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Definition of CARBON CYCLE
: the cycle of carbon in the earth's ecosystems in which carbon dioxide is fixed by photosynthetic organisms to form organic nutrients and is ultimately restored to the inorganic state (as by respiration, protoplasmic decay, or combustion)
: a cycle of thermonuclear reactions in which four hydrogen atoms synthesize into a helium atom by the catalytic action of carbon with the release of nuclear energy and which is held to be the source of most of the energy radiated by the sun and stars
: the cycle of carbon in living beings in which carbon dioxide is fixed by photosynthesis to form organic nutrients and is ultimately restored to the inorganic state by respiration and protoplasmic decay
Circulation through nature of carbon in the form of the simple element and its compounds. The source of carbon in living things is carbon dioxide (CO) from air or dissolved in water. Algae and green plants (producers) use CO in photosynthesis to make carbohydrates, which in turn are used in the processes of metabolism to make all other compounds in their tissues and those of animals that consume them. The carbon may pass through several levels of herbivores and carnivores (consumers). Animals and, at night, plants return the CO to the atmosphere as a by-product of respiration. The carbon in animal wastes and in the bodies of organisms is released as CO in a series of steps by decay organisms (decomposers), chiefly bacteria and fungi (seefungus). Some organic carbon (the remains of organisms) has accumulated in Earth's crust in fossil fuels, limestone, and coral. The carbon of fossil fuels, removed from the cycle in prehistoric times, is being returned in vast quantities as CO via industrial and agricultural processes, some accumulating in the oceans as dissolved carbonates and some staying in the atmosphere (seegreenhouse effect).