
The oxygen cycle is the biogeochemical cycle that describes the movement of oxygen within its three main reservoirs: the atmosphere (air), the total content of biological matter within the biosphere (the global sum of all ecosystems), and the lithosphere (Earth`s crust). Failures in the oxygen cycle within the hydrosphere (the combined mass of wa....
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http://en.wikipedia.org/wiki/Oxygen_cycle

circulation of oxygen in various forms through nature. Free in the air and dissolved in water, oxygen is second only to nitrogen in abundance among ... [1 related articles]
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http://www.britannica.com/eb/a-z/o/43

Cyclic movement of oxygen in different chemical forms from the environment, to organisms, and then back to the environment.
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the continuous movement of oxygen. Oxygen is a major and vital component of all living matter. Most comes from the process of photosynthesis carried out by plants
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http://www.epaw.co.uk/EPT/glossary.html

the process by which oxygen released into the atmosphere by photosynthetic organisms is taken up by aerobic organisms while the carbon dioxide released as a by-product of repiration is taken up for photosynthesis.
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