Alveolus
One of the deadend, multilobed air sacs that constitute the gas exchange surface of the
lungs. Or One of the milk-secreting sacs of epithelial tissue in the mammary glands.
Amino acids
An organic molecule possessing both carboxyl and amino groups. Amino acids serve as the
monomers of proteins.
Amino group
A functional group that consists of a nitrogen atom bonded to two hydrogen atoms; can act
as a base in solution, accepting a hydrogen ion and acquiring a charge of +1.
Aminoacyl
A family of enzymes, at least one for each amino acid, that catalyze the attachment of an
amino acid to its specific tRNA molecule.
Amitosis
Direct cell devision, that is, the cell divides by simple cleavage of the nucles without
formation of spireme spindle figures or chromosomes.
Ammonification
The process by which decomposers break down proteins and amino acids, releasing the
excess nitrogen in the form of ammonia (NH 3 ) or ammonium ion (NH 4 +).
Amniocentesis
A technique for determining genetic abnormalities in a fetus by the presence of certain
chemicals or defective fetal cells in the amniotic fluid, obtained by aspiration from a needle
inserted into the uterus.
Amnion
The innermost of four extraembryonic membranes; encloses a fluid-filled sac in which the
embryo is suspended.
Amniote
A vertebrate possessing an amnion surrounding the embryo; reptiles, birds, and mammals
are amniotes.
Amniotic egg
A shelled, water-retaining egg that enables reptiles, birds, and egg-laying mammals to
complete their life cycles on dry land.
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