Carbon dioxide content is about 0.03 per cent in the atmosphere. Carbon dioxide cycle involves the cycling of carbon, which is a basic component of all the organic compounds that build all the living things. Atmospheric CO2 and dissolved CO2 in the water are the basic sources of carbon. Carbon moves from the atmosphere to producers, to consumers and from both to the decomposers and finally back to the atmosphere. Carbon is also found in water, fossil fuels and sedimentary rocks.
In the first step, CO2 is fixed in photosynthesis, mainly by the green plants. The plants synthesise simple sugar by photosynthesis, which later gets converted into polysaccharides and fats and stored in their tissues. When plants are feed by animals, these are digested and synthesised in other forms. Much of the carbon is returned back to the atmosphere in the form of gas, as a result of respiration in producers and consumers. Carbon from animal excreta, in the protoplasm of plant and animals, is released by bacterial and fungal activities, when they die. CO2 is also released by the burning of fossil fuels, which enters the atmosphere.