Three stages of cellular respiration and which yields the most ATP?

• A. Glycolysis, Krebs cycle, and electron transport chain; the electron transport chain yields the most ATP.
• B. Fermentation, glycolysis, and photosynthesis; glycolysis yields the most ATP.
• C. Glycolysis, beta-oxidation, and electron transport chain; beta-oxidation yields the most ATP.
• D. Glycolysis, Krebs cycle, and fermentation; Krebs cycle yields the most ATP.

Answer: (B)

Energy production across cellular respiration happens in three stages: glycolysis, the Krebs cycle, and the electron transport chain. Among these, the electron transport chain yields the most ATP. Glycolysis occurs in the cytoplasm and makes a small amount of ATP plus NADH. The Krebs cycle, inside mitochondria, generates a bit more ATP and additional NADH and FADH2. The heavy lifting comes from the electron transport chain, where NADH and FADH2 donate electrons to pumps that build a proton gradient. ATP synthase uses that gradient to produce a large amount of ATP—roughly 26–28 ATP from NADH/FADH2, adding up to around 30–32 ATP per glucose in many cells. So, while the first two stages contribute energy, the electron transport chain is the main source of ATP.

The other processes mentioned aren’t stages of cellular respiration. Fermentation happens when oxygen is scarce and yields far less ATP. Photosynthesis is the process of building sugars using light, not energy production in respiration. Beta-oxidation feeds fatty acids into metabolism but isn’t listed as one of the three respiration stages.