Boldface terms within a definition are also defined in this glossary.

Figures and tables that illustrate defined terms are noted in parentheses.

G-1

AAA ATPase family A group of proteins that couple hydrolysis of ATP with large molecular movements usually associated with unfolding of protein substrates or the disassembly of multisubunit protein complexes.

ABC superfamily A large group of integral membrane proteins that often function as ATP-powered membrane transport proteins to move diverse molecules (e.g., phospholipids, cholesterol, sugars, ions, peptides) across cellular membranes. (Figure 11-15)

acetylcholine (ACh) Neurotransmitter that functions at vertebrate neuromuscular junctions and at various neuron-neuron synapses in the brain and peripheral nervous system. (Figure 22-25)

acetyl CoA Small, water-soluble metabolite comprising an acetyl group linked to coenzyme A (CoA). The acetyl group is transferred to citrate in the citric acid cycle and is used as a carbon source in the synthesis of fatty acids, steroids, and other molecules. (Figure 12-15)

acid Any compound that can donate a proton (H⁺). The carboxyl and phosphate groups are the primary acidic groups in biological macromolecules.

actin Abundant structural protein in eukaryotic cells that interacts with many other proteins. The monomeric globular form (G-actin) polymerizes to form actin filaments (F-actin). In muscle cells, F-actin interacts with myosin during contraction. See also microfilament. (Figure 17-5)

action potential Rapid, transient, all-or-none electrical activity propagated in the plasma membrane of excitable cells (e.g., neurons and muscle cells) as the result of the selective opening and closing of voltage-gated Na⁺ and K⁺ channels. (Figures 22-2 and 22-9)

activation domain A region of an activator transcription factor that will stimulate transcription when fused to a DNA-binding domain.

activation energy The input of energy required to (overcome the barrier to) initiate a chemical reaction. By reducing the activation energy, an enzyme increases the rate of a reaction. (Figure 2-30)

activation loop A region in most protein-tyrosine kinases, containing a tyrosine residue that, when phosphorylated, increases kinase activity.

activator Specific transcription factor that stimulates transcription.

active site Specific region of an enzyme that binds a substrate molecule(s) and promotes a chemical change in the bound substrate. (Figure 3-23)

active transport Protein-mediated movement of an ion or small molecule across a membrane against its concentration gradient or electrochemical gradient driven by the coupled hydrolysis of ATP.(Figure 11-2, [1]; Table 11-1)

adapter proteins Adapter proteins physically link one protein to another protein by binding to both of them. Adapter proteins directly or indirectly (via additional adapters) connect cell-adhesion molecules or adhesion receptors to elements of the cytoskeleton or to intracellular signaling proteins.

adenosine triphosphate (ATP) See ATP.

adenylyl cyclase One of several enzymes that is activated by binding of certain ligands to their cell-surface receptors and catalyzes formation of cyclic AMP (cAMP) from ATP; also called adenylate cyclase. (Figures 15-25 and 15-26)

adhesion receptor Protein in the plasma membrane of animal cells that binds components of the extracellular matrix, thereby mediating cell-matrix adhesion. The integrins are major adhesion receptors. (Figure 20-1, [5])

ADP (adenosine diphosphate) The product, along with inorganic phosphate, of ATP hydrolysis by ATPases.

aequorin A bioluminescent protein, isolated from Aequorea victoria, that is activated by binding calcium ions.

aerobic Referring to a cell, organism, or metabolic process that utilizes gaseous oxygen (O₂) or that can grow in the presence of O₂.

aerobic oxidation Oxygen-requiring metabolism of sugars and fatty acids to CO₂ and H₂O coupled to the synthesis of ATP.

aerobic respiration See aerobic oxidation.

afferent neurons Nerves that transmit signals from peripheral tissues to the central nervous system.

agonist A molecule, often synthetic, that mimics the biological function of a natural molecule (e.g., a hormone).

Agrin A glycoprotein synthesized by developing motor neurons that increases MuSK kinase activity in a muscle cell, facilitating development of a neuromuscular junction. (Figure 22-23)

Akt A cytosolic serine/threonine kinase that is activated following binding to PI 3,4-bisphosphate and PI 3,4,5-trisphosphate; also called protein kinase B.

allele One of two or more alternative forms of a gene. Diploid cells contain two alleles of each gene, located at the corresponding site (locus) on homologous chromosomes.

