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.
