Question

during alcohol fermentation in yeast, nad+ is regenerated from nadh by which of the following processes?

• oxidation of ethanol to acetaldehyde
• oxidation of pyruvate to acetyl coa
• reduction of pyruvate to lactate
• reduction of acetaldehyde to ethanol
• reduction of acetyl coa to ethanol

Explanation:

1. Recall alcohol fermentation in yeast: Pyruvate is converted to acetaldehyde (releasing CO₂), then acetaldehyde is reduced to ethanol. During this reduction of acetaldehyde to ethanol, NADH donates electrons (is oxidized) to regenerate NAD⁺.
2. Analyze other options:

• Oxidation of ethanol to acetaldehyde would consume NAD⁺, not regenerate it (opposite of what's needed).
• Oxidation of pyruvate to acetyl - CoA is part of aerobic respiration, not fermentation.
• Reduction of pyruvate to lactate is lactic acid fermentation (in muscle cells, bacteria), not yeast alcohol fermentation.
• Reduction of acetyl - CoA to ethanol is not a step in alcohol fermentation.

Answer:

D. reduction of acetaldehyde to ethanol (assuming the options are labeled A - E with D being "reduction of acetaldehyde to ethanol" as per the options list; if the original options had labels, use the correct label with the text. For example, if the fourth option was labeled D, then D. reduction of acetaldehyde to ethanol)