Which metabolic pathway is common to both fermentation and aerobic cellular respiration?

• A. Glycolysis
• B. Beta-oxidation
• C. Electron transport chain
• D. Citric acid cycle

Answer: (A)

Glycolysis is the step shared by both fermentation and aerobic cellular respiration. It takes place in the cytoplasm and converts glucose into pyruvate, producing a small amount of ATP and NADH. This pathway does not require oxygen, so it functions whether oxygen is present or not. In fermentation, the pyruvate from glycolysis is converted to lactate or ethanol, with NADH recycled back to NAD+ to keep glycolysis running. In aerobic respiration, pyruvate from glycolysis enters the mitochondria to feed the full oxidative pathways (pyruvate to acetyl-CoA, the citric acid cycle, and the electron transport chain), generating much more ATP. The other options—beta-oxidation, the electron transport chain, and the citric acid cycle—are not common to fermentation, since beta-oxidation is fatty acid breakdown, and the electron transport chain and citric acid cycle occur in aerobic respiration (and are not part of the fermentation process).