You could say photosynthesis, which gives O 2 into the athmosphere, and various burns (like the slow burns in metabolism or actual fires) which make it go into C O 2 .
The two major processes in the oxygen cycle are photosynthesis and respiration. Photosynthesis produces oxygen from carbon dioxide and water, while respiration uses oxygen to release energy from glucose. Together, these processes maintain the oxygen levels in the environment.
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