Part 6 — Electron Transport System (ETS) & Oxidative Phosphorylation (25 MCQs)

Part 6 — Electron Transport System (ETS) & Oxidative Phosphorylation (25 MCQs)

1. Where is the electron transport chain located in plant cells?
   A. Cytosol
   B. Inner mitochondrial membrane
   C. Chloroplast thylakoid membrane only
   D. Cell membrane only
   Answer: B. Inner mitochondrial membrane
   Explanation:

• A/D: Not correct.

• B (Correct): Plant mitochondrial ETS resides in the inner mitochondrial membrane (analogous to animal mitochondria).

• C: Chloroplast thylakoid membranes contain photosynthetic ETS but mitochondrial ETS is in mitochondria.

2. Which complexes of the mitochondrial ETS pump protons across the inner membrane to establish proton gradient?
   A. Complexes I, III, and IV (I, III, IV)
   B. Complex II only
   C. Complex I only
   D. Complex V only
   Answer: A. Complexes I, III, and IV
   Explanation:

• A (Correct): These complexes translocate protons; Complex II (succinate dehydrogenase) does not pump protons. Complex V uses the proton gradient to make ATP.

• B/C/D: Incorrect.

3. Which mobile electron carrier transports electrons between Complex I/II and Complex III?
   A. Ubiquinone (coenzyme Q, Q)
   B. Cytochrome c
   C. ATP synthase
   D. Oxygen
   Answer: A. Ubiquinone (coenzyme Q, Q)
   Explanation:

• A (Correct): Ubiquinone shuttles electrons from complexes I/II to III.

• B: Cytochrome c carries electrons from complex III to IV.

• C/D: Not carriers between these complexes.

4. Which carrier moves electrons from Complex III to Complex IV?
   A. Cytochrome c
   B. Ubiquinone
   C. NADH
   D. ATP
   Answer: A. Cytochrome c
   Explanation:

• A (Correct): Cytochrome c is a small soluble protein in intermembrane space carrying electrons to complex IV.

• B/C/D: Incorrect.

5. What is the final electron acceptor in the mitochondrial electron transport chain?
   A. NAD⁺
   B. Oxygen
   C. CO₂
   D. Water
   Answer: B. Oxygen
   Explanation:

• A/C/D: Not final acceptor.

• B (Correct): Oxygen accepts electrons and with protons forms water at Complex IV.

6. What is chemiosmotic theory (mitchell)?
   A. ATP synthesis is driven by proton motive force (PMF) across membrane through ATP synthase.
   B. ATP is produced directly by electron transfer without proton translocation.
   C. ATP is produced in the cytosol only.
   D. ATP is synthesized by substrate-level phosphorylation only.
   Answer: A. ATP synthesis is driven by proton motive force (PMF) across membrane through ATP synthase.
   Explanation:

• A (Correct): Mitchell’s chemiosmotic hypothesis posits proton gradient energy is harnessed by ATP synthase.

• B/C/D: Incorrect.

7. Which complex contains cytochrome a₃ and Cu centers and reduces O₂ to water?
   A. Complex I
   B. Complex II
   C. Complex IV (cytochrome c oxidase)
   D. Complex III
   Answer: C. Complex IV (cytochrome c oxidase)
   Explanation:

• C (Correct): Cytochrome c oxidase contains heme a and a₃ and Cu centers to reduce oxygen.

• A/B/D: Other complexes.

8. Which compound is an uncoupler of oxidative phosphorylation that dissipates the proton gradient and increases O₂ consumption without ATP synthesis?
   A. Oligomycin
   B. DNP (2,4-dinitrophenol)
   C. Cyanide
   D. Rotenone
   Answer: B. DNP (2,4-dinitrophenol)
   Explanation:

• A: Oligomycin inhibits ATP synthase (reduces O₂ consumption).

• B (Correct): DNP shuttles protons across inner membrane, collapsing proton gradient; increases respiration but no ATP produced.

• C/D: Inhibitors at ETS, not uncouplers.

9. Which inhibitor blocks electron flow at Complex IV by binding to heme iron, preventing O₂ reduction?
   A. Rotenone
   B. Antimycin A
   C. Cyanide (CN⁻)
   D. Oligomycin
   Answer: C. Cyanide (CN⁻)
   Explanation:

• A: Blocks Complex I.

• B: Blocks Complex III.

• C (Correct): Cyanide binds cytochrome a₃ preventing electron transfer to oxygen.

• D: Inhibits ATP synthase, not ETS.

