The oxygen cycle is a biogeochemical cycle in which oxygen moves in the biosphere among the lithosphere (land), hydrosphere (water) and atmosphere (air). It is an essential cycle in maintaining a constant level of oxygen in the atmosphere and is one of the main processes that allow the existence of life.
This movement includes the conversion of atmospheric oxygen into carbon dioxide by animal respiration and then back into oxygen by green plants during photosynthesis. Some oxygen is also produced when the ultraviolet radiation in sunlight reacts with water vapor in the atmosphere. The oxygen cycle and the carbon cycle are dependent on each other because the latter releases oxygen, mainly by plants, for use in the oxygen cycle, and the former emits carbon dioxide, largely by animals, that returns to the carbon cycle.