The cycling of oxygen between the biotic and abiotic components of the environment (see biogeochemical cycle). The oxygen cycle is closely linked to the carbon cycle and the water cycle (see hydrological cycle). In the process of respiration oxygen is taken in by living organisms and released into the atmosphere, combined with carbon, in the form of carbon dioxide. Carbon dioxide enters the carbon cycle or is taken up by plants for photosynthesis. During photosynthesis oxygen is evolved by the chemical splitting of water and returned to the atmosphere. In the upper atmosphere, ozone is formed from oxygen and dissociates to release oxygen (see also ozone layer).