© 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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