【abiotic】
nonliving, as in the physical environment

【absorption】
the process by which water and dissolved substances pass through a membrane

【acetylcholine】
a transmitter substance released from the axons of nerve cells at the synapse

【active immunity】
protective immunity to a disease in which the individual produces antibodies as a result of previous exposure to the antigen

【adaptation】
a behavioral or biological change that enables an organism to adjust to its environment

【adaptive radiation】
the production of a number of different species from a single ancestral species

【adenosine phosphate】
adenosine disphosphate（ADP） and adenosine triphoshate（ATP）, which are energy storage molecules

【ADH（vasopressin）】
a hormone that regulates water reabsorption

【adipose】
fatty tissue, fat-storing tissue, or fat within cells

【adrenal cortex】
the outer part of the adrenal gland that secretes many hormones, including cortisone and aldosterone

【adrenal medulla】
the inner part of the adrenal gland that secretes adrenalin

【adrenocorticotrophic hormone】
usually referred to as ACTH and secreted by the anterior lobe of the pituitary gland; stimulates the adrenal cortex to produce its characteristic hormones

【aerobe】
an organism that requires oxygen for respiration and can live only in the presence of oxygen

【aerobic】
requiring free oxygen from the atmosphere for normal activity and respiration