USA Biology Olympiad (USABO) Practice Exam

During cellular respiration, which process produces the most ATP?

1. Glycolysis

2. Krebs cycle

3. Electron transport chain

4. Fermentation

The correct answer is: Electron transport chain

The process that produces the most ATP during cellular respiration is the electron transport chain. This occurs in the inner mitochondrial membrane and involves a series of protein complexes that transfer electrons derived from NADH and FADH2, which are produced in earlier stages of respiration, such as glycolysis and the Krebs cycle. As electrons move through these protein complexes, protons are pumped from the mitochondrial matrix into the intermembrane space, creating a proton gradient. This gradient represents potential energy, which is utilized by ATP synthase to synthesize ATP when protons flow back into the matrix. The electron transport chain generates around 28 to 30 ATP molecules per glucose molecule, making it the most productive stage in terms of ATP yield. In contrast, glycolysis and the Krebs cycle produce only a small number of ATP—2 ATP from glycolysis and 2 ATP from the Krebs cycle—while fermentation produces significantly less ATP, typically yielding just 2 ATP per glucose molecule. Therefore, the electron transport chain's ability to leverage the proton gradient to drive ATP synthesis accounts for its position as the greatest producer of ATP in cellular respiration.