Oxygen Cycle

© Marco Bleeker, 1995-1998

The carbon and oxygen cycles are closely related, because they are directly associated with photosynthesis and respiration processes.  Oxygen constitutes about 21% of the atmosphere.  In free form, oxygen (O2) is essential for aerobic respiration (eg by animals),  where it is returned to the atmosphere as carbon-dioxide.   Oxygen is also a by-product of photosynthesis.   The oxygen liberated from photosynthesis comes from the water molecules used in the process.   The carbon-dioxide in the process does not liberate oxygen but is turned into carbohydrate for the plant's use.


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