C3 photosynthesis • The form of photosynthesis in which 3-phosphoglycerate is the first stable product, and ribulose bisphosphate is the CO2 receptor.
C4 photosynthesis • The form of photosynthesis in which oxaloacetate is the first stable product, and phosphoenolpyruvate is the CO2 acceptor. C4 plants also perform the reactions of C3 photosynthesis.
Calcitonin • A hormone produced by the thyroid gland; it lowers blood calcium and promotes bone formation. (Contrast with parathormone.)
Calmodulin (cal mod´ joo lin) • A calcium-binding protein found in all animal and plant cells; mediates many calcium-regulated processes.
calorie [L. calor: heat] • The amount of heat required to raise the temperature of one gram of water by one degree Celsius (1°C) from 14.5°C to 15.5°C. In nutrition studies, "Calorie" (spelled with a capital C) refers to the kilocalorie (1 kcal = 1,000 cal).
Calvin-Benson cycle • The stage of photosynthesis in which CO2 reacts with RuBP to form 3PG, 3PG is reduced to a sugar, and RuBP is regenerated, while other products are released to the rest of the plant.
Calyx (kay´ licks) [Gr. kalyx: cup] • All of the sepals of a flower, collectively.
CAM • See crassulacean acid metabolism.
Cambium (kam´ bee um) [L. cambiare: to exchange] • A meristem that gives rise to radial rows of cells in stem and root, increasing them in girth; commonly applied to the vascular cambium which produces wood and phloem, and the cork cambium, which produces bark.
cAMP (cyclic AMP) • A compound, formed from ATP, that mediates the effects of numerous animal hormones. Also needed for the transcription of catabolite-repressible operons in bacteria. Used for communication by cellular slime molds.
Canopy • The leaf-bearing part of a tree. Collectively the aggregate of the leaves and branches of the larger woody plants of an ecological community.
Capillaries [L. capillaris: hair] • Very small tubes, especially the smallest blood-carrying vessels of animals between the termination of the arteries and the beginnings of the veins.
Capsid • The protein coat of a virus.
Carbohydrates • Organic compounds with the general formula CnH2mOm . Common examples are sugars, starch, and cellulose.
Carboxylic acid (kar box sill´ ik) • An organic acid containing the carboxyl group, -COOH, which dissociates to the carboxylate ion, -COO-.
Carcinogen (car sin´ oh jen) • A substance that causes cancer.
Cardiac (kar´ dee ak) [Gr. kardia: heart] • Pertaining to the heart and its functions.
Carnivore [L. carn: flesh + vovare: to devour] • An organism that feeds on animal tissue. (Contrast with detritivore, herbivore, omnivore.)
Carotenoid (ka rah´ tuh noid) [L. carota: carrot] • A yellow, orange, or red lipid pigment commonly found as an accessory pigment in photosynthesis; also found in fungi.
Carpel (kar´ pel) [Gr. karpos: fruit] • The organ of the flower that contains one or more ovules.
Carrier • (1) In facilitated diffusion, a membrane protein that binds a specific molecule and transports it through the membrane. (2) In respiratory and photosynthetic electron transport, a participating substance such as NAD that exists in both oxidized and reduced forms. (3) In genetics, a person heterozygous for a recessive trait.
Carrying capacity • In ecology, the largest number of organisms of a particular species that can be maintained indefinitely in a given part of the environment.
Cartilage • In vertebrates, a tough connective tissue found in joints, the outer ear, and elsewhere. Forms the entire skeleton in some animal groups.
Casparian strip • A band of cell wall containing suberin and lignin, found in the endodermis. Restricts the movement of water across the endodermis.
Catabolism [Ge. kata: down + ballein: to throw] • Degradational reactions of metabolism, in which complex molecules are broken down. (Contrast with anabolism.)
Catalyst (cat´ a list) [Gr. kata-, implying the breaking down of a compound] • A chemical substance that accelerates a reaction without itself being consumed in the overall course of the reaction. Catalysts lower the activation energy of a reaction. Enzymes are biological catalysts.
Cation (cat´ eye on) • An ion with one or more positive charges. (Contrast with anion.)
Caudal [L. cauda: tail] • Pertaining to the tail, or to the posterior part of the body.
cDNA • See complementary DNA.
