Science / Biology Glossary

Endoplasmic Reticulum (ER): A network of membranous tubules in the cytoplasm of a cell; involved in the production of phospholipids, proteins, and other functions. Rough ER is studded with ribosomes; smooth ER is not.

Endoskeleton: An internal supporting skeleton with muscles on the outside; in vertebrates, consists of the skull, spinal column, ribs, and appendages.

Endosperm: A food storage tissue that provides nutrients to the developing embryo in angiosperms; formed from the triploid cell produced when a sperm cell fertilizes the central cell. Some endosperm is . . . View Full Definition

Endosymbiosis: Theory that attempts to explain the origin of the DNA-containing mitochondria and chloroplasts in early eukaryotes by the engulfing of various types of bacteria that were not digested but be . . . View Full Definition

Endothermic: A reaction that gives off energy. The product is in a lower energy state than the reactants.

Endotherms: Animals that have the ability to maintain a constant body temperature over a wide range of environmental conditions.

Endothermy: The internal control of body temperature; the ability to generate and maintain internal body heat.

Energy: The ability to bring about changes or to do work.

Energy Flow: The movement of energy through a community via feeding relationships.

Energy Of Activation: The minimum amount of energy required for a given reaction to occur; varies from reaction to reaction.

Entropy: The degree of disorder in a system. As energy is transferred from one form to another, some is lost as heat; as the energy decreases, the disorder in the system&emdash;and thus the entropy&e . . . View Full Definition

Enzymes: Protein molecules that act as catalysts in biochemical reactions.

Eon: The longest umit of geological time.

Epidermis: 1. The outermost layer of skin consisting of several layers of epithelial cells&emdash;notably, keratinocytes&emdash;and, in the inner layer of the epidermis, basal cells and melanocytes. 2. . . . View Full Definition

Epididymis: A long, convoluted duct on the testis where sperm are stored.

Epiglottis: A fiap of tissue that closes off the trachea during swallowing.

Epinephrine: A hormone produced by the adrenal medulla and secreted under stress; contributes to the fight or fiight response.

Epistasis: The masking of the effects of one gene by the action of another, example: widows peak masked by the baldness gene.

Epithelial Tissue: Cells in animals that are closely packed in either single or multiple layers, and which cover both internal and external surfaces of the animal body. Also referred to as epithelium.

Epoch: Subdivision of a geological period.

Eras: One of the major divisions of the geologic time scale.

Erythrocytes: Red blood cells; doubly concave, enucleated cells that transport oxygen in the blood.

Esophagus: The muscular tube extending between and connecting the pharynx to the stomach.

Estrogen: A female sex hormone that performs many important functions in reproduction.

Ethylene: A gaseous plant hormone that stimulates fruit ripening and the dropping of leaves.

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