# During aerobic respiration. H_2O is formed. Where does the oxygen atom for the formation of the water come from?

All that can be said is that this oxygen atom was originally one of the atoms in the reactants of the process (the ${C}_{2} {H}_{12} {O}_{6}$ or the ${O}_{2}$). It is not possible to be certain as to which of these molecules was the source of that particular oxygen atom.
So, it was either part of the food that was consumed to produce the glucose, or the oxygen that was transported from the lungs via the blood to the cell. But which one? Impossible to say. One ${H}_{2} O$ molecule will contain an oxygen atom from the ${O}_{2}$ and the next could contain one from the sugar.