Why do yeast make alcohol?

Ever wonder why yeast make alcohol? Probably not, I realize, but you should. Yeast throw off ethanol in the process of metabolizing sugar, so alcohol is a byproduct of survival; fair enough. But alcoholic fermentation is, in fact, a surprisingly inefficient way to get energy. The standard oxygen-requiring way of breaking down sugar used by most cells, our own included, wrings somewhere between 30 and 38 ATP (38 is the ideal number; it’s probably never quite that high in practice) out of a single glucose molecule. (ATP is the cellular currency in which energy is transferred and spent.) Nevertheless, alcoholic fermentation has the distinct advantage of not needing oxygen and so it makes perfectly good, intuitive sense for Saccharomyces cerevisiae to use it when oxygen isn’t available.

Here’s the quirk: S. cerevisiae uses inefficient alcoholic fermentation even when it does have access to oxygen, even though it has the machinery for the much, much more energetically worthwhile aerobic metabolic process. Yeast will only switch to aerobic metabolism when the amount of sugar available for them to eat is very low. Why? A good question, and one microbiologists haven’t had much success answering.

Our best hypothesis according to a brand-new review on the subject comes in two parts:

  1. Alcoholic fermentation lets yeast act fast to use up the “public goods” while squirreling away private resources for later. Every microorganism you’ll encounter in grape juice can consume sugar. Very few can also consume (and get energy out of) ethanol, but yeast can. So, by converting sugar to ethanol, S. cerevisiae can starve out other microbes and leave itself with a food source for later.
  2. As an additional and maybe even bigger benefit, ethanol is toxic to most yeast and bacteria at concentrations that Saccharomyces can tolerate with relative ease

Possibly the most bizarre thing? We don’t know much about what determines the circumstances under which S. cerevisiae, our long-time compatriot and coworker, produces alcohol versus making energy in some other way. We’ve looked at when and where different yeast genes are expressed and when and where it makes different byproducts but, like so much else in the wonderful and frustrating world of modern-day genetics, putting together the whole story is still a work-in-progress.

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3 thoughts on “Why do yeast make alcohol?

  1. I was under the impression is the reason yeast makes alcohol was because fermentation, as opposed to respiration, doesn’t require mitochondria and if the yeast cell doesn’t need to expend the energy to make mitochondria it won’t.

    • I’ve never encountered that explanation, Kurt. An interesting idea, though I don’t know how the energetic balance would play out in the end. I’ll have to keep looking on that one.

      • Obviously respiration will provide more ATP in the long run, but short term, when there is lots of fermentable sugar available the yeast does not need to conserve potential fuel. I didn’t do that well in Wine Micro so my analysis may be off base though.

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