Science / Biology Glossary

Atrioventricular (AV) Node: Tissue in the right ventricle of the heart that receives the impulse from the atria and transmits it through the ventricles by way of the bundles of His and the Purkinje fibers.

Atrioventricular (AV) Valve: The valve between each auricle and ventricle of the heart.

Auricle: The chamber of the heart that receives blood from the body returned to the heart by the veins. Also referred to as atrium.

Autonomic System: The portion of the peripheral nervous system that stimulates smooth muscle, cardiac muscle, and glands; consists of the parasympathetic and sympathetic systems.

Autosomes: The chromosomes other than the sex chromosomes. Each member of an autosome pair (in diploid organisms) is of similar length and in the genes it carries.

Autotrophic: Refers to organisms that synthesize their nutrients and obtain their energy from inorganic raw materials.

Autotrophs: Organisms that synthesize their own nutrients; include some bacteria that are able to synthesize organic molecules from simpler inorganic compounds.

Auxins: A group of hormones involved in controlling plant growth and other functions; once thought responsible for phototropism by causing the cells on the shaded side of a plant to elongate, thereb . . . View Full Definition

Axial Skeleton: The skull, vertebral column, and rib cage; one of the two components of the skeleton in vertebrates.

Axillary Buds: Buds borne in the axil (where the leaf meets the stem) of a stem.

Axons: Long fibers that carry signals away from the cell body of a neuron.

B Cells: Type of lymphocyte responsible for antibody-mediated immunity; mature in the bone marrow and circulate in the circulatory and lymph systems where they transform into antibody-producing plasm . . . View Full Definition

B Memory Cells: Long-lived B cells that are produced after an initial exposure to an antigen and play an important role in secondary immunity. They remain in the body and facilitate a more rapid responce if . . . View Full Definition

Bacteriophages: Viruses that attack and kill bacterial cells; composed only of DNA and protein.

Bark: The outer layer of the stems of woody plants; composed of an outer layer of dead cells (cork) and an inner layer of phloem.

Barr Body: Inactivated X-chromosome in mammalian females. Although inactivated, the Barr body is replicated prior to cell division and thus is passed on to all descendant cells of the embryonic cell th . . . View Full Definition

Basal Body: A structure at the base of a cilium or flagellum; consists of nine triplet microtubules arranged in a circle with no central microtubule.

Base: A substance that lowers the hydrogen ion concentration in a solution.

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