Figure 18. - The carbon cycle, one of the most important to man, is also known as the carbon dioxide cycle. It can also be considered an oxygen cycle. In this illustration, the biological fixing of carbon dioxide by green plants (photosynthesis) and the transfer of organic carbon to Consumers is shown by green arrows; the return of carbon to the atmosphere (combustion, oxidation, decomposition, respiration) is shown in red. The passive transfer of carbon dioxide between atmosphere and oceans is shown in blue.