Respiration

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Note: Many facts have been simplified to make them easier to understand.
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There was another way that bacteria could survive the Great Oxygen Poisoning. Fermentation created waste products, such as acids and alcohols. By letting oxygen slowly burn these molecules some bacteria could make the oxygen safe. They survived. We call them aerobic bacteria, or just aerobes.

They not only survived but prospered. Burning releases energy. The aerobes had discovered a new energy source! Aerobes could get more energy out of their food than anaerobes, and they quickly became the commonest bacteria. So oxygen became a help instead of a poison.
The burning process is called respiration. If you compare this diagram with the one for photosynthesis you should see that the energy which is released by respiration had originally come from the Sun. This is why the Sun is so very important for life. It supplies almost all the energy used by living things, whether bacteria, plants or animals.