own food by oxidizing inorganic substances.
  592. cyanobacteria
    A type of single-celled photoautotroph; the first to use a noncyclic pathway of photosynthesis, which slowly enriched the early atmosphere with oxygen.
  593. endosymbiosis
    [Endo�, within + symbiosis, living together] An intimate, permanent ecological interaction in which one species lives and reproduces in the other�s body to the benefit of one or both.
  594. eukaryotic cells
    Type of cell that starts life with a nucleus and other membranebound organelles.
  595. heterotrophs
    [Gk. heteros, other, + trophos, feeder] Organism that cannot make its own food; feeds on other organisms, their wastes, or their remains.
  596. prokaryotic cells
    [L. pro, before, + Gk. karyon, kernel] A single-celled organism, often walled, that does not have the organelles characteristic of eukaryotic cells. Only bacteria and archaeans are prokaryotic.
  597. Proterozoic
    Era between 2.5 million to 544 million years ago. An oxygen-rich early atmosphere formed, sparking the Cambrian explosion of biodiversity.
  598. proto-cells
    Presumed stage of chemical evolution that preceded living cells.
  599. RNA world
    Model for a time prior to the evolution of DNA; a self-replicating system chemically evolved in which RNA strands were templates for protein synthesis.
  600. stromatolites
    Fossilized remains of domeshaped mats of shallow-water communities, cyanobacterial species especially, that were infiltrated with dissolved minerals and fine sediments. Some are 3 billion years old.
  601. Chapter 21
  602. AIDS
    Acquired immune deficiency syndrome. A set of chronic disorders that develops after prolonged infection by HIV has weakened the immune system.
  603. Archaeans
    Domain of prokaryotic species; one of two lineages that evolved shortly after life originated. Archaeans have many unique molecular and biochemical traits but also share some traits with bacteria and other traits with eukaryotic species.
  604. bacillus
    Rod-shaped prokaryotic cell.
  605. bacterial chromosome
    A circular, doublestranded molecule of prokaryotic DNA.
  606. bacteriophages
    One of a class of viruses that infects bacteria.
  607. cell wall
    Of many cells (not animal cells), a semirigid but permeable structure that surrounds the plasma membrane; helps a cell retain its shape and resist rupturing.
  608. chemoautotrophs
    Any prokaryotic cell that makes its own food by oxidizing inorganic substances.
  609. coccus
    A spherical prokaryotic cell.
  610. conjugation
    Among prokaryotic species, a mode of gene transfer that is possible when one of the cells has an F plasmid. Also a sexual reproductive mode among some single-celled eukaryotes.
  611. cyanobacteria
    A type of single-celled photoautotroph; the first to use a noncyclic pathway of photosynthesis, which slowly enriched the early atmosphere with oxygen.
  612. endospores
    Of certain bacteria, a resting structure enclosing a bit of cytoplasm and the DNA; resists heat, irradiation, drying, acids, disinfectants, and boiling water. It germinates when conditions favor growth and a bacterium emerges from it.
  613. epidemic
    Rapid spread, then subsidence, of a disease within a population.
  614. extreme halophiles
    Bacterium or archaean adapted to an extremely salty habitat.
  615. extreme thermophile
    Bacterium or archaean adapted to a hot aquatic habitat; e.g., a hot spring or hydrothermal vent.6
  616. infection
    Invasion and multiplication of a pathogen or parasite in a host. Disease follows if defenses are not mobilized fast enough against the tissue disruptions.
  617. lysis
    Gross damage to a cell wall, plasma membrane, or both that lets cytoplasm leak out; causes cell death.
  618. lysogenic pathway
    A latent period that extends many viral replication cycles. Viral genes are integrated into host chromosome and may remain inactivated through many host cell divisions before being replicated.
  619. lytic pathway
    A rapid viral replication pathway that ends with lysis of host cell.
  620. methanogens
    Any bacterium or archaean that produces methane gas as by-product of anaerobic reactions.
  621. pathogen
    [Gk. pathos, suffering, + gene�s, origin] A virus, bacterium, fungus, protist, or parasitic worm that infects an organism and multiplies in it, thus causing disease.
  622. photoautotrophs
    Any photosynthetic autotroph; e.g., nearly all plants, most algae, and a few bacteria.
  623. plasmid
    A small, circular bacterial DNA molecule having a few genes; replicated independently of the bacterial chromosome.
  624. prions
    A type of protein particle normally in vertebrate nervous systems that turns infectious when its shape changes.
