Science / Biology Glossary

Abiogenesis: An early theory that was held that some organisms originated from nonliving material.

Abnormal Hemoglobin: Hemoglobin molecule with a different shape due to an altered amino acid sequence (ultimately caused by an altered DNA base sequence), such as in the inherited disease sickle-cell anemia.

Abscisic Acid: A plant hormone that promotes dormancy in perennial plants and causes rapid closure of leaf stomata when a leaf begins to wilt.

Absolute Time: One of the two types of geologic time (relative time being the other), with a definite age date established mostly by the decay of radioactive elements, although ages may also be obtained by . . . View Full Definition

Absorption: The process by which the products of digestion are transferred into the bodys internal environment, enabling them to reach the cells.

Absorptive Feeders: Animals such as tapeworms that ingest food through the body wall. These animals can be very dangerous if injested.

Acetyl Coa: An intermediate compound formed during the breakdown of glucose by adding a two-carbon fragment to a carrier molecule (Coenzyme A or CoA).

Acetylcholine: A chemical released at neuromuscular junctions that binds to receptors on the surface of the plasma membrane of muscle cells, causing an electrical impulse to be transmitted. The impulse ult . . . View Full Definition

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 sulfur dioxide and nitrogen oxides released during the combustion of fossil fuels combine with water and oxyge . . . View Full Definition

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 as a result of infection by the human immunodeficiency virus (HIV), which attacks helper T cells, crippling the immune system and greatly reducing the . . . View Full Definition

Actin: The protein from which microfilaments are composed; forms the contractile filaments of sarcomeres in muscle cells.

Action Potential: A reversal of the electrical potential in the plasma membrane of a neuron that occurs when a nerve cell is stimulated; caused by rapid changes in membrane permeability to sodium and potassium.

Active Transport: Transport of molecules against a concentration gradient (from regions of low concentration to regions of high concentration) with the aid of proteins in the cell membrane and energy from ATP.

Adaptation: Tendency of an organism to suit its environment; one of the major points of Charles Darwins theory of evolution by natural selection: organisms adapt to their environment. Those organisms be . . . View Full Definition

Adaptive Radiation: The development of a variety of species from a single ancestral form; occurs when a new habitat becomes available to a population. Evolutionary pattern of divergence of a great many taxa fro . . . View Full Definition

Adenine: One of the four nitrogen-containing bases occurring in nucleotides, the building blocks of the organic macromolecule group known as nucleic acids (DNA and RNA). Adenine is also the base in t . . . View Full Definition

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