acetyl CoA

Small, water-soluble metabolite comprising an acetyl group linked to coenzyme A (CoA); formed during oxidation of pyruvate, fatty acids, and amino acids. Its acetyl group is transferred to citrate in the citric acid cycle. (Figure 16-10)

acetylcholine (ACh)

Neurotransmitter that functions at vertebrate neuromuscular junctions and at various neuron-neuron synapses in the brain and peripheral nervous system.


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


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 microfilaments.

action potential

Rapid, transient, all-or-none electrical activity that is propagated in the plasma membrane of excitable cells such as neurons and muscle cells. Action potentials, or nerve impulses, allow long-distance signaling in the nervous system. (Figure 21-14)

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 2-27)

active site

Region of an enzyme molecule where the substrate binds and undergoes a catalyzed reaction.

active transport

Energy-requiring movement of an ion or small molecule across a membrane against its concentration gradient or electrochemical gradient. Energy is provided by the coupled hydrolysis of ATP or the cotransport of another molecule down its electrochemical gradient.

adenosine triphosphate

See ATP.

adenylyl cyclase

Membrane-bound enzyme that catalyzes formation of cyclic AMP (cAMP) from ATP; also called adenylate cyclase. Binding of certain ligands to their cell-surface receptors leads to activation of adenylyl cyclase and a rise in intracellular cAMP. (Figure 20-15)


Referring to a cell, organism, or metabolic process that utilizes 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.

Mercy College of Ohio

Copyright © 2017 · All Rights Reserved · Mercy College of Ohio