  625. prokaryotic fission
    Cell reproduction mechanism of prokaryotic cells only.
  626. reverse transcription
    A viral enzyme that catalyzes the assembly of free nucleotides into a strand of DNA on an RNA template.
  627. spirillum
    A spiral-shaped prokaryotic cell.
  628. virus
    A noncellular infectious agent of DNA or RNA, a protein coat and, in some types, an outer lipid envelope; it can be replicated only after its genetic material enters a host cell and subverts the host�s metabolic machinery.
  629. Chapter 22
  630. algal blooms
    Rapid, huge increases in algal population sizes after nutrient enrichment of an aquatic habitat.
  631. alveolates
    A type of single-celled eukaryote that has many tiny, membrane-bound sacs just beneath the plasma membrane; e.g., a ciliate, apicomplexan, or dinoflagellate.
  632. amoebas
    A single-celled amoebozoan that moves on pseudopods. All are predatory or parasitic; none forms colonies.
  633. binary fission
    Asexual reproductive mode of certain invertebrates; the body splits spontaneously, then both parts grow what is missing. See also Prokaryotic fission.
  634. brown algae
    A stramenopile; a multicelled marine photoautotroph with an abundance of the pigment fucoxanthin; e.g., kelps.
  635. ciliates
    An alveolate having arrays of cilia at its surface; traditionally called a ciliated protozoan. Most are free-living predators.
  636. coccolithophores
    A single-celled marine autotroph having calcium carbonate plates; one of the chrysophytes that are abundant in phytoplankton and the leading source of calcium deposits on the seafloor.
  637. conjugation
    Among prokaryotic species, a mode of gene transfer that is possible when one of the cells has an F plasmid. Also a sexual reproductive mode among some single-celled eukaryotes.
  638. contractile vacuole
    [L. contractus, to draw together] Organelle in some single-celled eukaryotes that collects excess water in the cell body and then expels it.
  639. cyst
    Of many microbes, a resting stage with a secreted cover. Also an abnormal, fluid-filled sac in skin with no opening.
  640. diatoms
    A single-celled photoautotroph with a perforated silica shell, which has two overlapping parts that fit together like a pillbox; one of the chrysophytes.
  641. euglenoids
    A single eukaryotic cell with a thick flagellum. Different kinds are colorless heterotrophs or photoautotrophs of aquatic habitats.
  642. foraminiferans
    Single-celled, predatory eukaryote with a richly perforated shell through which thin pseudopods project.
  643. kinetoplastids
    A colorless flagellate; the only eukaryote with mitochondrial DNA massed inside a mitochondrion almost as long as the cell; e.g., Trypanosoma.
  644. microtubule
    Largest cytoskeletal element; a filament of tubulin subunits. Contributes to cell shape, growth, and motion.
  645. parasite
    [Gk. para, alongside, + sitos, food] Organism that withdraws nutrients from a living host, which it usually does not kill outright.
  646. pellicle
    A thin, flexible, protein-rich body covering of some single-celled eukaryotes.
  647. plankton
    Aquatic community of mostly microscopic autotrophs and heterotrophs.
  648. plasmodium
    Of plasmodial slime molds, a multinucleated mass that forms whena single diploid cell undergoes rounds of mitosis without cytoplasmic division.
  649. protists
    Informal name for all structurally simple eukaryotes, which are now being classified as monophyletic groups.
  650. pseudopod
    A dynamic lobe of membraneenclosed cytoplasm; functions in motility and phagocytosis by amoebas, amoeboid cells, and many white blood cells.
  651. radiolarian
    A single-celled predatory eukaryote that has pseudopods projecting from a perforated silica shell
  652. red algae
    An aquatic, mostly multicelled photoautotroph having an abundance of phycobilins that masks its chlorophyll a.
  653. red tide
    An algal blood that turns the water near coasts rust-red or brown.
  654. slime molds
    An amoebozoan; one of the free-living, amoebalike cells that also cluster into a migrating mass, differentiate, and form reproductive structures.
  655. stramenopiles
    A single-celled or multicelled eukaryote with four outer membranes and 2 flagella, one with thin filaments projecting from it.
  656. Chapter 23
  657. angiosperms
    [Gk. angeion, vessel + spermia, seed] A flowering plant; its egg-containing ovules mature into seeds within closed, protected chambers called ovaries.
  658. bryophytes
    Nonvascular land plant. The haploid stage dominates its life cycle, and its sperm require standing water to reach eggs. A moss, liverwort, or hornwort.
