Atmospheres contain about 21 percentage of oxygen. Almost all living organisms need oxygen. They use oxygen during the process of creating energy or process of photosynthesis. Just as water moves from sky to earth and back into hydrological cycle, oxygen is also cycled through the environment. Plants mark the beginning of oxygen cycle. Plant produce oxygen during the process of photosynthesis. Humans and animals are form the other half of the oxygen cycle, they breath oxygen. So the oxygen created in plants are used up by humans and animals. There is continuous exchange of oxygen between the atmosphere and all water surface on the earth. The total amount of oxygen in the biosphere is relatively constant, so that the oxygen cycle may get stable.