carbon cycle, the steps by which carbon in the form of carbon dioxide is extracted from and returned to the atmosphere by living organisms, especially human beings. The process starts with the photosynthetic production of carbohydrates by plants, progresses through the consumption of carbohydrates by animals and human beings, and ends with the exhalation of carbon dioxide by those same animals and human beings and with the release of carbon dioxide during the decomposition of dead plants and animals. Various chemical processes intervene between the ingestion of carbohydrates and the release of carbon dioxide. Carbohydrate metabolism starts with the movement of glucose through plasma membranes and subsequently involves glycolysis, the processes of the citric acid cycle, electron transport, and oxidative phosphorylation. See also citric acid cycle.
