Oxygen’s key role is to function as a terminal electron acceptor. In other words, it is like the positive terminal on a battery. Food (like carbohydrates) function as the negative terminal. Electrons flow from food to oxygen, and cells capture some of this energy along the way, similar to how a light bulb captures the energy in an electrical circuit. In this regards oxygen was the better method for us over chlorine, fluorine, nitrogen or sulphur which also store energy but not as well.
Oxygen levels are still tightly controlled in your cells and tissues in order to prevent oxidative damage. One of the most abundant proteins in your blood, catalase, serves to reverse the oxidation of water to hydrogen peroxide that would otherwise quickly kill you. We still need carbon and hydrogen and life can form with less oxygen than we need … but it wouldn’t be us.
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