In the absence of oxygen, which process allows NAD+ to be regenerated for glycolysis?

• A. Anaerobic respiration.
• B. Photosynthesis.
• C. Fermentation.
• D. Citric acid cycle.

Answer: (C)

When oxygen isn’t available, the cell can’t rely on the electron transport chain to recycle NADH back to NAD+. Glycolysis keeps running only if NAD+ is available, so a system is needed to regenerate it. Fermentation does exactly that by transferring the electrons from NADH to an organic molecule, turning NADH back into NAD+ so glycolysis can continue and produce a small amount of ATP. In muscles or some microbes, this shows up as lactic acid fermentation, where pyruvate becomes lactate, while in yeast it becomes ethanol and carbon dioxide. This recycling of NAD+ is what allows glycolysis to keep supplying ATP when respiration isn’t an option.