Although pyruvate is the end product of glycolysis and no more ATP is generated by fermentation of pyruvate to ethanol, yeast produce ethanol under anaerobic conditions because

A. pyruvate export requires more energy than ethanol export.

B. glycolysis substrate-level phosphorylation changes pyruvate to ethanol.

C. electrons (and protons) are then added to ethanol to make lactate.

D. pyruvate to ethanol fermentation decreases deleterious CO2 levels.

E. transfer of electrons from NADH to pyruvate to make ethanol regenerates NAD+, which is necessary for new rounds of glycolysis to proceed.

E. transfer of electrons from NADH to pyruvate to make ethanol regenerates NAD+, which is necessary for new rounds of glycolysis to proceed.

Explanation:

In anaerobic conditions, cells only obtain ATP from glycolysis. During this metabolic process, 2 NAD + molecules are reduced to produce NADH.

During fermentation, pyruvate is first converted into acetaldehyde which is then reduced by the electrons provided by NADH to produce ethanol. This process regenerates the NAD + that will be reused to continue glycolysis.

If fermentation didn't happen, the NAD + wouldn't regenerate and glycolysis would stop, causing cell death because of lack of ATP.