Chemiosmotic coupling: membrane potential vs proton gradient In mitochondria, can the electrical membrane potential (ΔΨ) and the chemical proton gradient (ΔpH) each, on their own, drive ATP synthesis from ADP + Pi?
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AAre both required to make ATP
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BAre sufficient, separately, to make ATP from ADP + Pi
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CReinforce one another when respiratory inhibitors are present
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DCancel one another when uncouplers are present
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EOnly ΔpH contributes; ΔΨ is irrelevant for ATP synthase
Answer
Correct Answer: Are sufficient, separately, to make ATP from ADP + Pi
Explanation
Introduction / Context:Peter Mitchell’s chemiosmotic theory explains how electron transport is coupled to ATP synthesis. The proton motive force (PMF) comprises two components: an electrical potential (ΔΨ) and a chemical gradient (ΔpH). This question probes whether either component alone can power ATP synthesis.
Given Data / Assumptions:
- ATP synthase utilizes the PMF across the inner mitochondrial membrane.
- PMF = ΔΨ + (2.303 * RT/F) * ΔpH (expressed in volts).
- Experimental systems can manipulate ΔΨ and ΔpH independently (e.g., K+ diffusion potentials, pH jumps).
Concept / Approach:If the total PMF is sufficient, ATP synthase can operate even when one component is near zero. Classic reconstitution experiments showed ATP formation driven solely by a pH gradient or solely by an electrical potential, provided that the magnitude of PMF crosses the catalytic threshold.
Step-by-Step Solution:Define PMF: PMF = ΔΨ + ΔpH term.Create ΔpH without ΔΨ (valinomycin/K+ to collapse ΔΨ): ATP synthesis still occurs if ΔpH is large.Create ΔΨ without ΔpH (nigericin to collapse ΔpH): ATP synthesis still occurs if ΔΨ is large.Therefore, either component can independently drive synthesis so long as PMF magnitude is adequate.
Verification / Alternative check:Submitochondrial particle and liposome experiments with bacteriorhodopsin or pH jumps reproduce ATP formation under isolated component conditions.
Why Other Options Are Wrong:(a) Overstates dependence; both are not simultaneously required.(c) Respiratory inhibitors reduce proton pumping; “reinforce” is misleading.(d) Uncouplers dissipate PMF; it is not that ΔΨ and ΔpH cancel each other, but that both are collapsed.(e) ΔΨ significantly contributes to PMF; it is not irrelevant.
Common Pitfalls:Assuming ATP synthase requires a fixed ΔpH; ignoring the electrical component; confusing inhibitors (e.g., rotenone) with uncouplers (e.g., FCCP).
Final Answer:Are sufficient, separately, to make ATP from ADP + Pi.