Acid
A substance that increases the hydrogen ion concentration in a solution.
Acid precipitation
Rain, snow, or fog that is more acidic than pH 5.6.
Acoelomate
A solid-bodied animal lacking a cavity between the gut and outer body wall.
Acrocentric
Having the centromere located near one end of the chromosome so that one chromosomal
arm is long and the other is short.
Acrosome
An organelle at the tip of a sperm cell that helps the sperm penetrate the egg.
ACTH
Abbreviation of adrenocorticotropic hormone.
Actin
A globular protein that links into chains, two of which twist helically about each other,
forming microfilaments in muscle and other contractile elements in cells.
Actinomorphic
Capable of being divided into equal halves along any diameter, as the flowers of the rose or
tulip; radially symmetrical.
Action potential
A rapid change in the membrane potential of an excitable cell, caused by stimulus-triggered,
selective opening and closing of voltage-sensitive gates in sodium and potassium ion
channels.
Activation energy
The energy that must be possessed by atoms or molecules in order to react.
Active site
The specific portion of an enzyme that attaches to the substrate by means of weak chemical
bonds.
Active transport
The movement of a substance across a biological membrane against its concentration or
electrochemical gradient, with the help of energy input and specific transport proteins.
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