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The biogeochemical cycle that describes the exchange of carbon in the various 'spheres' (e.g. biosphere) of the earth is aptly called the carbon cycle (much like the water cycle or the nitrogen cycle).
[Attached is an image that briefly illustrates the cycle]
Carbon cycle involves the following:
- atmosphere - this contains, usually carbon in the form of carbon dioxide and methane, both contribute to the ability of the atmosphere to retain heat, thus maintain temperature of the earth. Carbon dioxide is the predominant carbon-containing gas in the atmosphere. It can either dissolve in the ocean and water bodies, or enter plants through photosynthesis.
- terrestrial biosphere - this includes a large family of carbon-containing compounds found in land dwelling species. For instance, the carbon compounds, such as lipids, and others, in humans, or glucose synthesized by plants. It enters the biosphere through photosynthesis for autotrophs (plants), and the other animals (e.g. humans, heterotrophs) through digestion of plants. It leaves the biosphere in a lot of ways, one of which is respiration, where it is returned to the atmosphere. It may also be stored in the soil (e.g. fossils).
- oceans and bodies of water- a lot of dissolved carbon compounds are found in bodies of water, including amounts of dissolved carbon dioxide. Other dissolved inorganic compounds are also found in the deep layers of oceans. It enters this area of the earth, as mentioned, via dissolution on the surface of the ocean (carbon dioxide), which is then converted to carbonates. It can also enter the ocean as dissolved organic compounds through various bodies such as rivers. Decomposition of dead animals in the deep part of the oceans also contribute to the carbon content of oceans.
At this point, we can see a rather complicated cycle for carbon. [It's more complicated than the water cycle, because carbon is incorporated into various compounds and is not just converted into a different state like water]. If we start with the atmosphere, we can see that carbon dioxide can either go to the biosphere through photosynthesis (carbon dioxide to carbon compounds such as glucose) or to the ocean (via dissolution on the surface). On the biosphere, carbon is transferred from organism to organism through digestion and feeding on each other (e.g. plant eaten by man...). At this point, it can exit the biosphere through respiration, and back to the atmosphere, or through decomposition and into the soil (where it will probably be stored for a longer time -- which could also eventually go back to the atmosphere via microbial respiration). It can also move from the biosphere to the ocean (via river transport, among other ways). Let's move to the dissolved carbon dioxide in the ocean. This can go back to the atmosphere via a continuous gas-exchange between the atmosphere and ocean bodies. Or it can also be stored in the deep ocean (as carbonate, or inorganic compounds).
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