  659. charophytes
    A type of photoautotroph once grouped with other green algae but now known to be more closely related to land plants; e.g., desmids, stoneworts.
  660. coal
    A nonrenewable energy source that formed more than 280 million years ago from submerged, undecayed, and slowly compacted plant remains.
  661. coevolution
    The joint evolution of two species interacting so closely that a change in the structure, function, or behavior of one exerts selection pressure on the other over the generations.
  662. cones
    Reproductive structure of certain seed-bearing plants; has clusters of scales with exposed ovules on their surface.
  663. conifers
    A type of gymnosperm adapted to conserve water through droughts and cold winters. Cone-producing woody trees or shrubs with thickly cuticled needlelike or scalelike leaves.
  664. cotyledons
    Seed leaf; part of a flowering plant embryo. In eudicots, two cotyledons absorb nutrients from endosperm, emerge aboveground as the seed germinates, and transfer nutrients that sustain early growth; photosynthetic before true leaves form. In most monocots, one small cotyledon helps transfer nutrients from endosperm to the embryo, but it remains underground when a seed germinates and is never photosynthetic.
  665. cuticle
    Of plants, a cover of transparent waxes and cutin on the outer wall of epidermal cells. Of annelids, a thin, flexible coat. Of arthropods, a lightweight exoskeleton hardened with chitin.
  666. cycad
    A gymnosperm of subtropical or tropical habitats; pollen-bearing and seedbearing strobili on separate plants.
  667. endosperm
    Nutritive tissue in the seeds of flowering plants only.
  668. eudicots
    A single eukaryotic cell with a crystalline rod reinforcing a thick flagellum. Different kinds are colorless heterotrophs or photoautotrophs of aquatic habitats.
  669. ferns
    A seedless vascular plant having fronds that often are divided in leaflets.
  670. flower
    A reproductive structure of fertile parts (stamens, carpels) nonfertile parts (sepals, petals), and a receptacle (modified base of floral shoot).
  671. fruits
    [L. after frui, to enjoy] Mature ovary, often with accessory parts, from a flower.
  672. gametophyte
    [Gk. phyton, plant] A haploid multicelled body in which haploid gametes form during the life cycle of plants and some algae.
  673. ginkgos
    A deciduous gymnosperm; its ancestors were diverse in dinosaur times.
  674. gnetophytes
    A type of woody, vinelike or shrubby gymnosperm.
  675. gymnosperms
    [Gk. gymnos, naked, + sperma, seed] A vascular plant that forms seeds on exposed surfaces of sporeproducing structures; e.g., conifers, cycads.
  676. hornworts
    Bryophyte having a horn-shaped sporophyte attached to a flat gametophyte.
  677. horsetails
    Seedless vascular plant having rhizomes, scale-like leaves, and hollow stems with silica-reinforced ribs.
  678. lignin
    Gluelike polymer deposited in secondary cell walls; makes some plant parts stronger, more waterproof, and less vulnerable to attacks.
  679. liverworts
    One of the bryophytes.
  680. lycophytes
    A type of seedless vascular plant, typically with true leaves, roots, and stems; e.g., a club moss.
  681. magnoliids
    One of three major flowering plant groups; e.g., magnolias, avocados.
  682. megaspores
    Haploid meiotic spore in ovary of seed-bearing plants; gives rise to a female gametophyte with egg cell.
  683. microspores
    Type of walled haploid spore of gymnosperms and angiosperms that gives rise to pollen grains.
  684. monocots
    Monocotyledon; flowering plant characterized by embryo sporophytes having one cotyledon; floral parts usually in threes (or multiples of three); and often parallel-veined leaves.
  685. mosses
    Most common kind of bryophyte.
  686. ovules
    [L. ovum, egg] Of seedbearing plants, an egg-containing female gametophyte surrounded by tissue layers; a mature ovule is a seed.
  687. peat bogs
    Compressed, soggy, acidic mat of accumulated remains of peat mosses.
  688. phloem
    Plant vascular tissue that distributes photosynthetic products through the plant body. Its conducting tubes are interconnecting, living cells assisted by companion cells that help load solutes into the tubes.
  689. pollen grains
    [L. pollen, fine dust] A tiny structure that forms from microspores; consists of a sturdy wall around a few cells that will develop into a mature, sperm-bearing, male gametophyte.
  690. pollination
    Arrival of pollen on a carpel�s stigma in a flower of the same species.
  691. pollinators
    Any agent that delivers pollen grains to the egg-containing structures in flowers of the same species; e.g., wind, water, or birds, bats, and other animals.
