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?

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.