G-2

allosteric Referring to proteins and cellular processes that are regulated by allostery.

allostery Change in the tertiary and/or quaternary structure of a protein induced by binding of a small molecule to a specific regulatory site, causing a change in the protein’s activity.

alpha carbon atom (C_α) In amino acids, the central carbon atom that is bonded to four different chemical groups (except in glycine) including the side chain, or R group. (Figure 2-4)

alpha (α) helix Common protein secondary structure in which the linear sequence of amino acids is folded into a right-handed spiral stabilized by hydrogen bonds between carboxyl and amide groups in the backbone. (Figure 3-4)

alternative splicing Process by which the exons of one pre-mRNA are spliced together in different combinations, generating two or more different mature mRNAs from a single pre-mRNA. (Figure 5-16)

amino acid An organic compound containing at least one amino group and one carboxyl group. In the amino acids that are the monomers for building proteins, an amino group and carboxyl group are linked to a central carbon atom, the α carbon, to which a variable side chain is attached. (Figures 2-4 and 2-14)

aminoacyl-tRNA Activated form of an amino acid, used in protein synthesis, consisting of an amino acid linked via a high-energy ester bond to the 3'-hydroxyl group of a tRNA molecule. (Figure 5-19)

amphipathic Referring to a molecule or structure that has both a hydrophobic and a hydrophilic part.

amphiphilic See amphipathic.

amphitelic attachment Describes the correct attachment of chromosomes to the mitotic spindle, where sister kinetochores attach to microtubules emanating from opposite poles. (Figure 19-22)

amplification An increase in signal intensity as a cellular signal is transduced.

amyotrophic lateral sclerosis (ALS) Lou Gehrig’s disease, characterized by progressive death of the motor neurons connecting the central nervous system to muscles.

anaerobic Referring to a cell, organism, or metabolic process that functions in the absence of gaseous oxygen (O₂).

anaphase Mitotic stage during which the sister chromatids (or duplicated homologs in meiosis I) separate and move apart (segregate) toward the spindle poles. (Figure 18-37)

anchoring junctions Specialized regions on the cell surface containing cell-adhesion molecules or adhesion receptors; includes adherens junctions and desmosomes, which mediate cell-cell adhesion, and hemidesmosomes, which mediate cell-matrix adhesion. (Figures 20-14 and 20-16)

anaerobic respiration Respiration in which molecules other than oxygen, such as sulfate or nitrate, are used as the final recipient of the electrons transported via the electron-transport chain.

anaphase-promoting complex or cyclosome (APC/C) A ubiquitin ligase that targets securin, mitotic cyclins, and other proteins for proteasomal degradation from the onset of anaphase until entry into the subsequent cell cycle.

aneuploidy Any deviation from the normal diploid number of chromosomes in which extra copies of one or more chromosomes are present or one of the normal copies is missing.

anion A negatively charged ion.

antagonist A molecule, often synthetic, that blocks the biological function of a natural molecule (e.g., hormone).

antibody A protein (immunoglobulin), normally produced in response to an antigen, that interacts with a particular site (epitope) on the same antigen and facilitates its clearance from the body. (Figure 3-21)

anticodon Sequence of three nucleotides in a tRNA that is complementary to a codon in an mRNA. During protein synthesis, base pairing between a codon and anticodon aligns the tRNA carrying the corresponding amino acid for addition to the growing polypeptide chain. (Figure 5-20)

antigen Any material (usually foreign) that elicits an immune response. For B cells, an antigen elicits formation of antibody that specifically binds the same antigen; for T cells, an antigen elicits a proliferative response, followed by production of cytokines or the activation of cytotoxic activity.

antigen-presenting cell (APC) Any cell that can digest an antigen into small peptides and display the peptides in association with class II MHC molecules on the cell surface where they can be recognized by T cells. Professional APCs (dendritic cells, macrophages, and B cells) constitutively express class II MHC molecules. (Figures 23-25 and 23-26)

antiport A type of cotransport in which a membrane protein (antiporter) transports two different molecules or ions across a cell membrane in opposite directions. See also symport. (Figure 11-2, [3C])

apical Referring to the tip (apex) of a cell, an organ, or other body structure. In the case of epithelial cells, the apical surface is exposed to the exterior of the body or to an internal open space (e.g., intestinal lumen, duct). (Figure 20-10)

apoptosis A genetically regulated process, occurring in specific tissues during development and disease, by which a cell destroys itself; marked by the breakdown of most cell components and a series of well-defined morphological changes; also called programmed cell death. See also caspases. (Figures 21-33 and 21-40)

