Suppose you discovered a mutant strain of spinach in which the thylakoid membranes were slightly permeable to protons, thus allowing a slow leakage (remember that normal membranes are not permeable to protons at all).How might this defect affect the yield of ATP and/or NADPH from the light reactions?单项选择题
A
a. ATP yield would increase.
B
b. ATP yield would decrease.
C
c. NADPH yield would increase.
D
d. NADPH yield would decrease.

Question

Suppose you discovered a mutant strain of spinach in which the thylakoid membranes were slightly permeable to protons, thus allowing a slow leakage (remember that normal membranes are not permeable to protons at all).How might this defect affect the yield of ATP and/or NADPH from the light reactions?单项选择题

A

a. ATP yield would increase.

B

b. ATP yield would decrease.

C

c. NADPH yield would increase.

D

d. NADPH yield would decrease.

BlackTom AI · Accepted Answer

Proton motive force across the thylakoid membrane powers ATP synthase, so making the membrane leaky to H+ directly lowers the driving force for ATP synthesis; therefore ATP yield would decrease.

1) Relevant knowledge points and concepts
- Light reactions: PSII splits water → releases protons into lumen; electrons flow through cytochrome b6f; PQ/PQH2 pump contributes to lumen H+ buildup; PSI reduces ferredoxin which then reduces NADP+ to NADPH.  
- Chemiosmosis: the proton gradient (ΔpH and electric potential) across the thylakoid membrane (proton motive force, PMF) drives ATP synthase (CF1-CF0) to make ATP.  
- Membrane impermeability to H+ is essential: uncontrolled H+ leakage dissipates PMF and uncouples electron transport from ATP production.

2) Step-by-step reasoning
- In wild-type thylakoids, protons pumped into the lumen create a strong PMF. Protons re-enter only via ATP synthase, coupling flow to ATP formation.  
- In the mutant, the membrane permits slow proton leakage from lumen to stroma, so fewer protons pass through ATP synthase for a given rate of electron transport.  
- Result: the PMF is lower at steady state and ATP synthase produces less ATP per unit light absorbed (reduced ATP yield).  
- NADPH formation is driven by electron transfer from PSI to NADP+ (via ferredoxin and FNR) and does not require the proton gradient; therefore NADPH yield from the light reactions is largely unchanged by passive proton leak (unless secondary regulatory feedbacks alter electron flow).

3) Calculations
- No numeric calculation required; qualitative cause-effect (decreased PMF → decreased ATP synthesis).

4) Why the correct answer is correct
- Because ATP synthase depends directly on the PMF; leaking H+ dissipates PMF so ATP production falls.

5) Why other options are incorrect
- a (ATP yield would increase): opposite of expected; leak reduces, not raises, PMF.  
- c (NADPH yield would increase): NADPH production does not benefit from H+ leak; it depends on electron flow, so increase is not expected.  
- d (NADPH yield would decrease): NADPH formation is not directly driven by the proton gradient; any decrease would be secondary and not the primary outcome.

6) Conclusion
- The best answer is b. ATP yield would decrease. This matches the provided correct answer.

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