This is a question for you biology and chemistry people out there:

If I remember high school biochemistry correctly, cost of the energy our body uses is obtained by reacting sugars with oxygen (low-carb diets notwithstanding). In that reaction, what fraction of the released energy is stored as potential energy in the sugar, as opposed to in the oxygen? (If such a question even makes sense)


10 comments to

  • I seem to think that the Oxygen is a catalyst and doesn’t count for the potential energy in the equation. I could be horribly off-base, though. I’ll ask the wonderful biochemist I live with. ;)

    • I’m positive that oxygen isn’t a catalyst.

      If I recall correctly, the full reaction is:

      C6H12O6 + 6 O2 => 6 CO2 + 6 H2O + heat

    • No, O2 in the reaction isn’t a catalyst, as it is consumed in the overall reaction, as asked (and later written). catalyst = unchanged over the course of the reaction.

      As for your question – you answered it for yourself in the following post. Most of the energy needed to run bodily processes is taken from sugar (and protein and fat.) That is why we eat. If one stopped eating, but still breathed, one would subsist for a while, as the body would break down anything resembling an energy source (Carbon-based glycogen, fats and proteins in tissues) and then one would go kaput. If one doesn’t take in external carbon sources, one *burns* through the stores and dies. Independent of how much O2 you breathe.

      As Oldest Song indicates below, Metabolism is a huge, intricate, confusing system. A more basic answer, I think, is to say that an unsaturated carbon, with four single bonds – say from a Snickers bar, is oxidized*** to CO2, and most of the energy that is released from that oxidation is ultimately stored in another form – ATP – after passing through many intermediates. If you take any higher level biochem or metabolics course, one no longer considers energy in the form of a potential, but rather as how many ATPs are synthesized. ‘Cause really, it is all about the ATP production.

      A business analogy – an international business might get paid in $US and $Canadian, pounds, rubles, krone, what have you. This is Carbon in various forms. then the company takes the money to the bank to exchange to US$. Here US$ are ATP and the bank is your metabolism (gut, enzymes, liver). Then the business can pay its employees in US$ for the work they do. Employees are muscles, immune system, and most importantly brain. As well as all of the thousands of other processes that go one that allow you to blink your eyes as you read this. In this analogy, O2 is one of the things that allows for the conversion of currencies to US$, but doesn’t add to the bottom line. The heat of the reaction is the interest taken by the bank for the transaction.

      Right, so. As to the O2, you should note that on the first page of the link you had in your next post, elements have no potential energy in their most stable form. O2 = 0. The little bit o’energy oxygen picks up is from the process of dissolving it.

      You might guess, I love love love talking about and teaching Biochemistry. Especially to non-Biology people. If I can make any of this clearer, or answer any other questions, let me know.


      *** really is burning, same chemical reaction, more controlled circumstances. Drak isn’t too far off, here. (I can’t believe I just wrote that!) dietary calories are different from metric calories (measure of heat) for some archaic reason that I learned once but quickly forgot. But all are related to joules, which is metric measure of energy as work.

  • It isn’t really sensical to consider either one or the other to have ‘given up’ the energy– the products, collectively, have less energy than the ingredients, collectively.

    If you give Oxygen some nice metallic sodium to burn instead, it’ll pursue that reaction instead.

    If you -have- water and CO2, it’ll cost you energy to turn it back into O2 and -either- a pile of coal and hydrogen gas -or- sugar.

    Mitocondria and/or cells can produce energy from a number of different things… kids get the symplified version in highschool, and apart from knowing that footnote, I don’t know much more about the various alternate chemical pathways… the formula you’ve presented above is sort of “black boxed” at the arrow.

  • Doesn’t make sense as stated. Basically, in the back box, glucose (6C) is broken down through glycolysis to two molecules of pyruvate (3C each) which go through the pyruvate dehydrogenase complex and are oxidatively decarboxylated to acetyl-CoA, which proceeds through the TCA cycle in another series of oxidative decarboxylations, so at the end of one cycle you have six C02 (from the decarboxylations) and a bunch of NADH (this is your reducing equivalent) and a little ATP. The CO2 is waste. The NADH is then processed through the cytochrome complex, reducing oxygen to water, generating more ATP, and and restoring NAD+. The oxygen reduction step is entirely separate from the step where the sugar is broken down.

    (Incidentally, there are a lot of intermediates between glucose and acetyl-coA and in the TCA cycle – the other things we get energy from are converted into some of these intermediates by other pathways and enter energy metabolism in that way.)

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