  692. rhizoids
    A rootlike absorptive structure.
  693. rhizomes
    A short absorptive stem that grows underground in a horizontally branching pattern, most often.
  694. rhyniophytes
    The first seedless vascular plants; originated in Gondwana lowlands.
  695. root systems
    Underground vascular plant structures that absorb water, mineral ions.
  696. seed
    A mature ovule.
  697. shifting cultivation
    A practice of cutting and burning trees, then tilling ashes into the soil of a small plot of land. Once called slash-and-burn agriculture.
  698. shoot systems
    Aboveground plant parts; e.g., stems, leaves, flowers.
  699. spores
    A structure of one or a few cells, often walled or coated, that protects and/or disperses a new sexual or asexual generation. Many bacteria as well as apicomplexans, fungi, and plants form spores.
  700. sporophyte
    [Gk. phyton, plant] A sporeproducing vegetative body of a plant or multicelled alga that grows by mitotic cell divisions from a zygote.
  701. stomata
    A gap between two plumped guard cells that lets water vapor and gases diffuse across the epidermis of a leaf or primary stem; diffusion stops when the cells lose water and collapse.
  702. strobilus
    Of certain nonflowering plants, a cluster of spore-producing structures.
  703. whisk ferns
    Seedless vascular plant having a branching form and no true roots; e.g., Psilotum.
  704. xylem
    [Gk. xylon, wood] Of vascular plants, a complex tissue that conducts water and solutes through tubes of interconnected walls of cells that are dead at maturity.
  705. Chapter 24
  706. ascospores
    Sexual spore of sac fungi.
  707. basidiospores
    Sexual spore of club fungi.
  708. chytrids
    A flagellated intracellular parasite of a fungal group, the microsporidians.
  709. club fungi
    Fungus that produces sexual spores in a club-shaped cell, a basidium.
  710. decomposers
    [L. dis�, to pieces] One of the prokaryotic or fungal heterotrophs that obtains carbon and energy by breaking down wastes or remains of organisms. The collective action of decomposers helps cycle nutrients to producers in ecosystems.
  711. extracellular digestion and absorption
    Mode of nutrition; the organism grows in or on organic matter, digests it with secreted enzymes, and absorbs digested bits.
  712. fungi
    Kingdom of fungi.
  713. gills
    Respiratory organ with a thin, moist, vascularized layer for gas exchange.
  714. hypha, hyphae
    Fungal filament having chitin-reinforced walls; component of a mycelium.
  715. lichen
    Mutualism between a fungus and one or more photoautotrophs.
  716. microsporidians
    Intracellular fungal parasite of aquatic habitats that forms flagellated spores; belongs to one of the most ancient eukaryotic lineages.
  717. mushrooms
    Aboveground reproductive structure produced by many club fungi.
  718. mutualism
    [L. mutuus, reciprocal] A type of symbiotic interaction that benefits both participants.
  719. mycelium, mycelia
    [Gk. mykes, fungus] Underground mesh of tiny, branching filaments (hyphae); the food-absorbing portion of most fungi.
  720. mycorrhiza -izae
    �Fungusroot.� A form of mutualism between a fungus and young plant roots. Hyphae withdraw some carbohydrates from the plant, which withdraws some absorbed mineral ions from hyphae.
  721. parasites
    [Gk. para, alongside, + sitos, food] Organism that withdraws nutrients from a living host, which it usually does not kill outright.
  722. sac fungi
    Fungus that produces sexual spores in sac-shaped cells; e.g., truffles.
  723. saprobes
    Heterotroph that extracts energy and carbon from nonliving organic matter and so causes its decay.
  724. spores
    A structure of one or a few cells, often walled or coated, that protects and/or disperses a new sexual or asexual generation. Many bacteria as well as apicomplexans, fungi, and plants form spores.
  725. symbiosis
    [Gk. sym, together, + bios, life, mode of life] An ecological interaction in which one or more individuals interact closely with individuals of a different species for some or all of the life cycle; e.g., mutualism, predation, parasitism.
  726. zygomycetes
    Type of parasitic or saprobic fungus in which diploid zygotes develop into zygospores, a type of thick-walled sexual spore in a thin, clear covering.
  727. zygospore
    Sexual spore of zygomycetes.
  728. Chapter 25
  729. animals
    Any multicelled heterotroph that ingests other organisms or their tissues, develops through a series of embryonic stages, and is motile during part or all of the life cyle. Most species have epithelial tissues and extracellular matrixes.
