Living things are the most important part of the carbon cycle.  Today, the atmosphere is only 0.004% carbon.  Photosynthesis and respiration occur thanks to carbon.  Here are the equations for each:

Photosynthesis: H2O + CO2 + Energy à C6H12O6 + O2

Respiration: C6H12O6 + O2 à H2O + CO2 + Energy

Each reaction uses the product of the other.  Plants use CO2  and energy (from the sun) during photosynthesis in order to make sugars and starches.  During photosynthesis plants take in CO2 and in return release oxygen.  Other organisms that use cellular respiration take in the oxygen and in return put out carbon. CO2 is also returned into the air when dead plants decay.

There are also two other sources of carbon.  They are oceans and rocks.  The oceans hold a large amount of CO2 because CO2 dissolves easily in water.  Carbon is stored in rocks like coal, oil, and limestone.  These rocks were formed from the bodies of dead animals.  Since the bodies of these organisms never completely decayed some carbon remained in them.  This CO2 is released into the air when we burn fossil fuels for energy.