Cecum (see´ cum) [L. caecus: blind] • A blind branch off the large intestine. In many nonruminant mammals, the cecum contains a colony of microorganisms that contribute to the digestion of food.
Cell adhesion molecules • Molecules on animal cell surfaces that affect the selective association of cells during development of the embryo.
Cell cycle • The stages through which a cell passes between one division and the next. Includes all stages of interphase and mitosis.
Cell division • The reproduction of a cell to produce two new cells. In eukaryotes, this process involves nuclear division (mitosis) and cytoplasmic division (cytokinesis).
Cell theory • The theory, well established, that organisms consist of cells, and that all cells come from preexisting cells.
Cell wall • A relatively rigid structure that encloses cells of plants, fungi, many protists, and most bacteria. The cell wall gives these cells their shape and limits their expansion in hypotonic media.
Cellular immune system • That part of the immune system that is based on the activities of T cells. Directed against parasites, fungi, intracellular viruses, and foreign tissues (grafts). (Contrast with humoral immune system.)
Cellular respiration • See respiration.
Cellulose (sell´ you lowss) • A straight-chain polymer of glucose molecules, used by plants as a structural supporting material.
Central dogma • The statement that information flows from DNA to RNA to polypeptide (in retroviruses, there is also information flow from RNA to cDNA).
Central nervous system • That part of the nervous system which is condensed and centrally located, e.g., the brain and spinal cord of vertebrates; the chain of cerebral, thoracic and abdominal ganglia of arthropods.
Centrifuge [L. fugere: to flee] • A device in which a sample can be spun around a central axis at high speed, creating a centrifugal force that mimics a very strong gravitational force. Used to separate mixtures of suspended materials.
Centriole (sen´ tree ole) • A paired organelle that helps organize the microtubules in animal and protist cells during nuclear division.
Centromere (sen´ tro meer) [Gr. centron: center + meros: part] • The region where sister chromatids join.
Centrosome (sen´ tro soam) • The major microtubule organizing center of an animal cell.
Cephalization (sef´ uh luh zay´ shun) [Gr. kephale: head] • The evolutionary trend toward increasing concentration of brain and sensory organs at the anterior end of the animal.
Cerebellum (sair´ uh bell´ um) [L.: diminutive of cerebrum: brain] • The brain region that controls muscular coordination; located at the anterior end of the hindbrain.
Cerebral cortex • The thin layer of gray matter (neuronal cell bodies) that overlays the cerebrum.
Cerebrum (su ree´ brum) [L.: brain] • The dorsal anterior portion of the forebrain, making up the largest part of the brain of mammals. In mammals, the chief coordination center of the nervous system; consists of two cerebral hemispheres.
Cervix (sir´ vix) [L.: neck] • The opening of the uterus into the vagina.
cGMP (cyclic guanosine monophosphate) • An intracellular messenger that is part of signal transmission pathways involving G proteins. (See G protein.)
Channel • A membrane protein that forms an aqueous passageway though which specific solutes may pass by simple diffusion; some channels are gated: they open and close in response to binding of specific molecules.
Chaperone protein • A protein that assists a newly forming protein in adopting its appropriate tertiary structure.
Chemical bond • An attractive force stably linking two atoms.
Chemiosmotic mechanism • The formation of ATP in mitochondria and chloroplasts, resulting from a pumping of protons across a membrane (against a gradient of electrical charge and of pH), followed by the return of the protons through a protein channel with ATPase activity.
Chemoautotroph • An organism that uses carbon dioxide as a carbon source and obtains energy by oxidizing inorganic substances from its environment. (Contrast with chemoheterotroph, photoautotroph, photoheterotroph.)
Chemoheterotroph • An organism that must obtain both carbon and energy from organic substances. (Contrast with chemoautotroph, photoautotroph, photoheterotroph.)
Chemoreceptor • A cell or tissue that senses specific substances in its environment.
Chemosynthesis • Synthesis of food substances, using the oxidation of reduced materials from the environment as a source of energy.
Chiasma (kie az´ muh) (plural: chiasmata) [Gr.: cross] • An X-shaped connection between paired homologous chromosomes in prophase I of meiosis. A chiasma is the visible manifestation of crossing over between homologous chromosomes.
