Question

why only oxygen is used by us for breaking of glucose

Answer 1

Much less energy is released during anaerobic respiration than during aerobic respiration. This is because the breakdown of glucose is incomplete. Anaerobic respiration produces an oxygen debt. This is the amount of oxygen needed to oxidise lactic acid to carbon dioxide and water.

Answer 2

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Without oxygen, organisms can split glucose into just two molecules of pyruvate. This releases only enough energy to make two ATP molecules. With oxygen, organisms can break down glucose all the way to carbon dioxide.

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