Question

during alcohol fermentation in yeast, nad+ is regenerated from nadh by which of the following processes?
reduction of acetaldehyde to ethanol
oxidation of ethanol to acetaldehyde
reduction of acetyl coa to ethanol
reduction of pyruvate to lactate
oxidation of pyruvate to acetyl coa

Explanation:

In alcohol fermentation in yeast, after glycolysis (which produces NADH), pyruvate is first decarboxylated to acetaldehyde. Then, to regenerate NAD⁺ (which is crucial for glycolysis to continue as NAD⁺ is a co - factor in glycolysis), acetaldehyde acts as an electron acceptor. NADH donates electrons to acetaldehyde. When a molecule accepts electrons (and in this case also hydrogen, as in the conversion of acetaldehyde (\(CH_3CHO\)) to ethanol (\(CH_3CH_2OH\))), it is reduced. So, the reduction of acetaldehyde to ethanol regenerates NAD⁺.

• Oxidation of ethanol to acetaldehyde would consume NAD⁺ (if it were a biological reaction in yeast fermentation context, which it is not the regenerative step for NAD⁺ in alcohol fermentation).
• Reduction of acetyl CoA to ethanol is not a step in alcohol fermentation in yeast.
• Reduction of pyruvate to lactate is a step in lactic acid fermentation (not alcohol fermentation in yeast).
• Oxidation of pyruvate to acetyl CoA is a step in aerobic respiration (link reaction) and does not regenerate NAD⁺ in the context of alcohol fermentation in yeast.

Answer:

reduction of acetaldehyde to ethanol