If humans did not breathe in o2, we would not make enough atp to meet our energy requirements have enough enzymes to catalyze reactions be able to synthesize organic molecules from inorganic molecules be able to perform lactic acid fermentation be affected because we can switch to alcoholic fermentation

If humans did not breathe in O2, we would not make enough ATP to meet our energy requirements

Explanation:

During the last step of aerobic cellular respiration, the respiratory chain where oxidative phosphorylation occurs, most of the ATP generated in cellular respiration occurs. This step is coupled with the reoxidation of the NADH and FADH2 molecules, which become NAD + and FAD, respectively. In this reoxidation, high energy electrons are released which, after losing their excess energy, reduce the oxygen gas to water molecules. The energy released gradually by the electrons during their transfer to the oxygen gas is used in the production of ATP.