For simplicity, I am only considering complete combustion.

I am ignoring variants such as deoxyribose, just pure carbohydrates.

Balancing the equation is easy due to the definition of carbohydrate: we already have the correct amount of oxygen for the hydrogen. We just need to supply the oxygen for the carbon.

$$\ce{C_mH_{2n}O_n + m O2 -> m CO2 + n H2O}$$

Here is the question: do we know whether the oxygen already in the carbohydrate is more likely to be in the water or the carbon dioxide? I expect that this could be determined with isotopic labelling.

If it might make a difference, consider combustion in a laboratory.

Subsidiary questions.

Might the use of isotopes affect the reaction? I know that biological processes can be sensitive to isotopes.

Might the answer be different in biological systems?

  • 1
    $\begingroup$ Not homework, just an old man's curiosity. $\endgroup$
    – badjohn
    Mar 16, 2022 at 10:15
  • 1
    $\begingroup$ In biological systems, it's relatively easy to track the atoms, but the answer is different from combustion because the reactions are quite different even though the products are the same. The atoms from oxygen gas end up only in the water product, whereas the oxygen atoms on the carbohydrate can either be retained on carbon or end up in phosphate groups or water in the medium. Some of the oxygen atoms in the CO2 product also come from water in the medium. $\endgroup$
    – Andrew
    Mar 16, 2022 at 15:55
  • $\begingroup$ @Andrew Of course, I should have thought of that, a very different route. $\endgroup$
    – badjohn
    Mar 16, 2022 at 17:59


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.