Which part of the Krebs cycle yields high energy, requires oxygen, and is most efficient?

• A. Aerobic metabolism
• B. Glycolysis
• C. Fermentation
• D. Anaerobic metabolism

Answer: (A)

Oxygen-enabled oxidative phosphorylation makes aerobic metabolism the most energy-efficient way to extract energy from glucose. In aerobic metabolism, the Krebs cycle supplies NADH and FADH2, which feed the electron transport chain. Oxygen acts as the final electron acceptor, allowing the chain to run smoothly and drive a large amount of ATP production. Because this downstream step efficiently converts the energy stored in NADH and FADH2 into ATP, the overall yield per glucose is much higher than in pathways that don’t rely on oxygen.

Glycolysis can occur without oxygen and provides only a small amount of ATP directly, with the rest of the energy depending on subsequent steps that may or may not occur. Fermentation also happens without oxygen and yields minimal ATP compared with aerobic respiration. Anaerobic metabolism denotes processes that do not require oxygen and generally produce less ATP than aerobic respiration.