Which process regenerates NAD+ under anaerobic conditions to sustain glycolysis?

• A. Fermentation
• B. Oxidative phosphorylation
• C. Citric acid cycle
• D. Photosynthesis

Answer: (A)

When oxygen isn’t available, glycolysis would run out of NAD+ unless NADH is rebuilt back into NAD+. Fermentation does just that, allowing glycolysis to continue and keep making ATP. In lactic acid fermentation, pyruvate accepts electrons from NADH to form lactate, regenerating NAD+. In alcoholic fermentation, pyruvate is converted to acetaldehyde and then to ethanol, also regenerating NAD+. This NAD+ recycling lets glycolysis proceed despite the lack of oxygen. Oxidative phosphorylation depends on an electron transport chain and oxygen, so it can’t operate anaerobically to regenerate NAD+. The citric acid cycle also relies on NAD+ and feeds NADH into respiration, so it doesn’t sustain glycolysis on its own without oxygen. Photosynthesis isn’t the mechanism that enables glycolysis to continue in anaerobic conditions.