Carbon is required in large amount as a basic building block for all organic matter. The ultimate source of carbon is carbon dioxide, converted to organic matter in the photosynthesis. The basic movement of carbon is from the atmospheric reservoir to producers, to consumers and from both these groups to the decomposers and then back to the reservoir. In atmosphere, concentration of CO2 is 320 ppm. CO2 is removed by photosynthesis and returned by respiration. As a result of respiration, CO2 level near the ground may rise to 0.05 % at night and drop to well below the average concentration of 0.032% during the day as CO2 is taken up by photosynthesis.
Carbon cycle is actually carbon – hydrogen - oxygen cycle or carbon dioxide -water- oxygen cycle. Plants and animals respire CO2, all the time with the help of atmospheric oxygen for getting energy from the reverse food i.e. carbohydrates, fats and proteins. Oxygen is replenished by the process of photosynthesis. This dissolved CO2 combines with water in the soil or in the aquatic ecosystem to form carbonic acid in a reversible reaction. In turn carbonic acid dissociates in a reversible reaction into hydrogen and bicarbonate ions HCO3. The later ion, in turn dissociates in another reversible reaction into hydrogen and carbonate ions.
Atmospheric CO2.
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Dissolved CO2 + H2O === H2CO3 === H+ + HCO3- === H+ + CO3-
Because all these reactions are reversible, the direction of the reaction depends on the concentration of critical components. Assimilation of the bicarbonate ion in photo-synthesis by aquatic plants would tend to shift the equilibrium.
The green plants in both aquatic and terrestrial ecosystems breakup water into hydrogen and oxygen in the process of photosynthesis.
6 CO2 + 12 H2O ------------ C6H12O6 + 6O2 + 6 H2O
When sugars are broken up in respiration and in the process of decay and fermentation, hydrogen atoms are oxidized to water again.
C6H12O6 + 6 O2 ---------- 6CO2 + 6H2O + energy
This is how cyclic circulation of oxygen and hydrogen takes place in the ecosystem.