Science / Biology Glossary

Annuals: Plants that grow and reproduce sexually during one year.

Antagonistic Muscles: A pair of muscles that work to produce opposite effects&emdash;one contracts as the other relaxes: for example, the bicep and tricep muscles on opposite sides of your upper arm.

Anther: The top of a stamens filament; divided into pollen sacs in which the pollen grains form.

Antibiotic Resistance: Tendency of certain bacteria to develop a resistance to commonly over-used antibiotics.

Antibiotics: Substances produced by some microorganisms, plants, and vertebrates that kill or inhibit the growth of bacteria.

Antibodies: Proteins produced by immune system cells that bind to foreign molecules and microorganisms and inactivate them.

Antibody-Mediated Immunity: Immune reaction that protects primarily against invading viruses and bacteria through antibodies produced by plasma cells; also known as humoral immunity.

Anticodon: A sequence of three nucleotides on the transfer RNA molecule that recognizes and pairs with a specific codon on a messenger RNA molecule; helps control the sequence of amino acids in a growi . . . View Full Definition

Antidiuretic Hormone (ADH): A hormone produced by the hypothalamus and released by the pituitary gland that increases the permeability of the renal tubule of the nephron and thereby increases water reabsorption; also k . . . View Full Definition

Antigenic Determinant: The site on an antigen to which an antibody binds, forming an antigen-antibody complex.

Antigens: Molecules carried or produced by microorganisms that initiate antibody production; mostly proteins or proteins combined with polysaccharides.

Antinutrients: Chemicals produced by plants as a defense mechanism; inhibit the action of digestive enzymes in insects that attack and attempt to eat the plants.

Anus: The posterior opening of the digestive tract.

Aorta: The artery that carries blood from the left ventricle for distribution throughout the tissues of the body. The largest diameter and thickest walled artery in the body.

Apical Meristem: A meristem (embryonic tissue) at the tip of a shoot or root that is responsible for increasing the plants length.

Apnea: A disorder in which breathing stops for periods longer than 10 seconds during sleep; can be caused by failure of the automatic respiratory center to respond to elevated blood levels of carbo . . . View Full Definition

Apocrine Glands: Sweat glands that are located primarily in the armpits and groin area; larger than the more widely distributed eccrine glands.

Appendicular Skeleton: The bones of the appendages (wings, legs, and arms or fins) and of the pelvic and pectoral girdles that join the appendages to the rest of the skeleton; one of the two components of the skel . . . View Full Definition

Appendix: Blind sac at the end of the large intestine that usually ruptures during final exams; a vestigial organ in humans.

Archaea: Proposed, but not widely accepted, sixth taxonomic kingdom that would include the archaebacteria.

Archaebacteria: Ancient (over 3.5 billion years old) group of prokaryotes; some biologists want to place this group into a separate Kingdom, the Archaea. Most currently place it within the Kingdom Monera.

Archaeocyathids: An extinct group of animals that were part of Cambrian-aged reef environments, but which were extinct by the close of the Cambrian Period.

Archean - Proterozoic Era: The period of time beginning 4.6 billion years ago with the formation of the Earth and ending 570 million years ago.

Aridity: The condition of receiving sparse rainfall; associated with cooler climates because cool air can hold less water vapor than warm air. Many deserts occur in relatively warm climates, however, . . . View Full Definition

Arrector Pili: A muscle running from a hair follicle to the dermis. Contraction of the muscle causes the hair to rise perpendicular to the skin surface, forming goose pimples.

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