why we breathe oxygen and not something like carbon dioxide or nitrogen?
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Different molecules have different tendencies to react with other molecules. Oxygen is a fairly high-energy substance that is happy to break apart and attach to other things. Carbon dioxide is a lower-energy substance that doesn’t usually want to break apart, you need to put energy *into* it to convince carbon dioxide to break apart.
So the other comment is partly right – this is because we evolved to use oxygen. But it’s also because oxygen is inherently more suitable to *burn for energy* in the body, just like oxygen is necessary if you want a fire to burn, and spraying a bunch of carbon dioxide at a fire is actually a strategy that can be used to extinguish a fire.
Could we have evolved to function without oxygen? Probably not in our current form, but bacteria and other forms of life *do* function without needing oxygen. But would we “breathe” carbon dioxide or nitrogen? Probably not. We would have evolved to get energy without “breathing” the way we do now.
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