Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

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Glossary
ABC transporters A large family of membrane transport
proteins that use the energy of ATP hydrolysis to transfer
peptides or small molecules across membranes. (Figure 11–16)
acetyl CoA Small water-soluble activated carrier molecule.
Consists of an acetyl group linked to coenzyme A (CoA) by an
easily hydrolyzable thioester bond. (Figure 2–38)
acetylcholine receptor (AChR) Membrane protein that
responds to binding of acetylcholine (ACh). The nicotinic AChR
is a transmitter-gated ion channel that opens in response
to ACh. The muscarinic AChR is not an ion channel, but a
G-protein-coupled cell-surface receptor.
acid A proton donor. Substance that releases protons (H + )
when dissolved in water, forming hydronium ions (H 3 O + ) and
lowering the pH. (Panel 2–2, pp. 92–93)
acid hydrolases Hydrolytic enzymes—including proteases,
nucleases, glycosidases, lipases, phospholipases,
phosphatases, and sulfatases—that work best at acidic pH;
these enzymes are found within the lysosome.
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, FADH 2 ,
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 CO 2 and H 2 O, 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.