action potential   Rapid, transient, self-propagating electrical excitation in the plasma membrane of a cell such as a neuron or muscle cell. Action potentials, or nerve impulses, make possible long-distance signaling in the nervous system. (Figure 11–31) activated carrier   Small diffusible molecule that stores easily exchangeable energy in the form of one or more energy-rich covalent bonds. Examples are ATP, acetyl CoA, FADH2, NADH, and NADPH. (Figure 2–31) activation energy   The extra energy that must be acquired by atoms or molecules in addition to their ground-state energy in order to reach the transition state required for them to undergo a particular chemical reaction. (Figure 2–21) activation-induced deaminase (AID) The enzyme catalyzing the processes of somatic hypermutation and immunoglobulin class switching in activated B cells.
active site Region of an enzyme surface to which a substrate molecule binds in order to undergo a catalyzed reaction.  (Figure 1–7) active transport   Movement of a molecule across a membrane or other barrier driven by energy other than that stored in the electrochemical or concentration gradient of the transported molecule.
adaptation (1) Adaptation (desensitization): adjustment of sensitivity following repeated stimulation. The mechanism that allows a cell to react to small changes in stimuli even against a high background level of stimulation. (2) Evolutionary adaptation: an evolved trait.
adaptive immune system   System of lymphocytes providing highly specific and long-lasting defense against pathogens in vertebrates. It consists of two major classes of lymphocytes: B lymphocytes (B cells), which secrete antibodies that bind specifically to the pathogen or its products, and T lymphocytes (T cells), which can either directly kill cells infected with the pathogen or produce secreted or cell-surface signal proteins that stimulate other host cells to help eliminate the pathogen. (Figure 24–2) adaptor protein, adaptor   General term for a protein that functions solely to link two or more different proteins together in an intracellular signaling pathway or protein complex.  (Figure 15–11) adenylyl cyclase (adenylate cyclase) Membrane-bound enzyme that catalyzes the formation of cyclic AMP from ATP. An important component of some intracellular signaling pathways.
adherens junction Cell junction in which the cytoplasmic face of the plasma membrane is attached to actin filaments. Examples include adhesion belts linking adjacent epithelial cells and focal contacts on the lower surface of cultured fibroblasts.
adhesins   Specific proteins or protein complexes of pathogenic bacteria that recognize and bind cell-surface molecules on the host cells to enable tight adhesion and colonization of tissues.
adhesion belt   Adherens junctions in epithelia that form a continuous belt (zonula adherens) just beneath the apical face of the epithelium, encircling each of the interacting cells in the sheet.
ADP (adenosine 5′-diphosphate) Nucleotide produced by hydrolysis of the terminal phosphate of ATP. Regenerates ATP when phosphorylated by an energy-generating process such as oxidative phosphorylation. (Figure 2–33) aerobic respiration   Process by which a cell obtains energy from sugars or other organic molecules by allowing their carbon and hydrogen atoms to combine with the oxygen in air to produce CO2 and H2O, respectively.
affinity maturation   Progressive increase in the affinity of antibodies for the immunizing antigen with the passage of time after immunization.
Agrin   Signal protein released by an axonal growth cone during formation of the synapse between it and a muscle cell.
AIRE (autoimmune regulator) A protein expressed by a subpopulation of epithelial cells in the thymus that stimulates the production of small amounts of self proteins characteristic of other organs, exposing developing thymocytes to these proteins for the purpose of self-tolerance.