Carbon is found in different forms on the Earth. It is found as carbon dioxide in the atmosphere, as carbonate and bicarbonate salts in various minerals, as well as it occurs in the elemental form of diamonds and graphite. All biotic factors are based on carbon-containing molecules like carbohydrates, proteins, fats, nucleic acids and vitamins. Carbon is incorporated into life-forms through photosynthesis which is performed in the presence of sunlight by all life-forms that contain chlorophyll. Photosynthesis converts CO2 from the atmosphere or dissolved in water into glucose molecules. These molecules are either converted into other substances or used to provide energy for the synthesis of other biologically important molecules.
The use of glucose to provide energy to living things involved the process of respiration in which oxygen may or may not be used to convert glucose back into CO2. This CO2 goes back into the atmosphere again. The process of combustion also adds to the level of carbon dioxide in the atmosphere. This percentage is said to have increased when human beings started cutting down trees, burning fossil fuels, etc. on a very large scale. Thus, carbon is cycled repeatedly through different forms by the various physical and biological activities and this cycle is therefore called carbon cycle.
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