apical dominance Suppression of the growth of the lateral branches of a plant through the activity of apical meristems.
apical meristem The group of plant cells at the tips of the roots and shoots that are able to keep dividing, and from which all tissues ultimately develop.
appendicular skeleton The division of the skeletal system consisting of the bones of the paired appendages, including the pelvic and pectoral girdles to which they are attached.
aqueous solution A solution in which water is the solvent. Because living things contain so much water, nearly all solutions important in biological systems are aqueous solutions.
aquifer The porous underground rock in which groundwater is stored.
Archaea The domain (major group) that contains only microscopic, single-celled organisms superficially similar to bacteria but genetically quite different. Many of the Archaea live in extreme environments such as boiling-hot vents on the ocean floor.
arithmetical increase An increase in numbers by an addition of a set number in each time period. The number added does not depend on the number already in existence, and natural populations do not show this type of increase.
arteriole The smallest type of artery, which branches into the capillary beds, where gases and nutrients are delivered to tissues.
artery A blood vessel that carries blood away from the heart.
artificial eutrophication Nutrient enrichment of a lake through human intervention. This can be intentional, or it can occur accidentally when, for example, fertilizers from lawns or agriculture are washed into the lake.
asexual reproduction Reproduction that occurs without the union of two reproductive cells (sexual reproduction). Offspring of asexual reproduction are genetically identical to their parents.
assortative mating Mating in which males and females that share a particular characteristic, such as short height, tend to mate with one another. Such mating tends to bring similar alleles together but does not directly alter allele frequencies in the population.
atmosphere The layer of gases that surround the Earth. The atmosphere is nonliving, but its presence enables life to exist on Earth.
atom A unit of matter, once thought to be the smallest particle in nature but now known to be made up of many smaller particles including electrons, and protons and neutrons which are in turn made up of quarks.
atomic number The number of protons in the nucleus of an atom. This number defines the element, so gold is gold because it has an atomic number of 79. The elements are ordered on the periodic table according to atomic number.
atomic weight The combined weight (mass) of protons and neutrons in an atom¡¯s nucleus.
ATP synthase The enzyme that enables the movement of hydrogen ions back across the mitochondrial inner membrane to power the production of ATP in the final process of the electron transport chain. The hydrogen ions flow through the enzyme, causing part of the enzyme to spin, thus driving the synthesis of ATP from ADP and phosphate.
autonomic nervous system A subdivision of the efferent division of the peripheral nervous system that provides involuntary regulation of the smooth muscles, cardiac muscles, and glands.
autosomal dominant disorder A dominant genetic disorder caused by a faulty allele that lies on an autosomal chromosome.
autosomal recessive disorder A disorder in which the gene or genes lie on one or more autosomes, rather than on the sex chromosomes, and two recessive, nonfunctional alleles are present. Sickle-cell anemia is one example.
autotroph Any organism that manufactures its own food. Plants are the most important autotrophs on Earth.
axial skeleton The division of the skeletal system that forms the central column, including the skull, vertebral column, and rib cage.
axon A single, large extension of the cell body of a neuron that carries signals away from the cell body toward other cells. The axons of some neurons in humans may be several feet long.