why do we need the carbon cycle

Dunnette, David A. , and Robert J. O Brien, eds. The Science of Global Change: The Impact of Human Activities on the Environment. Washington, DC: American Chemical Society, 1992. Hamblin, W. K. , and E. H. Christiansen. Earth s Dynamic Systems. 9th ed. Upper Saddle River: Prentice Hall, 2001. Levi, Barbara Gross, David Hafemeister, and Richard Scribner. Global Warming: Physics and Facts. New York: American Institute of Physics, 1992. Matthews, John A. , E. M. Bridges, and Christopher J. Caseldine. The Encyclopaedic Dictionary of Environmental Change.

New York: Edward Arnold, 2001. Tolbert, N. E. , and Jack Preiss, eds. Regulation of Atmospheric Carbon by Photosynthetic Carbon Metabolism. New York: Oxford University Press, 1994.
Most of the chemicals that make up living tissue contain carbon. When organisms die the carbon is recycled so that it can be used by future generations. The model that describes the processes involved is called the carbon cycle. Carbon enters the atmosphere as carbon dioxide from respiration and combustion.

Carbon dioxide is absorbed by producers to make carbohydrates in photosynthesis. Animals feed on the plant passing the carbon compounds along the food chain. Most of the carbon they consume is exhaled as carbon dioxide formed during respiration. The animals and plants eventually die. The dead organisms are eaten by decomposers and the carbon in their bodies is returned to the atmosphere as carbon dioxide. In some conditions decomposition is blocked. The plant and animal material may then be available as fossil fuel in the future for combustion.

In the sea, marine animals may convert some of the carbon in their diet to calcium carbonate which is used to make their shells. Over time the shells of dead organisms collect on the seabed and form limestone. Due to Earth movements this limestone may eventually become exposed to the air where it's weathered and the carbon is released back into the atmosphere as carbon dioxide. Volcanic action may also release carbon dioxide.

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