Chitin (kye´ tin) [Gr. kiton: tunic] • The characteristic tough but flexible organic component of the exoskeleton of arthropods, consisting of a complex, nitrogen-containing polysaccharide. Also found in cell walls of fungi.
Chlorophyll (klor´ o fill) [Gr. kloros: green + phyllon: leaf] • Any of a few green pigments associated with chloroplasts or with certain bacterial membranes; responsible for trapping light energy for photosynthesis.
Chloroplast [Gr. kloros: green + plast: a particle] • An organelle bounded by a double membrane containing the enzymes and pigments that perform photosynthesis. Chloroplasts occur only in eukaryotes.
Choanocyte (cho´ an oh cite) • The collared, flagellated feeding cells of sponges.
Cholecystokinin (ko´ lee sis to kai nin) • A hormone produced and released by the lining of the duodenum when it is stimulated by undigested fats and proteins. It stimulates the gallbladder to release bile and slows stomach activity.
Chorion (kor´ ee on) [Gr. khorion: afterbirth] • The outermost of the membranes protecting mammal, bird, and reptile embryos; in mammals it forms part of the placenta.
Chorionic villus sampling • A medical procedure that extracts a portion of the chorion from a pregnant woman to enable genetic and biochemical analysis of the embryo. (Contrast with amniocentesis.)
Chromatid (kro´ ma tid) • Each of a pair of new sister chromosomes from the time at which the molecular duplication occurs until the time at which the centromeres separate at the anaphase of nuclear division.
Chromatin • The nucleic acid-protein complex found in eukaryotic chromosomes.
Chromatophore (krow mat´ o for) [Gr. kroma: color + phoreus: carrier] • A pigment-bearing cell that expands or contracts to change the color of the organism.
Chromosome (krome´ o sowm) [Gr. kroma: color + soma: body] • In bacteria and viruses, the DNA molecule that contains most or all of the genetic information of the cell or virus. In eukaryotes, a structure composed of DNA and proteins that bears part of the genetic information of the cell.
Chylomicron (ky low my´ cron) • Particles of lipid coated with protein, produced in the gut from dietary fats and secreted into the extracellular fluids.
Chyme (kime) [Gr. kymus, juice] • Created in the stomach; a mixture of ingested food with the digestive juices secreted by the salivary glands and the stomach lining.
Cilium (sil´ ee um) (plural: cilia) [L. cilium: eyelash] • Hairlike organelle used for locomotion by many unicellular organisms and for moving water and mucus by many multicellular organisms. Generally shorter than a flagellum.
Circadian rhythm (sir kade´ ee an) [L. circa: approximately + dies: day] • A rhythm in behavior, growth, or some other activity that recurs about every 24 hours under constant conditions.
Circannual rhythm (sir can´ you al) [L. circa: approximately + annus: year) • A rhythm of behavior, growth, or some other activity that recurs on a yearly basis.
Citric acid cycle • A set of chemical reactions in cellular respiration, in which acetyl CoA reacts with oxaloacetate to form citric acid, and oxaloacetate is regenerated. Acetyl CoA is oxidized to carbon dioxide, and hydrogen atoms are stored as NADH and FADH2. Also called the Krebs cycle.
Class • In taxonomy, the category below the phylum and above the order; a group of related, similar orders.
Class I MHC molecules • These cell surface proteins participate in the cellular immune response directed against virus-infected cells.
Class II MHC molecules • These cell surface proteins participate in the cell-cell interactions (of helper T cells, macrophages, and B cells) of the humoral immune response.
Class switching • The process whereby a plasma cell changes the class of immunoglobulin that it synthesizes. This results from the deletion of part of the constant region of DNA, bringing in a new C segment. The variable region is the same as before, so that the new immunoglbulin has the same antigenic specificity.
Clathrin • A fibrous protein on the inner surfaces of animal cell membranes that strengthens coated vesicles and thus participates in receptor-mediated endocytosis.
Clay • A soil constituent comprising particles smaller than 2 micrometers in diameter.
Cleavages • First divisions of the fertilized egg of an animal.
Cline • A gradual change in the traits of a species over a geographical gradient.
