For this reaction, found in typical biochemistry textbook:

$C_6H_{12}O_6 + 6O_2 \to 6CO_2 + 6H_2O$

I am interested in where do the oxygen atoms of $6O_2$ go. I think they go to $6H_2O$, but this is not sufficient for balance. So Does oxygen atom from $6CO_2$ also get into the carbon dioxide. Experimentally these can be confirmed by iostope labeling experiments. Reference to any such experiments will be appreciated.

I am interested in this question because the central metabolism can be understood as a electron transfer process where glucose is donor and oxygen is acceptor. We need to calculate, effectively how many electrons glucose transfers to oxygen in case of full oxidation.

  • $\begingroup$ This reaction does not actually happen. This reaction denotes combustion, not respiration. $\endgroup$ – WYSIWYG Nov 4 '14 at 6:31
  • $\begingroup$ The reaction provided is correct...albeit it's just an empirical formula....& respiration can be theoretically called an example of controlled combustion.... $\endgroup$ – souvik bhattacharya Nov 5 '14 at 11:04

I think this experiment (PDF file) will help you understand the basic concept about the fate of oxygen in aerobic respiration. Basically the result is:

  1. The oxygen of respiratory carbon dioxide is in exchange equilibrium with body water.
  2. Utilized molecular oxygen is converted to body water.

In respect to calculation of electrons donated to oxygen, just calculating how many NADH2 and how many FADH2 is reduced in how many steps during ETC will be able to answer your question.


Glucose catabolism is a multistep process involving a series of reactions. The reaction you gave is simply the overall, balanced equation; it doesn't actually happen like that in living cells. All diatomic oxygen is converted to water in the electron transport chain, but water is also consumed and produced throughout the preceding steps, which is why the equation doesn't balance that way.

Check out these images from Wikipedia and note all of the compounds entering and exiting the process.


TCA Cycle

Mitochondrial Electron Transport Chain

  • 1
    $\begingroup$ I think it is high time for biochemistry books to change this misguiding summary of respiration. $\endgroup$ – WYSIWYG Nov 4 '14 at 10:42
  • $\begingroup$ @WYSIWYG I agree, or at the very least explain what it actually represents. $\endgroup$ – canadianer Nov 4 '14 at 17:02
  • $\begingroup$ @WYSIWYG Can you please give some reference journal articles that calls for updating the current text and also offers a more realistic picture. $\endgroup$ – DurgaDatta Nov 5 '14 at 6:59
  • $\begingroup$ @DurgaDatta The equation is a gross oversimplification of a complex process perhaps only useful in introductory biology. That it was the cause of your misunderstanding illustrates his point. The images I linked to offer a more realistic picture. $\endgroup$ – canadianer Nov 5 '14 at 7:05
  • $\begingroup$ @DurgaDatta. you can refer to Principles of Biochemistry - Lehninger $\endgroup$ – WYSIWYG Nov 5 '14 at 9:02

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