My biology teachers never explained why animals need to breathe oxygen, just that we organisms die if we don't get oxygen for too long. Maybe one of them happened to mention that its used to make ATP. Now in my AP Biology class we finally learned the specifics of how oxygen is used in the electron transport chain due to its high electronegativity. But I assume this probably isn't the only reason we need oxygen.
What other purposes does the oxygen we take in through respiration serve? Does oxygen deprivation result in death just due to the halting of ATP production, or is there some other reason as well? What percentage of the oxygen we take in through respiration is expelled later through the breath as carbon dioxide?
Answer
Oxygen is actually highly toxic to cells and organisms – reactive oxygen species cause oxidative stress, essentially cell damage and contributing to cell ageing. A lot of anaerobic organisms have never learned to cope with this and die almost immediately when exposed to oxygen. One classical example of this is C. botulinum.
Oxygen is incorporated in several molecules in the cell (for instance riboses and certain amino acids) but as far as I know, all of this comes into the cell as metabolic products, not in the form of pure oxygen.
The oxygen ($\ce{O2}$) we breathe is completely used up during aerobic respiration. The stoichiometry of this is given by the following simplified equation:
$$\ce{C_6H12O6 + 6 O2 -> 6 CO2 + 6 H2O + heat}$$
WYSIWYG’s answer goes into more detail.
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