In aerobic respiration, which stage yields NADH and FADH2 that donate electrons to the electron transport chain?

• A. Glycolysis
• B. Pyruvate Oxidation
• C. Krebs Cycle
• D. Fermentation

Answer: (C)

In aerobic respiration, the electron carriers that feed the electron transport chain are NADH and FADH2, which deliver high-energy electrons to power ATP synthesis. The stage that produces both of these carriers is the Krebs cycle. As acetyl-CoA is oxidized in the cycle, each turn yields three NADH and one FADH2 (along with one GTP/ATP). These carriers then transfer electrons to the electron transport chain—NADH donates to Complex I and FADH2 donates to Complex II—propelling the proton motive force that drives ATP synthase.

Glycolysis does generate NADH but not FADH2, and its NADH must travel to the mitochondrion via shuttle systems, not directly to the chain as both carriers. Pyruvate oxidation makes NADH but not FADH2. Fermentation bypasses the electron transport chain entirely and does not produce NADH or FADH2 for feeding into it.