The global carbon cycle is made up of the myriad biological and chemical processes on land and in the ocean that remove carbon from the atmosphere, transform carbon among various chemical forms, and release carbon back to the atmosphere. The main form of carbon involved in air-land and air-sea exchanges is carbon dioxide (CO2). Carbon in various forms has pivotal roles in biology, in part due to its unique chemistry, and the global carbon cycle is required for life on Earth. In turn, biological processes are critical to the cycle.
A major component of the carbon cycle is photosynthesis in land plants, in which sunlight drives CO2 assimilation in cellular organelles called chloroplasts. The direct products of the assimilation, which involves about 15% of the CO2 in the atmosphere each year, are sugars. These sugars, and the chemical energy they contain, are the basis of all plant and animal activities on land and within soils. Some of the carbon obtained by plants in photosynthesis, and by animals through eating plants or other animals, is released back to the atmosphere as CO2 through metabolism. Cellular respiration is a key part of that metabolism in both plants and animals. The remainder of the carbon is stored in plants and animals in carbohydrates, proteins, lipids, lignins, and other organic compounds. Six molecules of carbon dioxide, each consisting of one atom of carbon and two of oxygen, combine with six molecules of water, each consisting of two atoms of hydrogen and one of oxygen, to form one molecule of glucose (sugar). The glucose molecule consists of six atoms of carbon, twelve of hydrogen, and six of oxygen. Six oxygen molecules, consisting of two oxygen atoms each, are also produced and are discharged into the atmosphere unless the plant needs energy for its life processes. In that case, the oxygen combines with the glucose immediately at the rate of six molecules of oxygen to one of glucose, releasing six molecules of carbon dioxide and six of water for each oxidized molecule of glucose. The carbon cycle is thereby completed as the plant obtains the energy that was stored by glucose formation. In 2001 it was reported that field experiments extending over several years thus far indicate that forests are not the insatiable sinks for CO2 that some studies of global warming had thought they might be, but the results are not yet the final word on the role that the world's vegetation might play in such warming.
Through excretion, shedding, other processes, and death, the carbon contained in land plants and animals is added to organic-matter "pools" in soils and to litter layers resting on top of soils. These carbon pools serve as food for soil organisms. A by-product of the metabolism of soil organisms is CO2, and that metabolism eventually returns nearly all the carbon contained in the organic matter of soil and litter back to the atmosphere as CO2. Soil organisms, and especially fungi, also form a wide range of complex carbon-containing molecules, called humic acids, that can reside in soils for thousands of years before they are decomposed to CO2.
The exchange of CO2 between air and sea is a process that in large part does not involve life forms, but once the gas is dissolved in the ocean it can be assimilated in photosynthesis by marine plants. Ocean photosynthesis occurs in surface waters where light is available. Marine animals then eat some of the marine plants. Just as respiration in land plants and animals releases CO2 to the atmosphere, respiration in marine plants and animals releases some CO2 back into the waters in which they live. Dead marine organisms, and material excreted from living organisms, contribute to the dissolved organic carbon (DOC) and particulate organic carbon (POC) in the ocean, although the great bulk of ocean carbon is dissolved inorganic carbon. Just as atmospheric CO2 can dissolve in ocean surface waters, dissolved CO2 can return to the atmosphere from surface waters; air-sea exchange is a two-way process. Some ocean DOC and POC is consumed by other marine animals as part of the carbon cycles occurring within the ocean, and some eventually reaches the sea floor as sediment. Calcium carbonate also occurs in the ocean, mediated mainly by marine organisms, and some of that carbonate also reaches the sea floor.
Coal, petroleum, and natural gas may be formed if large amounts of dead plants and animals are shielded from decomposition over millions of years by burial under sediments, where the oxygen required for decomposition is limited. Some existing deposits of such fossil carbon are concentrated enough to be extracted and used by humans for fuel. When these fuels are burned, carbon is returned to the atmosphere as CO2 and once again enters the carbon cycle, after millions of years of immobilization.
Jeffrey S. Amthor
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