Mitochondrial bioenergetics — During electron transport, protons are pumped at all major sites EXCEPT which complex?

Introduction / Context:
The proton motive force (PMF) that drives ATP synthesis is created by proton pumping linked to electron transfer across the inner mitochondrial membrane. Not every respiratory complex pumps protons. Identifying which complexes contribute directly to the PMF is a core concept in oxidative phosphorylation.

Given Data / Assumptions:

• Main respiratory complexes: I (NADH dehydrogenase), II (succinate dehydrogenase), III (cytochrome bc1), IV (cytochrome c oxidase).
• Mobile carriers: ubiquinone (CoQ) and cytochrome c shuttle electrons.
• ATP synthase uses the PMF to produce ATP from ADP and Pi.

Concept / Approach:
Complexes I, III, and IV translocate protons from the matrix to the intermembrane space. Complex II transfers electrons from succinate to ubiquinone but does not pump protons. Thus, electron entry via FADH2 (Complex II) yields less ATP than entry via NADH (Complex I) because fewer protons are pumped per electron pair.

Step-by-Step Solution:

Verification / Alternative check:
Stoichiometric models and inhibitors (rotenone, antimycin A, cyanide) demonstrate that only I, III, and IV contribute to proton pumping, matching observed ATP yields.

Why Other Options Are Wrong:

• Complex I, III, and IV each pump protons and contribute to the PMF.

Common Pitfalls:
Assuming all electron-transfer steps pump protons; Complex II is an exception despite its central role in the TCA cycle and ETC.

Final Answer:
Complex II