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The reverse of this process, used by autotrophs (self-feeders), is known as photosynthesis . Respiration, unlike photosynthesis, takes advantage of a natural tendency of oxygen to combine with other molecules. Oxygen is very reactive, and when combining with other substances tends to release large amounts of energy. Thus life forms which employ the process of respiration to run their metabolisms are taking advantage of the most chemically efficient and energetic pathway nature provides to produce energy. It also takes advantage of the large amount of excess oxygen which has accumulated in the atmosphere.

The reverse of this process, used by autotrophs (self-feeders), is known as photosynthesis . Photosynthesis and respiration are balanced processes in the Earth's biosphere. Autotrophs produce the oxygen which heterotrophs need to survive.

It's a good thing that there is so much oxygen in the atmosphere or animal life as we know it would be impossible!