I know it might sound a bit absurd, but my teacher gave us some "answers to question", that is, given one answer, write at least one question the answer to which is the given one.

I've solved most of the answers, but there's one I just can't figure out: "because the metabolites in the cell are in ionized form", and the topic is anaerobic respiration and fermentation. No additional details from her.

I'm in high school (a "scientific" school in Italy), if that can help for the knowledge level.

Any idea?

  • $\begingroup$ Hmmm, scientific Jeopardy, interesting idea... $\endgroup$ Apr 29, 2020 at 10:38
  • $\begingroup$ "Why does ethanol escape from cells but metabolites do not when anaerobic respiration and fermentation are happening?". BTW: Not all metabolites are in ionized form (e.g. glucose), but many are, including neutral molecules attached to charged carriers (phosphorylated glucose, fatty acids with CoA etc). $\endgroup$
    – Karsten
    Apr 29, 2020 at 12:44
  • $\begingroup$ That's a pretty valid question, thanks! Any other idea? $\endgroup$ Apr 29, 2020 at 13:35

1 Answer 1


Interesting reverse question. I could figure out three questions:

  • Why milk changes from a liquid to a gel when it turns into yoghurt?
  • Why venous blood has lower pH than arterial blood?
  • Why do we get tired on intense physical exertion?

The answer to the first one would be because the metabolite of lactose fermentation in the lactobacilli cells is lactic acid, that in ionized form releases $\ce{H+}$, lowering the medium pH until the isoelectric point of the milk proteines is reached, causing their agglutination. The answer to the second one is that because the venous blood carries away carbonic acid produced as a metabolite in cellular respiration, that has two ionized forms, $\ce{HCO3-}$(bicarbonate ion) and $\ce{CO3^2-}$(carbonate ion). The answer to the third one would be that, as in lactobacilli case, our body resorts to anaerobic lactic fermentation in activities that require a large power output, beyond the oxygen delivery capacity of the circulatory system. So lactic acid builds up in the muscles, lowering the pH and causing the fatigue sensation. This last explanation was broadly accepted, but seems to be controversial in light of recent studies, as we can see in the abstract for this article:

In addition, many textbooks report that muscle fatigue is mainly the result of a decrease in pH within the muscle cell due to a rise in hydrogen ion concentration ([H+]) resulting from anaerobic metabolism and the accumulation of lactic acid.6–8 Recent literature, however, contradicts this assertion.

  • $\begingroup$ All good questions, but none of these are related to "anaerobic respiration and fermentation". The third one comes the closest, but we are not anaerobic organisms! $\endgroup$ Apr 29, 2020 at 21:55
  • $\begingroup$ We are not obligate anaerobes, but in fact most living things can rely on some degree of fermentation, even us, as a backup of sorts. $\endgroup$
    – ksousa
    May 2, 2020 at 16:50

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