Why do we need oxygen?

Inside mitochondria, the powerhouses of cells, oxygen acts as the final electron acceptor in the chain of reactions that produce ATP, the molecule cells use for energy. Without oxygen, cells can make only a small amount of ATP through anaerobic processes, which is why intense exercise leads to muscle fatigue. Aerobic respiration can yield around 30 ATP molecules per glucose molecule, compared with 2 from anaerobic glycolysis. The brain is especially sensitive: it uses around 20 percent of the body’s oxygen, and brain cells begin to die after a few minutes without it. Air is about 21 percent oxygen.