  730. annelids
    A bilateral invertebrate having a highly segmented body; major groups are polychaetes, oligochaetes, and leeches. Except in leeches, segments have clusters of chitin-reinforced bristles.
  731. arthropods
    Type of invertebrate having a hardened exoskeleton and specialized segments with jointed appendages; e.g., millipedes, spiders, lobsters, insects.
  732. bilateral symmetry
    Body plan in which the main axis divides the body into two halvesthat are mirror images of one another.
  733. brain
    Of most nervous systems, a major integrating center that receives, processes, and often stores sensory input, and issues coordinated commands for responses.
  734. budding
    A dormant shoot, mostly meristematic tissue and often sheathed in small, young leaves. A lateral (axillary) bud forms in a leaf axil; a terminal bud forms at a shoot tip and is the main zone of primary growth.
  735. centipedes
    Venomous predatory arthropod with many segments having paired legs.
  736. cephalization
    [Gk. kephalikos, head] During the evolution of most kinds of animals, the increasing concentration of sensory structures and nerve cells at the anterior end of the body.
  737. cephalopods
    Soft-bodied mollusk with a closed circulatory system. Moves by jet propulsion of water from a siphon; e.g., squids, octopuses, chambered nautilus.
  738. choanoflagellates
    Single-celled eukaryote having a microvilli collar around a single flagellum at their anterior end. A sister taxon of animals and fungi.
  739. cnidarians
    A type of radial invertebrate having epithelial tissues and a saclike gut. The only animal that makes nematocysts.
  740. coelom, coelomate
    Between the gut and body wall, a cavity lined with peritoneum.
  741. collar cells
    A sponge cell having a ring of food-trapping villi around a flagellum.
  742. complete digestive system
    A tubular digestive system having a mouth at one end and an anus at the other.
  743. connective tissue
    Most abundant type of animal tissue. Soft connective tissues differ in the amounts and arrangements of fibroblasts, fibers, ground substance. Adipose tissue, cartilage, bone tissue, and blood are specialized types.
  744. crustaceans
    One of the abundant �insects of the seas,� mostly marine arthropods having a hardened, flexible exoskeleton and pairs of jointed appendages.
  745. cuticle
    Of plants, a cover of transparent waxes and cutin on the outer wall of epidermal cells. Of annelids, a thin, flexible coat. Of arthropods, a lightweight exoskeleton hardened with chitin.
  746. deuterostomes
    [Gk. deuteros, second, + stoma, mouth] A bilateral animal of a lineage characterized in part by events of embryonic development, as when the second indentation to appear on the early embryo�s surface becomes the anus; e.g., an echinoderm or a chordate.
  747. echinoderm
    One of the deuterostomes; a radial invertebrate with some bilateral features and calcified spines or plates on the body wall; e.g., sea stars.
  748. ectoderm
    [Gk. ecto, outside, + derma, skin] First-formed, outer primary tissue layer of animal embryos; gives rise to nervous tissues and outer layer of the integument.
  749. endoderm
    Inner primary tissue layer of animal embryos; source of the inner gut lining and organs derived from it.
  750. epidermis
    Outermost tissue layer of plants and nearly all animals.
  751. epithelium
    Animal tissue covering external and internal body surfaces. A key innovation that favored larger, more complex bodies; cells started interacting as functional units.
  752. exoskeleton
    [Gk. skle�ros, hard, stiff] An external skeleton; e.g., a hardened cuticle.
  753. extracellular matrix
    Secretions and other deposits on or between cells of a tissue.
  754. flatworms
    One of the simplest existing animals with organ systems that form from three primary tissue layers.
  755. ganglia
    Distinct cluster of cell bodies of neurons.
  756. gastrodermis
    Glandular epithelium that lines the gut of many invertebrates.
  757. gonads
    Primary reproductive organ in animals; produces gametes.
  758. gut
    A sac or tube in which food is digested. Also the gastrointestinal tract from the stomach onward.
  759. hermaphrodites
    An individual with male and female gonads.
  760. heterotrophs
    [Gk. heteros, other, + trophos, feeder] Organism that cannot make its own food; feeds on other organisms, their wastes, or their remains.
  761. hydrostatic skeleton
    A fluid-filled cavity against which a contractile force can act.
  762. incomplete digestive system
    Saclike gut in which both food intake and waste output occur through a single openin.
  763. larvae, nymphs, pupae
    An immature stage between the embryo and adult in the life cycle of many animals.
