Oxidative Phosphorylation Questions and Answers

Answer: C

The compound that does not block electron transport is Oligomycin.

Oligomycin inhibits ATP synthase by binding to its F_o proton channel. It blocks phosphorylation, not the electron transport chain itself; in tightly coupled mitochondria electron flow slows only indirectly, because the proton gradient builds up, and it resumes at once when an uncoupler is added.

The other three are true electron transport inhibitors: rotenone blocks Complex I (NADH to ubiquinone), antimycin A blocks Complex III at the Q_i site, and cyanide binds the haem a₃-Cu_B centre of cytochrome c oxidase (Complex IV).

Answer: D

Plants tolerate rotenone because they have additional NAD(P)H dehydrogenases that are resistant to rotenone.

Plant mitochondria possess alternative, type II NAD(P)H dehydrogenases on both faces of the inner membrane. They pass electrons from NADH or NADPH directly to ubiquinone without going through Complex I and without pumping protons, and they are insensitive to rotenone. When rotenone blocks Complex I, electrons still reach the rest of the chain through these bypass enzymes, so respiration continues.

Plant Complex I itself is rotenone-sensitive, and tolerance is not attributed to degradation of the compound or to specific efflux channels. Insects and fish lack the bypass, which is why rotenone kills them.

Answer: B

The correct match is P-iv, Q-i, R-ii, S-iii.

• Oligomycin blocks the proton channel of the F_O part of ATP synthase.
• Antimycin A blocks the cytochrome bc₁ complex (complex III) at the Q_i site.
• DCMU (diuron) binds the Q_B site of photosystem II and stops electron flow from it.
• Valinomycin is a K⁺ ionophore that carries potassium across membranes and dissipates the membrane potential.

The other options interchange these targets, for example giving oligomycin to complex III.