Cyanobacteria, into larger bacterial cells led to their permanent incorporation as forerunners of the plastids (e.g. chloro-plasts) seen in modern eukaryotes. Similarly, other symbiotic aerobic bacteria gave rise to the mitochondria. Chloroplasts and mitochondria contain some circular DNA and small ribosomes, both more similar to those of bacteria than those of eukaryotes, and a double membrane envelope. The chloroplasts of some groups of algae have a triple envelope, suggesting more than one symbiotic event. See Bacteria; eukaryote; symbiosis.
endotherm An animal that can generate heat internally to raise its body temperature, e.g. birds and mammals. See homoio-thermy. Compare ectotherm.
energetic efficiency The ratio of energy stored or useful work done to energy intake.
hydroelectric and other renewable sources hydroelectric and other renewable sources
energy The capacity for doing work. Energy has several forms, some of which are convertible; they include chemical energy (energy stored in chemical compounds), electromagnetic energy, potential energy (the capability of doing work), kinetic energy (movement), mechanical energy, radiant energy (light), and thermal energy (heat). Energy is measured in joules.
energy budget The comparison between the energy entering and the energy leaving the body of an animal, a particular trophic level, or an industrial process. It is an indication of the efficiency of energy use. See also production efficiency.
energy conservation The strategy for reducing the energy usage by individuals, households, industrial units, or modes of transport. For example, the reduction in energy use for heating by installing insulation, double-glazing, and energy-efficient boilers and the use of low-energy lighting. The strategy can also be applied to the unnecessary use and discarding of objects that use energy in their manufacture, such as plastic bags.
energy efficiency The use of processes and fuel-consuming devices designed to ensure that they make efficient use of energy: that the energy input is low in relation to energy output. For example, the introduction of condensing boilers for heating, combined heat and power plants that burn waste, low-energy light bulbs, and the development of vehicles that make efficient use of fuel. See also energy budget.
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