Abiogenesis:  An early theory that was held that some organisms originated from nonliving mate . . .
Abnormal Hemoglobin:  Hemoglobin molecule with a different shape due to an altered amino acid sequence . . .
Abscisic Acid:  A plant hormone that promotes dormancy in perennial plants and causes rapid clos . . .
Absolute Time:  One of the two types of geologic time (relative time being the other), with a de . . .
Absorption:  The process by which the products of digestion are transferred into the bodys in . . .
Absorptive Feeders:  Animals such as tapeworms that ingest food through the body wall. These animals . . .
Acetyl Coa:  An intermediate compound formed during the breakdown of glucose by adding a two- . . .
Acetylcholine:  A chemical released at neuromuscular junctions that binds to receptors on the su . . .
Acid:  A substance that increases the number of hydrogen ions in a solution. . . .
Acid Rain:  The precipitation of sulfuric acid and other acids as rain. The acids form when . . .
Acoelomates:  Animals that do not have a coelom or body cavity; e.g., sponges and flatworms. . . .
Acquired Immunodeficiency Syndrome (Aids):  A collection of disorders that develop a . . .
Actin:  The protein from which microfilaments are composed; forms the contractile filame . . .
Action Potential:  A reversal of the electrical potential in the plasma membrane of a neuron that o . . .
Active Transport:  Transport of molecules against a concentration gradient (from regions of low con . . .
Adaptation:  Tendency of an organism to suit its environment; one of the major points of Char . . .
Adaptive Radiation:  The development of a variety of species from a single ancestral form; occurs whe . . .
Adenine:  One of the four nitrogen-containing bases occurring in nucleotides, the building . . .
Adenosine Diphosphate (ADP):  Lower energy form of ATP, having two (in . . .
Adenosine Triphosphate (ATP):  A common form in which energy is stored . . .