eccrine glands Sweat glands that are linked to the sympathetic nervous system and are widely distributed over the body surface.
ecological niche The role an organism occupies and the function it performs in an ecosystem; closely associated with feeding.
ecological time A timescale that focuses on community events that occur on the order of tens to hundreds of years.
ecology The study of how organisms interact with each other and their physical environment.
ecosystem The community living in an area and its physical environment.
ecotones Well-deÞned boundaries typical of closed communities.
ecotype A subdivision of a species; a stage in the formation of a species such that reproductive isolation has occurred.
ectoderm The outer layer of cells in embryonic development; gives rise to the skin, brain, and nervous system. Also, the outermost tissue layer in þatworms.
ectotherms Animals with a variable body temperature that is determined by the environment. Examples: fish, frogs, and reptiles.
effector In a closed system, the element that initiates an action in response to a signal from a sensor. In human systems, a muscle or gland often serves as an effector.
ejaculatory duct In males, a short duct that connects the vas deferens from each testis to the urethra.
electron A subatomic particle with a negative charge. Electrons circle the atom's nucleus in regions of space known as orbitals.
electron acceptor A molecule that forms part of the electron transport system that transfers electrons ejected by chlorophyll during photosynthesis. Part of the energy carried by the electrons is transferred to ATP, part is transferred to NADPH, and part is lost in the transfer system.
electron transport 1) A series of coupled oxidation/reduction reactions where electrons are passed like hot potatoes from one membrane-bound protein/enzyme to another before being finally attached to a terminal electron acceptor (usually oxygen or NADPH). ATP is formed by this process. 2) coupled series of oxidation/reduction reactions during which ATP is generated by energy transfer as electrons move from high reducing state to lower reducing state. PICTURE 1 | PICTURE 2
electrostatic attraction The attraction between atoms of opposite charge that holds the atoms together in ionic bonds.
element A substance composed of atoms with the same atomic number; cannot be broken down in ordinary chemical reactions.
elongation During protein synthesis, the growth of the polypeptide chain through the addition of amino acids; the second step in translation. PICTURE 1 | PICTURE 2
embryo Term applied to the zygote after the beginning of mitosis that produces a multicellular structure.
embryo sac Alternate term applied to the angiosperm female gametophyte contained within a megaspore. PICTURE
emphysema Lung disease characterized by shortness of breath, often associated with smoking.
endergonic Chemical reactions that require energy input to begin.
endochondral ossification The process by which human bones form from cartilage.
endocrine system One of eleven major body organ systems in animals; a system of glands that works with the nervous system in controlling the activity of internal organs, especially the kidneys, and in coordinating the long-range response to external stimuli. PICTURE
endocytosis The incorporation of materials from outside the cell by the formation of vesicles in the plasma membrane. The vesicles surround the material so the cell can engulf it.
endoderm The inner layer of cells in embryonic development that gives rise to organs and tissues associated with digestion and respiration. Also, the inner tissue layer in þatworms.
endodermis A layer of cells surrounding the vascular cylinder of plants. PICTURE
endometrium The inner lining of the uterus. PICTURE
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. PICTURE
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 solid (as in corn), some is liquid (as in coconut). PICTURE
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 became permanent additions to the ancestral "eukaryote". PICTURE
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. PICTURE
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&emdash;increases.
enzymes Protein molecules that act as catalysts in biochemical reactions. PICTURE
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. The outer layer of cells in the plant body, often covered by a waxy cuticle.
epididymis A long, convoluted duct on the testis where sperm are stored. PICTURE
epiglottis A þap of tissue that closes off the trachea during swallowing.
epinephrine A hormone produced by the adrenal medulla and secreted under stress; contributes to the "Þght or þight" response.
epistasis The masking of the effects of one gene by the action of another, example: widow's 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. PICTURE
eras One of the major divisions of the geologic time scale. PICTURE
erythrocytes Red blood cells; doubly concave, enucleated cells that transport oxygen in the blood. PICTURE
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.
Eubacteria The subunit of the Monera that includes the true bacteria such as E. coli. One of the three major groups of prokaryotes in the Kingdom Monera. The eubacteria have cell walls containing peptidoglycan.
Euglenoids Term applied to a division of protozoans that have one long flagellum, no cell wall, and which may have chloroplasts.
eukaryote A type of cell found in many organisms including single-celled protists and multicellular fungi, plants, and animals; characterized by a membrane-bounded nucleus and other membraneous organelles; an organism composed of such cells. The first eukaryotes are encountered in rocks approximately 1.2-1.5 billion years old. PICTURE
euphotic zone The upper part of the marine biome where light penetrates and photosynthesis occurs; usually extends to about 200 meters below the water surface.
eutrophication "Runaway" growth of aquatic plants that occurs when agricultural fertilizers containing phosphorus and nitrogen run off into lakes and ponds; also ultimately increases the plant death rate with the result that the bacterial decomposition of the dead plants uses up oxygen, causing Þsh and other organisms to suffocate.
evaporation The part of the hydrologic cycle in which liquid water is converted to vapor and enters the atmosphere.
evolution 1) The change in life over time by adaptation, variation, over-reproduction, and differential survival/reproduction, a process referred to by Charles Darwin and Alfred Wallace as natural selection. 2) Descent with modification.
evolutionary tree A diagram showing the evolutionary history of organisms based on differences in amino acid sequences. Organisms with fewer differences are placed closer together while those with more differences are further apart. PICTURE
excretion The process of removing the waste products of cellular metabolism from the body.
excretory system One of eleven major body systems in animals; regulates the volume and molecular and ionic constitution of internal body þuids and eliminates metabolic waste products from the internal environment.
exine Outer covering of pollen grains, often containing sporopollenin, an acid-resistant polysaccharide that allows pollen grains to become fossils.
exocytosis The process in which a membrane-enclosed vesicle Þrst fuses with the plasma membrane and then opens and releases its contents to the outside.
exon The DNA bases that code for an amino acid sequence. Exons are separated by introns that code for no amino acid sequences. PICTURE
exoskeleton A hard, jointed, external covering that encloses the muscles and organs of an organism; typical of many arthropods including insects.
exothermic A reaction where the product is at a higher energy level than the reactants.
exponential rate An extremely rapid increase, e.g., in the rate of population growth.
expression In relation to genes, the phenotypic manifestation of a trait. Expression may be age-dependent (e.g., Huntington disease) or affected by environmental factors (e.g., dark fur on Siamese cats).
extinction The elimination of all individuals in a group, both by natural (dinosaurs, trilobites) and human-induced (dodo, passenger pigeon, liberals [:)]) means.
extracellular digestion A form of digestion found in annelids, crustaceans, and chordates including vertebrates; takes place within the lumen of the digestive system, and the resulting nutrient molecules are transferred into the blood or body þuid.
extracellular route Path taken by water through the root in which water moves through the spaces between cell walls of the cortex parenchyma. PICTURE
eyespot 1. A pigmented photoreceptor in euglenoids. The eyespot senses light and orients the cell for maximum rates of photosynthesis. PICTURE 2. Term applied to a photosenstive area in starfish.
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