Succinate dehydrogenase (SDH/Complex II) is unique among the ETC complexes because it does NOT pump protons across the inner mitochondrial membrane. The reason is:
- A SDH transfers electrons directly to ubiquinol but the free energy released is insufficient to pump protons ✓
- B SDH is located on the outer mitochondrial membrane, away from the proton gradient
- C SDH oxidises succinate to fumarate, releasing only enough energy to reduce FAD to FADH2, which enters the ETC at a lower energy level
- D SDH lacks the cytochrome subunits required for proton translocation
Correct answer: A. SDH transfers electrons directly to ubiquinol but the free energy released is insufficient to pump protons
Explanation
Complex II (SDH) oxidises succinate to fumarate in the TCA cycle, with FADH2 as the electron carrier. FADH2 directly reduces ubiquinone (CoQ) to ubiquinol, delivering electrons to the Q-pool. The free energy change of succinate oxidation (ΔG ≈ -37 kJ/mol) is insufficient to drive proton translocation against the electrochemical gradient — in contrast with Complexes I, III, and IV which have larger ΔG values enabling proton pumping. This is why FADH2-linked substrates yield less ATP (~1.5 ATP) compared to NADH-linked (~2.5 ATP per pair of electrons).
Reference: Harper's Illustrated Biochemistry, 32nd ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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