The Carbon Cycle Carbon—the element that defines life—enters the biota through photosynthesis while the oxygen released in the process makes possible aerobic respiration of all living things. Molecules that contain carbon are the major constituents of living tissues, but the amount of carbon in active biosphere cycling is minor compared to the amount held in abiotic reservoirs such as sedimentary rocks, fossil fuel deposits, and deep sea sediments. Respiration and photosynthesis are the driving forces of the carbon cycle. Carbon enters the biosphere as atmospheric carbon dioxide (CO 2 ), which is incorporated by photosynthetic organisms into carbohydrates. It leaves, also as CO 2 , through the respiration of organisms. The carbon cycle is thus bound with that of oxygen and, through oxidation-reduction reactions, to other elements of importance to organisms. Removal of CO
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