Cloaca (klo ay´ kuh) [L. cloaca: sewer] • In some invertebrates, the posterior part of the gut; in many vertebrates, a cavity receiving material from the digestive, reproductive, and excretory systems.
Clonal anergy • When a naive T cell encounters a self-antigen, the T cell may bind to the antigen but does not receive signals from an antigen-presenting cell. Instead of being activated, the T cell dies (becomes anergic). In this way, we avoid reacting to our own tissue-specific antigens.
Clonal deletion • In immunology, the inactivation or destruction of lymphocyte clones that would produce immune reactions against the animal’s own body.
Clonal selection • The mechanism by which exposure to antigen results in the activation of selected T- or B-cell clones, resulting in an immune response.
Clone [Gr. klon: twig, shoot] • Genetically identical cells or organisms produced from a common ancestor by asexual means.
Cnidocytes • The feeding cells of cnidarians, within which nematocysts are housed.
Coacervate (ko as´ er vate) [L. coacervare: to heap up] • An aggregate of colloidal particles in suspension.
Coacervate drop • Drops formed when a mixture of large proteins and polysaccharides is shaken in water. The interiors of these drops, which are often very stable, contain most of the proteins and polysaccharides.
Coated vesicle • Vesicle, sometimes formed from a coated pit, with characteristic "bristly" surface; its membrane contains distinctive proteins, including clathrin.
Coccus (kock´ us) [Gr. kokkos: berry, pit] • Any of various spherical or spheroidal bacteria.
Cochlea (kock´ lee uh) [Gr. kokhlos: a land snail] • A spiral tube in the inner ear of vertebrates; it contains the sensory cells involved in hearing.
Codominance • A condition in which two alleles at a locus produce different phenotypic effects and both effects appear in heterozygotes.
Codon • A "triplet" of three nucleotides in messenger RNA that directs the placement of a particular amino acid into a polypeptide chain. (Contrast with anticodon.)
Coefficient of relatedness • The probability that an allele in one individual is an identical copy, by descent, of an allele in another individual.
Coelom (see´ lum) [Gr. koiloma: cavity] • The body cavity of certain animals, which is lined with cells of mesodermal origin.
Coelomate • Having a coelom.
Coenocyte (seen´ a sight) [Gr.: common cell] • A "cell" bounded by a single plasma membrane, but containing many nuclei.
Coenzyme • A nonprotein molecule that plays a role in catalysis by an enzyme. The coenzyme may be part of the enzyme molecule or free in solution. Some coenzymes are oxidizing or reducing agents.
Coevolution • Concurrent evolution of two or more species that are mutually affecting each other’s evolution.
Cohort (co´ hort) [L. cohors: company of soldiers] • A group of similar-age organisms, considered as it passes through time.
Collagen [Gr. kolla: glue] • A fibrous protein found extensively in bone and connective tissue.
Collecting duct • In vertebrates, a tubule that receives urine produced in the nephrons of the kidney and delivers that fluid to the ureter for excretion.
Collenchyma (cull eng´ kyma) [Gr. kolla: glue + enchyma: infusion] • A type of plant cell, living at functional maturity, which lends flexible support by virtue of primary cell walls thickened at the corners. (Contrast with parenchyma, sclerenchyma.)
Colon [Gr. kolon: large intestine] • The large intestine.
Commensalism • The form of symbiosis in which one species benefits from the association, while the other is neither harmed nor benefited.
Common bile duct • A single duct that delivers bile from the gallbladder and secretions from the pancreas into the small intestine.
Communication • A signal from one organism (or cell) that alters the pattern of behavior in another organism (or cell) in an adaptive fashion.
Community • Any ecologically integrated group of species of microorganisms, plants, and animals inhabiting a given area.
Companion cell • Specialized cell found adjacent to a sieve tube member in flowering plants.
Comparative analysis • An approach to studying evolution in which hypotheses are tested by measuring the distribution of states among a large number of species.
Comparative genomics • Computer-aided comparison of DNA sequences between different organisms to reveal genes with related functions.
Compensation point • The light intensity at which the rates of photosynthesis and of cellular respiration are equal.
Competitive inhibitor • A substance, similar in structure to an enzyme’s substrate, that binds the active site and thus inhibits a reaction.
