CBSE Class 9 Answered

The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of the Earth.

Carbon is found in various forms on Earth. It occurs in elemental form as well as combined state. All life forms are based on carbon -containing molecules.  Plants absorb CO₂ from the atmosphere during photosynthesis to make the substances they need for growth. The process of photosynthesis incorporates the carbon atoms from carbon dioxide into sugars, which are either converted into other substances or used to provide energy.The living organisms release CO₂ back in to the atmosphere during respiration. When the living organisms die the carbon is returned to the soil during decomposition.                                                    

Another major exchange of CO₂ occurs between the oceans and the atmosphere. The dissolved CO₂ in the oceans is used by marine biota in photosynthesis.

Another process that adds to the carbon dioxide in the atmosphere is the process of combustion where fuels are burnt to provide energy.

Thus carbon is cycled repeatedly through different forms by the various physical and biological activities.