Carbon is important constituents of all organic compounds. It forms 49% of dry weight of the organisms. Earth atmosphere contains 0.03% carbon dioxide gas.
The carbon cycle involves exchange of carbon between biosphere, lithosphere, hydrosphere and atmosphere of earth. This cycle involves:
(i) Carbon dioxide is taken from atmosphere by green plants which in presence of sunlight convert it into carbohydrate by process of photosynthesis.
(ii) Afterwards carbon moves along the food chain through consumers (herbivores, carnivores) at different trophic level.
(iii) Carbon return to the atmosphere in a no. of ways:
(a) Both animals and plants respire and release carbon -dioxide back into the atmosphere.
(b) Bacteria and fungi break down the dead animals and plants into carbon dioxide.
(c) Combustion of fossil fuels like coal, petroleum into carbon dioxide.
(d) Volcanic eruption.
This is how carbon cycle goes on forever.