apoptosome Large, disk-shaped heptamer of mammalian Apaf-1, a protein that assembles in response to apoptosis signals and serves as an activation machine for initiator and effector caspases. (Figure 21-41)

aptamer Region of single-stranded RNA or DNA ∼70–120 bp long that folds into a complex tertiary structure that binds a small molecule specifically. (Figure 9-8)

aquaporins A family of membrane transport proteins that allow water and a few other small uncharged molecules, such as glycerol, to cross biomembranes. (Figure 11-8)

archaea Class of prokaryotes that constitutes one of the three distinct evolutionary lineages of modern-day organisms; also called archaebacteria and archaeans. In some respects, archaeans are more similar to eukaryotes than to bacteria (eubacteria). (Figure 1-1)

G-3

associated constant (K_a) See equilibrium constant.

aster Structure composed of microtubules (astral fibers) that radiate outward from a centrosome during mitosis. (Figure 18-37)

astrocytes Star-shaped glial cells in the brain and spinal cord that perform many functions, including support of endothelial cells that form the blood-brain barrier, maintain extracellular ion composition, and provide nutrients to neurons.

asymmetric carbon atom A carbon atom bonded to four different atoms or chemical groups; also called chiral carbon atom. The bonds can be arranged in two different ways, producing stereoisomers that are mirror images of each other. (Figure 2-4)

asymmetric cell division Any cell division in which the two daughter cells receive the same genes but otherwise inherit different components (e.g., mRNAs, proteins) from the parental cell. (Figure 21-23b)

ATM/ATR Two related proteins kinases that are activated by DNA damage. Once active, they phosphorylate other proteins to initiate the cell’s response to DNA damage.

ATP (adenosine triphosphate) A nucleotide that is the most important molecule for capturing and transferring free energy in cells. Hydrolysis of each of the two phosphoanhydride bonds in ATP releases a large amount of free energy that can be used to drive energy-requiring cellular processes. (Figure 2-31)

ATPase One of a large group of enzymes that catalyze hydrolysis of ATP to yield ADP and inorganic phosphate with release of free energy. See also Na⁺/K⁺ ATPase and ATP-powered pump.

ATP-powered pump Any transmembrane protein that has ATPase activity and couples hydrolysis of ATP to the active transport of an ion or small molecule across a biomembrane against its electrochemical gradient; often simply called pump. (Figure 11-9)

ATP synthase Multimeric protein complex, bound to inner mitochondrial membranes, thylakoid membranes of chloroplasts, and the bacterial plasma membrane, that catalyzes synthesis of ATP during oxidative phosphorylation and photosynthesis; also called F₀F₁ complex. (Figure 12-26a)

ATR See ATM/ATR.

Aurora B kinase Destabilizes faulty microtubule-kinetochore interactions by phosphorylating microtubule-binding components within the kinetochore.

Aurora kinases Serine/threonine kinases that play a crucial role in cell division by controlling chromatid segregation. Aurora B kinase destabilizes faulty microtubule-kinetochore interactions by phosphorylating microtubule-binding components within the kinetochore.

autocrine Referring to signaling mechanism in which a cell produces a signaling molecule (e.g., growth factor) and then binds and responds to it.

autophagosome A large region of cytoplasm including multiple ribosomes and mitochondria engulfed in a closed membrane during periods of cell starvation for amino acids. The autophagosomes fuse with lysosomes, where the autophagosome constituents are broken down into amino acids and other nutrients that are transported into the cytoplasm.

autophagy Literally, “eating oneself”; the process by which cytosolic proteins and organelles are delivered to the lysosome, degraded, and recycled. Autophagy involves the formation of a double-membrane vesicle called an autophagosome or autophagic vesicle. (Figure 14-35)

autoradiography Technique for visualizing radioactive molecules in a sample (e.g., a tissue section or electrophoretic gel) by exposing a photographic film (emulsion) or two-dimensional electronic detector to the sample. The exposed film is called an autoradiogram or autoradiograph.

autosome Any chromosome other than a sex chromosome.

axon Long process extending from the cell body of a neuron that is capable of conducting an electric impulse (action potential), generated at the junction with the cell body, toward its distal, branching end (the axon terminus). (Figure 22-1)

axonal transport Motor protein–mediated transport of organelles and vesicles along microtubules in axons of nerve cells. Anterograde transport occurs from cell body toward axon terminal); retrograde transport, from axon terminal toward cell body. (Figures 18-16 and 18-17)

axoneme Bundle of microtubules and associated proteins present in cilia and flagella and responsible for their structure and movement. (Figure 18-31)