On-Line Biology Book: GLOSSARY

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habitat disruption  A disturbance of the physical environment in which a population lives.

hair bulb  The base of a hair; contains cells that divide mitotically to produce columns of hair cells.

hair root  The portion of a hair that extends from the skin's surface to the hair bulb.

hair shaft  The portion of a hair that extends above the skin's surface.

half-life  The time required for one-half of an original unstable radioactive element to be converted to a more stable daughter element.

halophiles A group of archaebacteria that are able to tolerate high salt concentrations.Medicine
Pharmaceutical
Biochemical
Medical-science

haploid  Cells that contain only one member of each homologous pair of chromosomes (haploid number = n). At fertilization, two haploid gametes fuse to form a single cell with a diploid number of chromosomes.

hardwoods Term applied to dicot trees, as opposed to softwoods, a term applied to gymnosperms.

Haversian canal  The central opening of compact bone; contains nerves and blood vessels. PICTURE

heart  The multicellular, chambered, muscular structure that pumps blood through the circulatory system by alternately contracting and relaxing. PICTURE

heartwood  Inner rings of xylem that have become clogged with metabolic by-products and no longer transport water; visible as the inner darker areas in the cross section of a tree trunk.

helper T cells  A type of lymphocyte that stimulates the production of antibodies by activating B cells when an antigen is present.

hemizygous  Having one or more genes that have no allele counterparts. Usually applied to genes on the male's X chromosome (in humans).

hemoglobin  A red pigment in red blood cells that can bind with oxygen and is largely responsible for the blood's oxygen-carrying capacity. Hemoglobin is composed of four polypeptide chains, two alpha (a) and two beta (b) chains.

hemophilia  A human sex-linked recessive genetic disorder that results in the absence of certain blood-clotting factors, usually Factor VII. Hemophiliacs suffer from an inability to clot their blood.

hepatitis B  A potentially serious viral disease that affects the liver; can be transmitted through sexual contact or through contact with infected blood.

herbaceous Term applied to a nonwoody stem/plant with minimal secondary growt