TEST BANK FOR Campbell Biology 9th Edition Glossary A

TEST BANK FOR Campbell Biology 9th Edition Glossary A 5′ cap - A modified form of guanine nucleotide added onto the 5′ end of a pre-mRNA molecule. A site - One of a ribosome's three binding sites for tRNA during translation. The A site holds the tRNA carrying the next amino acid to be added to the polypeptide chain. (A stands for aminoacyl tRNA.) ABC hypothesis - A model of flower formation identifying three classes of organ identity genes that direct formation of the four types of floral organs. abiotic - Nonliving; referring to the physical and chemical properties of an environment. abortion - The termination of a pregnancy in progress. abscisic acid (ABA) - A plant hormone that slows growth, often antagonizing the actions of growth hormones. Two of its many effects are to promote seed dormancy and facilitate drought tolerance. absorption - The third stage of food processing in animals: the uptake of small nutrient molecules by an organism's body. absorption spectrum - The range of a pigment's ability to absorb various wavelengths of light; also a graph of such a range. abyssal zone - The part of the ocean's benthic zone between 2,000 and 6,000 m deep. acanthodian - Any of a group of ancient jawed aquatic vertebrates from the Silurian and Devonian periods. accessory fruit - A fruit, or assemblage of fruits, in which the fleshy parts are derived largely or entirely from tissues other than the ovary. acclimatization - Physiological adjustment to a change in an environmental factor. acetyl coenzyme A (acetyl CoA) - The entry compound for the citric acid cycle in cellular respiration, formed from a fragment of pyruvate attached to a coenzyme. acetylcholine - One of the most common neurotransmitters; functions by binding to receptors and altering the permeability of the postsynaptic membrane to specific ions, either depolarizing or hyperpolarizing the membrane. acid - A substance that increases the hydrogen ion concentration of a solution. acid precipitation - Rain, snow, or fog that is more acidic than pH 5.2. acoelomate - A solid-bodied animal lacking a cavity between the gut and outer body wall. acrosomal reaction - The discharge of hydrolytic enzymes from the acrosome, a vesicle in the tip of a sperm, when the sperm approaches or contacts an egg. acrosome - A vesicle in the tip of a sperm containing hydrolytic enzymes and other proteins that help the sperm reach the egg. actin - A globular protein that links into chains, two of which twist helically about each other, forming microfilaments in muscle and other kinds of cells. action potential - An electrical signal that propagates (travels) along the membrane of a neuron or other excitable cell as a nongraded (all-or-none) depolarization. action spectrum - A graph that profiles the relative effectiveness of different wavelengths of radiation in driving a particular process. activation energy - The amount of energy that reactants must absorb before a chemical reaction will start; also called free energy of activation. activator - A protein that binds to DNA and stimulates gene transcription. In prokaryotes, they bind in or near the promoter; in eukaryotes, they generally bind to control elements in enhancers. active immunity - Long-lasting immunity conferred by the action of B cells and T cells and the resulting B and T memory cells specific for a pathogen. It can develop as a result of natural infection or immunization. active site - The specific region of an enzyme that binds the substrate and that forms the pocket in which catalysis occurs. active transport - The movement of a substance across a cell membrane against its concentration or electrochemical gradient, mediated by specific transport proteins and requiring an expenditure of energy. adaptation - Inherited characteristic of an organism that enhances its survival and reproduction in a specific environment. adaptive immunity - A vertebrate-specific defense that is mediated by B lymphocytes (B cells) and T lymphocytes (T cells). It exhibits specificity, memory, and self-nonself recognition. Also called acquired immunity. adaptive radiation - Period of evolutionary change in which groups of organisms form many new species whose adaptations allow them to fill different ecological roles in their communities. addition rule - A rule of probability stating that the probability of any one of two or more mutually exclusive events occurring can be determined by adding their individual probabilities. adenylyl cyclase - An enzyme that converts ATP to cyclic AMP in response to an extracellular signal. adhesion - The clinging of one substance to another, such as water to plant cell walls by means of hydrogen bonds. adrenal gland - One of two endocrine glands located adjacent to the kidneys in mammals. Endocrine cells in the outer portion (cortex) respond to adrenocorticotropic hormone (ACTH) by secreting steroid hormones that help maintain homeostasis during long-term stress. Neurosecretory cells in the central portion (medulla) secrete epinephrine and norepinephrine in response to nerve signals triggered by short-term stress. adrenocorticotropic hormone (ACTH) - A tropic hormone that is produced and secreted by the anterior pituitary and that stimulates the production and secretion of steroid hormones by the adrenal cortex. aerobic respiration - A catabolic pathway for organic molecules, using oxygen (O2) as the final electron acceptor in an electron transport chain and ultimately producing ATP. This is the most efficient catabolic pathway and is carried out in most eukaryotic cells and many prokaryotic organisms. age structure - The relative number of individuals of each age in a population. aggregate fruit - A fruit derived from a single flower that has more than one carpel. AIDS (acquired immunodeficiency syndrome) - The symptoms and signs present during the late stages of HIV infection, defined by a specified reduction in the number of T cells and the appearance of characteristic secondary infections. alcohol fermentation - Glycolysis followed by the reduction of pyruvate to ethyl alcohol, regenerating NAD+ and releasing carbon dioxide. aldosterone - A steroid hormone that acts on tubules of the kidney to regulate the transport of sodium ions (Na+) and potassium ions (K+). . . . . . . .