The atmospheric carbon dioxide is virtually the only source of carbon which is the basic constituent of all the organic compounds.
This gas is used by all plants in photosynthesis and the end products (organic substances) of this complex process are used in two ways.
One fraction is used in the construction of more living matter. The carbon and oxygen so supplied by CO2 remain in living matter until death.
Decay subsequently returns CO2 to the atmosphere, and this completes one possible carbon cycle. Another fraction of the organic substances is used as fuel in respiration by both plants and animals.
This process releases CO2 as a by-product. Such CO2 may now be used in photosynthesis again, or it may return to the environment, completing the second possible carbon cycle (Fig. 5.6). Thus, photosynthesis and respiration are the two major processes that drive the global Carbon Cycle, with CO2 as the main vehicle of flux between atmosphere, hydrosphere and biota.
The CO2 content of the atmosphere is replenished not only through biological oxidation (respiration), but also through non-living combustion i.e., forest fires and burning of industrial fuels release CO2 into the air. Occasionally volcanic eruptions also add CO2 to the atmosphere.