Water is one of the end products of aerobic respiration. what is the source of the oxygen atom used in formation of the water?

Hello! 

The source of the oxygen atom used in the formation of the water in aerobic respiration is molecular oxygen (O₂).

The general chemical equation for Aerobic Respiration ("Aerobic" means "in the presence of oxygen") is:

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

C₆H₁₂O₆ is Glucose, a common sugar. Aerobic Respiration occurs in a series of steps, the last one being called "Oxidative Phosphorylation". In this last step, molecular oxygen (O₂) from the air is used to generate energy and H₂O. 

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