In most organisms, oxygen slows down glycolysis, and pyruvate is directed towards the TCA cycle rather than fermentative pathways (this is called the Pasteur Effect). Some strains of the brewers’ yeast Saccharomyces cerevisiae display the opposite effect, sometimes called the Crabtree Effect (or, more correctly, glucose repression), in which high concentrations of glucose accelerate glycolysis and the production of ethanol, even in the presence of oxygen. In other words, the organism generates ATP by fermentation even when oxygen is available.
For a yeast species that displays the Pasteur Effect, treatment with carbon monoxide accelerates glycolysis and ethanol production. That is, the yeast behaves as if oxygen is absent and now displays the Crabtree Effect.

(A) What is the explanation for this effect of carbon monoxide?
[Tip: you will need to do some research on the modes of action of CO, and the enzymes of respiration].
The ATP yield of fermentation is much lower than that of oxidative phosphorylation.

(B) Why do you think some yeasts make energy by ethanol fermentation even in the presence of oxygen?

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