Which stage yields the most ATP per glucose molecule?

• A. Electron transport chain
• B. Glycolysis
• C. Krebs cycle
• D. Fermentation

Answer: (A)

Most ATP per glucose comes from the electron transport chain through oxidative phosphorylation. In this stage, NADH and FADH2 produced earlier donate electrons to the chain in the mitochondria; as electrons move through the chain, protons are pumped across the membrane, creating a gradient that drives ATP synthase to make ATP. This process yields the bulk of the energy, typically around 26–28 ATP per glucose in many cells, and adding the smaller amounts from glycolysis and the Krebs cycle brings the total close to 30–32 ATP per glucose. Glycolysis and the Krebs cycle contribute only small amounts of direct ATP (2 ATP each) and mainly provide the high-energy carriers that feed the chain. Fermentation bypasses the chain entirely and makes only about 2 ATP per glucose, since it doesn’t maximize ATP production.