On-Line Biology Book: GLOSSARY

D

dark reactions  The photosynthetic process in which food (sugar) molecules are formed from carbon dioxide from the atmosphere with the use of ATP; can occur in the dark as long as ATP is present.

death rate  The ratio between deaths and individuals in a specified population at a particular time.

decay series Most radioisotopes do not decay into a stable daughter element in one single decay, but rather through a series of radioactive intermediaries.

deciduous Term applied to trees that lose the leaves and have a dormancy period at least once per year.

deletion  The loss of a chromosome segment without altering the number of chromosomes.

dendrites  Short, highly branched fibers that carry signals toward the cell body of a neuron. PICTURE

dendrochronology  The process of determining the age of a tree or wood used in structures by counting the number of annual growth rings.

deoxyribonucleic acid (DNA)  A nucleic acid composed of two polynucleotide strands wound around a central axis to form a double helix; the repository of genetic information. Nucleic acid that functions as the physical carrier of inheritance for 99% of all species. The molecule is double-stranded and composed of two strands in an antiparallel and complementary arrangement. The basic unit, the nucleotide, consists of a molecule of deoxyribose sugar, a phosphate group, and one of four nitrogenous bases. PICTURE 1 PICTURE 2

deoxyribose Five-carbon sugar found in nucleotides of DNA. PICTURE

depth diversity gradient  The increase in species richness with increasing water depth until about 2000 meters below the surface, where species richness begins to decline.

dermal system Plant organ system that provides the covering for the plant.

dermis  One of the two layers of skin; a connective tissue layer under the epidermis containing elastic and collagen fibers, capillary networks, and nerve endings.

desert biome  Characterized by dry conditions and plants and animals that have adapted to those conditions; found in areas where local or global influences block rainfall.

desmosome  A circular region of membrane cemented to an adjacent membrane by a molecular glue made of polysaccharides; found in tissues that undergo stretching.

deuterostomes  Animals in which the first opening that appears in the embryo becomes the anus while the mouth appears at the other end of the digestive system. Main groups include chordates and echinoderms.

Devonian Period of geologic time from 410 - 360 million years before the present. Life on land diversified, with the amphibians appearing late in this period. Plants underwent major changes, including the development of forests and seeds. In the water, fish diversified into all modern groups as well as numerous now-extinct forms.

diabetes mellitus, Types I and II  A disorder associated with defects in insulin action. Type I diabetes is characterized by inadequate insulin secretion; Type II diabetes is characterized by impaired insulin secretion in response to elevated blood glucose levels or by loss of sensitivity to insulin by target cells.

diaphragm  A dome-shaped muscle that separates the thoracic and abdominal cavities.

diastole  The filling of the ventricle of the heart with blood. PICTURE

diatomaceous earth  Fossilized deposits of diatoms; used for abrasives, polishes and as a filtering agent.

dicots  One of the two main types of flowering plants; characterized by having two cotyledons, floral organs arranged in cycles of four or five, and leaves with reticulate veins; include trees (except conifers) and most ornamental and crop plants. PICTURE