Competition • In ecology, use of the same resource by two or more species, when the resource is present in insufficient supply for the combined needs of the species.
Competitive exclusion • A result of competition between species for a limiting resource in which one species completely eliminates the other.
Competitive inhibitor • A substance, similar in structure to an enzyme’s substrate, that binds the active site and inhibits a reaction.
Complement system • A group of eleven proteins that play a role in some reactions of the immune system. The complement proteins are not immunoglobulins.
Complementary base pairing • The A-T (or A-U), T-A (or U-A), C-G and G-C pairing of bases in double-stranded DNA, in transcription, and between tRNA and mRNA.
Complementary DNA (cDNA) • DNA formed by reverse transcriptase acting with an RNA template; essential intermediate in the reproduction of retroviruses; used as a tool in recombinant DNA technology; lacks introns.
Complete metamorphosis • A change of state during the life cycle of an organism in which the body is almost completely rebuilt to produce an individual with a very different body form. Characteristic of insects such as butterflies, moths, beetles, ants, wasps, and flies.
Compound • (1) A substance made up of atoms of more than one element. (2) Made up of many units, as the compound eyes of arthropods (as opposed to the simple eyes of the same group of organisms).
Condensation reaction • A reaction in which two molecules become connected by a covalent bond and a molecule of water is released. (AH + BOH Æ AB + H2O.)
Cones • (1) In the vertebrate retina: photoreceptors responsible for color vision. (2) In gymnosperms: reproductive structures consisting of many sporophylls packed relatively tightly.
Conidium (ko nid´ ee um) [Gr. konis: dust] • An asexual fungus spore borne singly or in chains either apically or laterally on a hypha.
Conifer (kahn´ e fer) [Gr. konos: cone + phero: carry] • One of the cone-bearing gymnosperms, mostly trees, such as pines and firs.
Conjugation (kahn´ jew gay´ shun) [L. conjugare: yoke together] • The close approximation of two cells during which they exchange genetic material, as in Paramecium and other ciliates, or during which DNA passes from one to the other through a tube, as in bacteria.
Connective tissue • An animal tissue that connects or surrounds other tissues; its cells are embedded in a collagen-containing matrix.
Connexon • In a gap junction, a protein channel linking adjacent animal cells.
Consensus sequences • Short stretches of DNA that appear, with little variation, in many different genes.
Constant region • The constant region in an immunoglobulin is encoded by a single exon and determines the function, but not the specificity, of the molecule. The constant region of the T cell receptor anchors the protein to the plasma membrane.
Constitutive enzyme • An enzyme that is present in approximately constant amounts in a system, whether its substrates are present or absent. (Contrast with inducible enzyme.)
Consumer • An organism that eats the tissues of some other organism.
Continental drift • The gradual drifting apart of the world’s continents that has occurred over a period of billions of years.
Convergent evolution • The evolution of similar features independently in unrelated taxa from different ancestral structures.
Cooperative act • Behavior in which two or more individuals interact to their mutual benefit. No conscious awareness by the actors of the effects of their behavior is implied.
Cooption • The act of capturing something for a particular use. In ecology refers to the diversion of ecological production for human use. Such production is said to be coopted.
Copulation • Reproductive behavior that results in a male depositing sperm in the reproductive tract of a female.
Corepressor • A low molecular weight compound that unites with a protein (the repressor) to prevent transcription in a repressible operon.
Cork • A waterproofing tissue in plants, with suberin-containing cell walls. Produced by a cork cambium.
Corolla (ko role´ lah) [L.: diminutive of corona: wreath, crown] • All of the petals of a flower, collectively.
Coronary (kor´ oh nair ee) • Referring to the blood vessels of the heart.
Corpus luteum (kor´ pus loo´ tee um) [L. corpus: body + luteum: yellow] A structure formed from a follicle after ovulation; it produces hormones important to the maintenance of pregnancy.
Cortex [L.: bark or rind] • (1) In plants, the tissue between the epidermis and the vascular tissue of a stem or root. (2) In animals, the outer tissue of certain organs, such as the adrenal cortex and cerebral cortex.
Corticosteroids • Steroid hormones produced and released by the cortex of the adrenal gland.
