All living things- plants and animals are made of organic compounds of which carbon is the essential element. In the atmosphere, carbon occurs as carbon dioxide. Green plants take carbon dioxide from the atmosphere and synthesize various organic compounds called carbon compounds. Herbivorous animals eat plants and the organic compounds of the plants then become part of animal tissues. Small carnivorous animals eat the herbivorous animals and thus, the organic compounds are transferred into their tissues. From small carnivorous animals, the organic compounds are transferred into the tissues of large carnivorous animals which feed upon the former. Some of the carbon withdrawn by photosynthesis goes back to the atmosphere when the living things respire. A large portion of carbon goes back as carbon dioxide when the decomposers (decay bacteria and fungi) decompose the dead bodies and excreta of the living things. Under certain conditions, the dead bodies undergo partial decomposition and form coal in the case of plants, and natural gas and petroleum mostly in the case of marine organisms. These products of decomposition convert carbon into carbon dioxide, on burning, and return that to the atmosphere.
Some rocks (for example- limestone and marble) contain carbon as carbonates. At some appropriate time, when suitable conditions have attained the carbonates of these rocks generate carbon dioxide under the action of acids formed and excreted by soil micro-organisms and plant roots.
