On-Line Biology Book: GLOSSARY

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abiogenesis  Early theory that 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 counting tree rings, decay of a specific type of atom, or annual sedimentary layers (such as varves in lakes or layers in a glacier). The term is in some disfavor because it suggests an exactness that may not be possible to obtain.

absorption  The process by which the products of digestion are transferred into the body's internal environment, enabling them to reach the cells.

absorptive feeders  Animals such as tapeworms that ingest food through the body wall.

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 ultimately leads to muscle contraction.

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).

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 oxygen in the atmosphere.

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 body's ability to fight infection; results in premature death brought about by various diseases that overwhelm the compromised immune system. PICTURE

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. PICTURE

adaptation Tendency of an organism to suit its environment; one of the major points of Charles Darwin's theory of evolution by natural selection: organisms adapt to their environment. Those organisms best adapted will have a greater chance of surviving and passing their genes on to the next generation.

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 from a common ancestral species as a result of novel adaptations or a recent mass extinction. Examples: mammals during the Cenozoic Era after the extinction of dinosaurs at the close of the Mesozoic Era flowering plants during the Cretaceous Period diversified because of their reproductive advantages over gymnosperm and non-seed plants that dominated the floras of the world at that time.

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 the energy carrying molecule ATP (adenosine triphosphate) which is the energy coin of the cell. PICTURE

adenosine diphosphate (ADP) Lower energy form of ATP, having two (instead of the three in ATP) phosphhate groups attached to the adenine base and ribose sugar. PICTURE

adenosine triphosphate (ATP)  A common form in which energy is stored in living systems; consists of a nucleotide (with ribose sugar) with three phosphate groups. The energy coin of the cell. PICTURE

adhesion  The ability of molecules of one substance to adhere to a different substance.

adrenocorticotropic hormone (ACTH)  A hormone produced by the anterior pituitary that stimulates the adrenal cortex to release several hormones including cortisol.

adventitious roots  Roots that develop from the stem following the death of the primary root. Branches from the adventitious roots form a fibrous root system in which all roots are about the same size; occur in monocots.

age structure  The relative proportion of individuals in each age group in a population.

aggregates  Fairly random associations of animals with little or no internal organization; form in response to a single stimulus and disperse when the stimulus is removed; one of the three broad classes of social organization.

albinism Genetic condition caused by the body's inability to manufacture pigments; an autosomal recessive trait.

aldosterone  A hormone secreted by the adrenal glands that controls the reabsorption of sodium in the renal tubule of the nephron.

alleles  Alternate forms of a gene.

allergens  Antigens that provoke an allergic reaction.

alpha decay Type of radioactive decay in which a radioisotope emits a large but slow-moving particle consisting of two protons and two neutrons.