Cost • See energetic cost, opportunity cost, risk cost.
Cotyledon (kot´ ul lee´ dun) [Gr. kotyledon: a hollow space] • A "seed leaf." An embryonic organ which stores and digests reserve materials; may expand when seed germinates.
Countercurrent exchange • An adaptation that promotes maximum exchange of heat or any diffusible substance between two fluids by the fluids flow in opposite directions through parallel tubes in close approximation to each other. An example is countercurrent heat exchange between arterioles and venules in the extremities of some animals.
Covalent bond • A chemical bond that arises from the sharing of electrons between two atoms. Usually a strong bond.
Crassulacean acid metabolism (CAM) • A metabolic pathway enabling the plants that possess it to store carbon dioxide at night and then perform photosynthesis during the day with stomata closed.
Crista (plural: cristae) • A small, shelflike projection of the inner membrane of a mitochondrion; the site of oxidative phosphorylation.
Critical night length • In the photoperiodic flowering response of short-day plants, the length of night above which flowering occurs and below which the plant remains vegetative. (The reverse applies in the case of long-day plants.)
Critical period • The age during which some particular type of learning must take place or during which it occurs much more easily than at other times. Typical of song learning among birds.
Cross section (also called a transverse section) • A section taken perpendicular to the longest axis of a structure.
Crossing over • The mechanism by which linked markers undergo recombination. In general, the term refers to the reciprocal exchange of corresponding segments between two homologous chromatids.
CRP • The cAMP receptor protein that interacts with the promoter to enhance transcription; a lowered cAMP concentration results in catabolite repression.
Crustacean (crus tay´ see an) • A member of the phylum Crustacea, such as a crab, shrimp, or sowbug.
Cryptic appearance [Gr. kryptos: hidden] • The resemblance of an animal to some part of its environment, which helps it to escape detection by predators.
Cryptochromes [Gr. kryptos: hidden + kroma: color] • Photoreceptors mediating some blue-light effects in plants and animals.
Culture • (1) A laboratory association of organisms under controlled conditions. (2) The collection of knowledge, tools, values, and rules that characterize a human society.
Cuticle • A waxy layer on the outer surface of a plant or an insect, tending to retard water loss.
Cyanobacteria (sigh an´ o bacteria) [Gr. kuanos: the color blue] • A division of photosynthetic bacteria, formerly referred to as blue-green algae; they lack sexual reproduction, and they use chlorophyll a in their photosynthesis.
Cyclic AMP • See cAMP.
Cyclins • Proteins that activate cyclin-dependent kinases, bringing about transitions in the cell cycle.
Cyclin-dependent kinase (cdk) • A kinase is an enzyme that catalzyes the addition of phosphate groups from ATP to target molecules. Cdk´s target proteins involved in transitions in the cell cycle and are active only when complexed to additional protein subunits, cyclins.
Cyst (sist) [Gr. kystis: pouch] • (1) A resistant, thick-walled cell formed by some protists and other organisms. (2) An abnormal sac, containing a liquid or semisolid substance, produced in response to injury or illness.
Cytochromes (sy´ toe chromes) [Gr. kytos: container + chroma: color] • Iron-containing red proteins, components of the electron-transfer chains in photophosphorylation and respiration.
Cytokinesis (sy´ toe kine ee´ sis) [Gr. kytos: container + kinein: to move] • The division of the cytoplasm of a dividing cell. (Contrast with mitosis.)
Cytokinin (sy´ toe kine´ in) [Gr. kytos: container + kinein: to move] • A member of a class of plant growth substances playing roles in senescence, cell division, and other phenomena.
Cytoplasm • The contents of the cell, excluding the nucleus.
Cytoplasmic determinants • In animal development, gene products whose spatial distribution may determine such things as embryonic axes.
Cytosine (site´ oh seen) • A nitrogen-containing base found in DNA and RNA.
Cytoskeleton • The network of microtubules and microfilaments that gives a eukaryotic cell its shape and its capacity to arrange its organelles and to move.
Cytosol • The fluid portion of the cytoplasm, excluding organelles and other solids.
Cytotoxic T cells • Cells of the cellular immune system that recognize and directly eliminate virus-infected cells. (Contrast with helper T cells, suppressor T cells.)