Glossary  ABCDEFGHIJKLMNOPQRSTUVWXZ        Biological TERMS 

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abiotic Pertaining to nonliving things.
abscission The detachment of plant parts, such as leaves, petals, or fruits, from the main plant.
acid Any substance that yields hydrogen ions when in solution. An acid has a number lower than 7 on the pH scale.
acrosome A structure, located on the front end of the head of a vertebrate sperm cell, that contains enzymes that help the sperm penetrate the accessory cells surrounding the oocyte.
action potential A transient reversal of electric potential across a cell membrane that results in a conducted nerve impulse.
activation energy The energy required to initiate a chemical reaction. Enzymes lower the activation energy of a reaction, thereby hugely speeding up the rate of the reaction.
active site The area of an enzyme, usually a groove or pocket, that binds the substrate and changes its shape to affect its reactivity.
actively acquired immunity Immunity resulting from natural exposure to an antigen in the environment or from deliberate exposure to an antigen.
active transport Transport of materials across the plasma membrane in which energy must be expended. Through active transport, solutes can be moved against their concentration gradient. The sodium-potassium pump is an example of active transport.
adaptation A modification in the structure or behavior of organisms over generations that makes them better suited to their environment.
adaptive radiation The rapid emergence of many species from a single species that has been introduced to a new environment. The different species specialize to fill available niches in the new environment.
adipose tissue Also known as fat, this type of loose connective tissue insulates the body and stores energy in addition to providing cushioning.
adrenocorticotropic hormone (ACTH) A hormone, secreted by the anterior pituitary, that stimulates release of steroid hormones from the outer cells of the adrenal gland.
adult stem cell A cell from a tissue of an adult that is capable of giving rise to many other types of cells or tissues.
afferent division The division of the peripheral nervous system that carries sensory information towards the central nervous system, having gathered information about the body or environment.
AIDS Acquired immune deficiency syndrome, a disease caused by the human immunodeficiency virus that attacks the body’s own immune system and leaves it unable to fight off even mild infections. 
alcoholic fermentation The process by which yeasts produce alcohol as a byproduct of glycolysis they perform in an oxygenless environment.
algal bloom An overabundance of algae in a lake as a result of an excess of nutrients. The many dead algae that fall to the bottom allow decomposing bacteria to flourish, using up so much oxygen that fish can suffocate.
alimentary canal Another term for the digestive tract, the muscular tube that runs through the body from mouth to anus and through which food passes.
alkaline Basic, as in solutions. Alkaline (basic) solutions have numbers above 7 on the pH scale.
allele One of the alternative forms of a single gene. In pea plants, a single gene codes for seed color, and it comes in two alleles: one codes for yellow seeds, the other for green seeds.
allergen A foreign substance that triggers an allergic reaction.These substances are usually derived from living things, including pollen, dust mites, foods, and fur.
allergies Immune system overreactions to antigens. 
allopatric speciation The type of speciation that occurs when a population is divided by geographical barriers and the resulting populations become separate species. This type of speciation is thought to be the most common in nature.
allosteric Denoting control of a chemical reaction through the effects of a second binding site on an enzyme. Allosteric reactions are important chiefly in negative feedback, when the product of a reaction binds to the enzyme that hastened the reaction, thereby changing the enzyme's shape and temporarily halting its ability to bind to more substrate.
alpha helix A common secondary structure of proteins that resembles a corkscrew or spiral staircase. Hair, nails, and horns are proteins made up almost entirely of alpha helices.