cell cycle (cell-division cycle) Reproductive cycle of a cell: the orderly sequence of events by which a cell duplicates its chromosomes and, usually, the other cell contents, and divides into two. (Figure 17–4) cell determination   Process whereby a cell progressively loses the potential to form other cell types, as development proceeds.
cell doctrine   The proposal that all living organisms are composed of one or more cells and that all cells arise from the division of other living cells.
cell memory Retention by cells and their descendants of persistently altered patterns of gene expression, without any change in DNA sequence. See also epigenetic inheritance.
cell plate   Flattened membrane-bounded structure that forms by fusing vesicles in the cytoplasm of a dividing plant cell and is the precursor of the new cell wall.
cell-cycle control system   Network of regulatory proteins that governs progression of a eukaryotic cell through the cell cycle.
cellulose Long, unbranched chains of glucose; major constituent of plant cell walls.
cellulose microfibril   Highly ordered crystalline aggregate formed from bundles of about 40 cellulose chains, arranged with the same polarity and stuck together in overlapping parallel arrays by hydrogen bonds between adjacent cellulose molecules.
central (primary) lymphoid organ Organ in which T or B lymphocytes are produced from precursor cells. In adult mammals, these are the thymus and bone marrow, respectively. (Figure 24–12) centriole   Short cylindrical array of microtubules, closely similar in structure to a basal body. A pair of centrioles is usually found at the center of a centrosome in animal cells. (Figure 16–48) centromere   Constricted region of a mitotic chromosome that holds sister chromatids together. This is also the site on the DNA where the kinetochore forms so as to capture microtubules from the mitotic spindle. (Figure 4–43) centrosome Centrally located organelle of animal cells that is the primary microtubule-organizing center (MTOC) and acts as the spindle pole during mitosis. In most animal cells it contains a pair of centrioles. (Figures 16–47 and 17–24) cerebral cortex   Outermost layer of the hemispheres of the brain; the most complex structure in the human body.
CG island Region of DNA in vertebrate genomes with a greater than average density of CG sequences; these regions generally remain unmethylated.
channel (membrane channel) Transmembrane protein complex that allows inorganic ions or other small molecules to diffuse passively across the lipid bilayer. (Figure 11–3) channelrhodopsin   Photosensitive protein forming a cation channel across the membrane that opens in response to light.
charge separation In photosynthesis, the light-induced transfer of a high-energy electron from chlorophyll to an acceptor molecule resulting in the formation of a positive charge on the chlorophyll and a negative charge on a mobile electron carrier.
chemical biology Name given to a strategy that uses large-scale screening of hundreds of thousands of small molecules in biological assays to identify chemicals that affect a particular biological process and that can then be used to study it.
chemical carcinogens   Disparate chemicals that are carcinogenic—due to the ability to cause mutations—when fed to experimental animals or painted repeatedly on their skin.
chemical group Certain combinations of atoms—such as methyl (–CH3), hydroxyl (–OH), carboxyl (–COOH), carbonyl  (–C=O), phosphate (–PO32–), sulfhydryl (–SH), and amino (–NH2) groups—that have distinct chemical and physical properties and influence the behavior of the molecule in which the group occurs.
chemiosmotic coupling (chemiosmosis) Mechanism in which an electrochemical proton gradient across a membrane  (composed of a pH gradient plus a membrane potential) is used to drive an energy-requiring process, such as ATP production or the rotation of bacterial flagella.
chemotaxis   Movement of a cell toward or away from some diffusible chemical.