Ethanol

Figure 8-8. The Embden-Meyerhoff-Parnas glycolytic pathway used by Saccharomyces cerevisiae and re-oxidize NAD via the lactate dehydroge-nase reaction. In contrast, in S. cerevisiae, this is done via a two-step series of reactions, in which pyruvate is first decarboxylated by pyruvate decarboxylase, generating acetalde-hyde and CO2.

Next, the acetaldehyde is reduced by one of several NADH-dependent alcohol dehydroge-nases to form ethanol and oxidized NAD. The net effect, then, of glycolytic metabolism by bakers' yeast is the formation of two moles of CO2 and two moles of ethanol per mole of glucose fermented (Figure 8-6).The oxidized and reduced forms of NAD are in balance, and the cell obtains a net gain of two moles of ATP. Yeast growth itself is quite modest during gly-colysis, because most of the glucose carbon is used for generating energy rather than for producing cell mass. Therefore, the initial population will increase only a few generations during the entire fermentation process.

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