According to this table, gasoline has a specific energy noticeably higher than fat and more than twice that of carbohydrates; even short-chain hydrocarbons like ethanol and even methanol offer an improvement in specific energy over carbohydrates. Given this, why are carbohydrates the most common short-to-medium-term energy storage form in biology?

  • $\begingroup$ Two quick reasons: (1) those compounds are toxic to cells, or may give toxic metabolites when metabolized; or (2) they might actually be great energy sources, but enzymes to process them have never evolved. $\endgroup$
    – Roland
    Aug 12 '15 at 16:50
  • $\begingroup$ I would argue that the toxicity is as much a result of the "choice" of energy storage as a cause, since toxicity is basically metabolic/physiological incompatibility; also, numerous organisms have evolved toxin immunity quite quickly where advantageous. (Also, the metabolites of hydrocarbons are simply water and carbon dioxide, the same as carbohydrates) Secondly, my question is essentially why those enzymes haven't evolved? Complexity seems an odd reason, since straight-chain hydrocarbons are simpler than carbohydrates, so what advantage selected for the latter rather than the former? $\endgroup$ Aug 12 '15 at 19:16
  • $\begingroup$ I agree with your comment about metabolic compatibility, but toxicity is not determined simply by elemental composition. The rest sounds similar to many questions on this forum of the form "why haven't cool function X evolved". Evolution is cannot plan ahead, so it gets stuck in local optima all the time. Photosynthesis somehow (randomly?) evolved to produce sugars from CO2, and thereafter sugars became a common currency. This may well be just a coincidence. But these are so early events in evolution that we really don't have any way of knowing. $\endgroup$
    – Roland
    Aug 12 '15 at 21:22
  • $\begingroup$ I fully understand that evolution tends to an immediate optimum rather than a long-term one, in which case the answer I am looking for would be the local optimum which made carbohydrate metabolism more (immediately) advantageous. Given that hydrocarbon-based metabolisms have evolved and currently exist, the implication is that carbohydrates offer or offered some selective advantage over hydrocarbons. $\endgroup$ Aug 14 '15 at 13:51

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