  764. malpighian tubules
    One of many small tubes that help insects on land dispose of toxic wastes without losing body water.
  765. mantle
    Body part of Molluscs. Covers fleshy visceral mass secretes the shell
  766. medusa
    [Gk. Medousa, one of three sisters in Greek mythology with snake-entwined hair] Of cnidarian life cycles, a free-swimming, bell-shaped stage, often with oral lobes and tentacles.
  767. mesoderm
    Primary tissue layers gives rise to many internal organs and part of the integument; pivotal in the evolution of large, complex animals.
  768. mesoglea
    Of cnidarians, a gelatinous matrix with scattered cells between the epidermis and gastrodermis; functions as a buoyant, deformable skeleton.
  769. metamorphosis
    [Gk. meta, change, + morphe, form] Major changes in body form of certain animals. Hormonally controlled growth, tissue reorganization, and remodeling of body parts leads to adult form.
  770. millipedes
    An arthropod with a great many unspecialized segments and paired legs; scavenges decaying plant material.
  771. mollusks
    Only invertebrate with a mantle draped over a soft, fleshy visceral mass; most have an external or internal shell; e.g., gastropods, bivalves, cephalopods.
  772. molting
    Periodic shedding of worn-out or too-small body structures. Permits an animal to grow in size or renew parts.
  773. nematocysts
    [Gk. nema, thread + kystis, pouch] A fluid-filled, jackin- the-box capsule housed in one of three types of sensory�effector cells in cnidarians. It has a mechanoreceptor projecting above the cell surface and a dischargeable, tubular thread, often with barbs or toxin-drenched. Only cnidarians make nematocysts.
  774. nephridium, -dia
    Of some invertebrates, one of many water-regulating units that help control the composition and volume of tissue fluid.
  775. nerve cords
    Of bilateral animals, a line of communication, usually paired, that runs parallel with the anterior�posterior axis. In large or long invertebrates, it often has one or more large axons. In chordates, it develops as a hollow, neural tube that gives rise to the spinal cord and brain.
  776. nerve net
    Nervous system of cnidarians and some other invertebrate groups; an asymmetrical mesh of sensory and motor neurons that controls simple movements. It activates epitheliomuscular cells arrayed as sheets or rings in the body wall.
  777. organ systems
    A set of organs that are interacting chemically, physically, or both in a common task.
  778. organs
    Body structure with definite form and function made of more than one tissue.
  779. parasites
    [Gk. para, alongside, + sitos, food] Organism that withdraws nutrients from a living host, which it usually does not kill outright.
  780. peritoneum
    The membrane that lines the coelom.
  781. pharynx
    A muscular tube. Invertebrate chordates use theirs in filter-feeding and respiration. In land vertebrates, it is the entrance to the esophagus and trachea.
  782. placozoans
    An asymmetric, soft-bodied animal with two simple tissues around a thin, inner matrix.
  783. planula
    Of cnidarians, a type of swimming or creeping larva, usually with a ciliated epidermis.
  784. polyp
    Vase-shaped, sedentary stage of cnidarian life cycles.
  785. proglottids
    One of many tapeworm body units that bud behind the scolex.
  786. protostomes
    [Gk. proto, first, + stoma, mouth] A bilateral animal of a branching lineage characterized partly by events in embryonic development, as when the first indentation to form on the early embryo�s surface becomes a mouth; e.g., mollusks, annelids, arthropods.
  787. pseudocoel
    False coelom; a main body cavity incompletely lined with tissue derived from mesoderm.
  788. radial symmetry
    Animal body plan with four or more roughly equivalent parts around an anterior�posterior axis.
  789. rotifers
    Bilateral, cephalized animal with a false coelom and a crown of cilia.
  790. roundworms
    Bilateral invertebrate with a false coelom and complete digestive system in a cylindrical body. Most are decomposers; many are parasites.
  791. signaling molecules
    Any secretion from one cell type that can alter the behavior of a different cell that bears a receptor or it; a means of cell communication.
  792. sponges
    Structurally, the simplest existing animal. Its asymmetrical body has a spiculereinforced matrix in two cell layers (not epithelium). Its phagocytic collar cells trap food from water flowing through pores in its wall.
  793. torsion
    A drastic twisting of the body, including the visceral mass, as certain molluscan embryos develop.
  794. tracheas
    An air-conducting tube used in respiration. Of land vertebrates, the windpipe.
  795. water-vascular system
    Of echinoderms, a system of tube feet connected to canals, through which controlled water flow can extend the feet in coordinated ways.

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