endosperm The tissue, serving as food storage, that surrounds an angiosperm embryo in the seed. The rice and wheat grains that we eat consist mostly of endosperm.

endothelium The innermost layer of blood and lymph vessels, made up of a type of flat epithelial cells. Capillary walls consist of one layer of endothelial cells, and exchange of gases, nutrients, and wastes occurs through these cells and the junctions between them.

endothermic Having a relatively stable internal body temperature and body heat that is generated internally, by the organism’s own metabolism. For example, mammals are endothermic.

energy The capacity to bring about movement against an opposing force.

energy flow model of ecosystems A conceptualization of ecosystems as units in which energy is first captured by given organisms and then transferred to other organisms. Because this capture and transfer can be quantified, the energy-flow model has been of great value in elucidating the workings of ecosystems.

entrain To initiate a new cycle of an organism's internal clock. Many biological rhythms are entrained by such environmental cues as sunlight and temperature.

environmental resistance All the forces in the environment that act to level off the rate of growth and limit population size. Limited food or sunshine, low temperature or rainfall, or predators are some components of environmental resistance.

enzyme A chemically active type of protein that speeds up, or in practical terms enables, chemical reactions in living things.

epicotyl All tissue of an embryonic or seedling plant above the cotyledons. The epicotyl gives rise to the first true leaves.

epidermis The outermost layer of skin in animals, or the outermost cell layer in plants.

epididymis The mass of tubules near the testis in which sperm complete their development and are stored.

epoch A division of a geologic period, which is in turn a division of an era. The Pleistocene is an epoch.

equilibrium species A species whose population size stays relatively stable and at or near its carrying capacity. Also known as a K-selected species.

era A large division of geologic time. The Precambrian, Paleozoic, Mesozoic, and Cenozoic are eras. An era includes several periods.

erythrocyte A blood cell, also known as a red blood cell, that transports oxygen to and carries carbon dioxide from every part of the body.

esophagus The muscular tube that begins at the pharynx and ends at the stomach, forming a portion of the digestive tract.

estrogen A class of hormones, produced by cells of the ovary, that supports egg development, growth of uterine lining, and development of female sex characteristics.

estuary An area where a river flows into the ocean, bringing freshwater and saltwater habitat together. Estuaries are among the most productive ecosystems on Earth.

Eukarya The domain (major group) that includes all organisms whose cells have a nucleus. This group includes plants, animals, fungi, and protists.

eukaryotic cell A cell containing a nucleus that is bound within a thin membrane and contains almost all of the cell’s DNA. Eukaryotic cells have internal specializations, and this allows different cells to specialize in a multicellular organism. Most familiar living things—your pets, your plants, you—are eukaryotes.

eusocial species A species of animal that is organized into a caste system in which there is a division of labor, such that different members of a population will consistently perform different tasks, and in which the young are raised through cooperative care, meaning care provided by many members of the group.

eutrophic Pertaining to a lake that has many nutrients, and therefore sustains many photosynthesizers. Eutrophic lakes often have abundant algal cover, cutting down visibility in the water.

evolution Any genetically based phenotypic change in a population of organisms over successive generations. Evolution can also be thought of as the process by which species of living things can undergo modification over successive generations, with such modification sometimes resulting in the formation of new species. Evolution is of central importance in biology because every living thing has been shaped by evolution.

exergonic Denoting a chemical reaction in which the reactants contain more energy than the products, so energy is released in the reaction. Exergonic reactions provide the energy for other, endergonic processes.