10. Which of the following statements is TRUE about ATP synthase (Complex V)?
    A. It pumps protons from matrix to intermembrane space.
    B. It synthesizes ATP using the proton flow from intermembrane space to matrix.
    C. It oxidizes NADH to NAD⁺.
    D. It reduces oxygen to water.
    Answer: B. It synthesizes ATP using the proton flow from intermembrane space to matrix.
    Explanation:

• A: Reverse direction from typical.

• B (Correct): Proton flow through F₀ into F₁ drives ATP synthesis.

• C/D: Functions of other complexes.

11. Which statement about P/O ratio is correct (classical values)?
    A. Represents ATP produced per O₂ molecule consumed; classical approx. NADH = 3 ATP, FADH₂ = 2 ATP.
    B. Is a measure of H⁺ pumped per electron.
    C. Equals the number of protons in matrix.
    D. Is the same regardless of NADH location (cytosol vs matrix).
    Answer: A. Represents ATP produced per O₂ molecule consumed; classical approx. NADH = 3 ATP, FADH₂ = 2 ATP.
    Explanation:

• A (Correct as the classical textbook approximation): P/O relates ATP made per oxygen (or per pair electrons) with older values; modern P/O are ~2.5 and 1.5.

• B/C/D: Not correct (D: cytosolic NADH shuttle can change practical ATP yield).

12. Which inhibitor blocks electron transfer from Complex I to ubiquinone?
    A. Antimycin A
    B. Rotenone
    C. CN⁻
    D. Oligomycin
    Answer: B. Rotenone
    Explanation:

• A: Blocks complex III.

• B (Correct): Rotenone inhibits electron transfer from NADH-Fe-S to ubiquinone at Complex I.

• C/D: Other actions.

13. Which mobile carrier accepts electrons directly from Complex III and transfers them to Complex IV?
    A. Ubiquinone (Q)
    B. Cytochrome c
    C. NADH
    D. ATP
    Answer: B. Cytochrome c
    Explanation:

• A: Q shuttles between I/II and III.

• B (Correct): Cytochrome c transports electrons from complex III to IV.

• C/D: Not mobile carriers in this context.

14. How many protons (classically) are pumped across the inner membrane per NADH oxidized (Complex I + III + IV)?
    A. 3
    B. 6–10 depending on model (classically 10 H⁺ per NADH)
    C. 0
    D. 100
    Answer: B. 6–10 depending on model (classically 10 H⁺ per NADH)
    Explanation:

• A/C/D: Not accurate.

• B (Correct): Classical textbooks often cite approx. 10 H⁺ per NADH (4 by I, 4 by III via Q cycle? Historically: I pumps 4 H⁺, III pumps 4 H⁺ (via Q cycle), IV pumps 2 H⁺ → total 10). Modern numbers vary; teaching often uses 10 H⁺.

15. Which statement best describes the role of oxygen in oxidative phosphorylation?
    A. Oxygen is a proton carrier.
    B. Oxygen is the final electron acceptor forming water, maintaining electron flow.
    C. Oxygen directly phosphorylates ADP.
    D. Oxygen acts as a coenzyme.
    Answer: B. Oxygen is the final electron acceptor forming water, maintaining electron flow.
    Explanation:

• A/C/D: Incorrect.

• B (Correct): By accepting electrons, oxygen allows continuous electron flow and proton pumping; without it electron flow halts.

16. Which event would cause a decrease in ATP synthesis by oxidative phosphorylation?
    A. Increased availability of ADP
    B. Decreased oxygen concentration
    C. Uncoupler presence (DNP) — leads to decreased ATP despite increased O₂ consumption
    D. Both B and C
    Answer: D. Both B and C
    Explanation:

• A: Increased ADP increases ATP synthesis (respiration).

• B (Correct): Low oxygen slows electron flow and ATP production.

• C (Correct): Uncouplers dissipate gradient — respiration may increase but ATP synthesis decreases.

• D (Correct): Both B and C reduce ATP synthesis.

17. Which molecule provides the immediate energy for phosphorylation of ADP by ATP synthase?
    A. Proton motive force (H⁺ gradient) across inner mitochondrial membrane
    B. Direct hydrolysis of glucose
    C. Light energy only
    D. Heat energy
    Answer: A. Proton motive force (H⁺ gradient)
    Explanation:

• A (Correct): Proton flow through ATP synthase (F₀) drives conformational changes in F₁ producing ATP.

• B/C/D: Incorrect.

