Water is the universal medium for life on earth, but the carbon forms the building block of life on earth. Being the abundant element in the universe, it is unparalleled in its ability to form molecules that are large, complex, and varied, making possible the diversity of organisms that have evolved on earth.
In a nonliving environment, carbon can exist as carbon dioxide (CO2), carbonate rocks, coal, petroleum, natural gas, and dead organic matter. On earth, carbon circulates through the land, ocean, and atmosphere, creating what is known as the carbon cycle. This global carbon cycle can be divided further into two separate cycles: the geological carbon cycles takes place over millions of years, whereas the biological or physical carbon cycle takes place from days to thousands of years.
In biological carbon cycle, the carbon enters the biosphere through the action of plants. Plants use solar energy and water to transform atmospheric CO2 to its own glucose and oxygen gas. The glucose is used for chemical energy, which the plant metabolizes. In its metabolism of food and respiration, plant eating animal consumes glucose (C6H12O6), which combines with oxygen (O2) to produce carbon dioxide (CO2), water (H2O),and energy (which is given off as heat). The animal has no need for the carbon dioxide and thus released it into the atmosphere. And the cycle continues with plant by the process of photosynthesis.