18. Which of the following is true about complex II (succinate dehydrogenase)?
    A. It pumps protons across membrane.
    B. It reduces ubiquinone to ubiquinol but does not pump protons.
    C. It oxidizes NADH directly.
    D. It synthesizes ATP.
    Answer: B. It reduces ubiquinone to ubiquinol but does not pump protons.
    Explanation:

• A: Complex II does not translocate protons.

• B (Correct): It transfers electrons from succinate to FAD then to Q, forming QH₂.

• C/D: Incorrect.

19. Which of the following is an inhibitor of F₁F₀-ATP synthase (complex V)?
    A. Oligomycin
    B. Rotenone
    C. Antimycin A
    D. Cyanide
    Answer: A. Oligomycin
    Explanation:

• A (Correct): Oligomycin blocks the proton channel (F₀) inhibiting ATP synthesis.

• B/C/D: ETS inhibitors at other complexes.

20. Respiratory control: When ADP concentration rises, what happens?
    A. Electron transport and oxygen consumption increase (state 3 respiration)
    B. Electron transport decreases
    C. ATP synthase is inhibited
    D. Oxygen is released by mitochondria
    Answer: A. Electron transport and oxygen consumption increase (state 3 respiration)
    Explanation:

• A (Correct): ADP stimulates ATP synthase, allowing proton flow and increased ETS activity (ADP control of respiration).

• B/C/D: Incorrect.

21. What is the role of the Q (ubiquinone) cycle at Complex III?
    A. It transfers electrons from QH₂ to cytochrome c with proton translocation and amplification of proton pumping (Q cycle mechanism).
    B. It creates ATP directly.
    C. It oxidizes oxygen directly.
    D. It is a site for NADH oxidation.
    Answer: A. It transfers electrons from QH₂ to cytochrome c with proton translocation and amplification of proton pumping (Q cycle mechanism).
    Explanation:

• A (Correct): Q-cycle explains how electrons from QH₂ reduce cytochrome c in two steps and contribute to proton gradient.

• B/C/D: Incorrect.

22. Which of the following statements about alternative oxidase (AOX) in plant mitochondria is correct?
    A. AOX transfers electrons from ubiquinol directly to oxygen, bypassing complexes III and IV, reducing proton pumping and ATP yield.
    B. AOX pumps extra protons, increasing ATP yield.
    C. AOX synthesizes ATP directly.
    D. AOX is the main respiratory enzyme in animals.
    Answer: A. AOX transfers electrons from ubiquinol directly to oxygen, bypassing complexes III and IV, reducing proton pumping and ATP yield.
    Explanation:

• A (Correct): AOX provides a cyanide-resistant pathway in plants that can maintain electron flow but with less ATP (heat production, stress responses).

• B/C/D: Incorrect.

23. Which experimental agent would decrease both proton gradient and ATP synthesis but increase O₂ consumption?
    A. Rotenone
    B. DNP (uncoupler)
    C. Oligomycin
    D. Cyanide
    Answer: B. DNP (uncoupler)
    Explanation:

• A/D: Block ETS → decrease O₂ consumption.

• C: Oligomycin blocks ATP synthase → decreases O₂ consumption.

• B (Correct): Uncoupler collapses gradient; ETS speeds up trying to re-establish gradient increasing O₂ consumption while ATP synthesis falls.

24. Which of the following best explains how oxidative phosphorylation yields more ATP than substrate-level phosphorylation alone?
    A. Electron transfer pumps protons to create PMF used by ATP synthase, producing many ATP per glucose.
    B. Substrate-level phosphorylation yields infinite ATP.
    C. Oxidative phosphorylation does not require oxygen.
    D. Oxidative phosphorylation directly converts glucose to ATP.
    Answer: A. Electron transfer pumps protons to create PMF used by ATP synthase, producing many ATP per glucose.
    Explanation:

• A (Correct): ETS oxidative phosphorylation couples electron flow to H⁺ pumping and ATP production via chemiosmosis, yielding large ATP amounts.

• B/C/D: Incorrect.

25. Which statement best describes ATP synthesis stoichiometry in mitochondria (modern estimates)?
    A. Each NADH yields ~2.5 ATP; each FADH₂ yields ~1.5 ATP (approximate modern P/O ratios)
    B. Each NADH yields exactly 4 ATP always
    C. FADH₂ yields more ATP than NADH
    D. No ATP is produced from NADH oxidation
    Answer: A. Each NADH yields ~2.5 ATP; each FADH₂ yields ~1.5 ATP (approximate modern P/O ratios)
    Explanation:

• A (Correct): Modern bioenergetics use these P/O approximations accounting for proton stoichiometry and ATP synthase coupling.

• B/C/D: Incorrect.

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