According to Peter Mitchell's chemiosmotic hypothesis, how is the proton gradient established across the thylakoid membrane during light reactions?
Answer and explanation
Correct answer: C. Protons accumulate in the thylakoid lumen; ATP synthase harnesses the proton gradient
1. Photolysis of water releases directly into the lumen.
2. Plastoquinone picks up from the stroma when reduced and releases them into the lumen upon oxidation by cytochrome .
3. Reduction of to by FNR consumes from the stroma.
2. Plastoquinone picks up from the stroma when reduced and releases them into the lumen upon oxidation by cytochrome .
3. Reduction of to by FNR consumes from the stroma.
- Step 1: In the chemiosmotic model, three distinct mechanisms generate high in the lumen and low in the stroma:
- Step 2: This creates a steep proton electrochemical gradient across the thylakoid membrane with lower pH inside the lumen.
Answer
Protons accumulate in the thylakoid lumen due to water splitting and plastoquinol proton translocation, while stromal protons are consumed in NADP+ reduction
Why not the others: 'Protons are actively pumped from the thylakoid lumen into the stroma by ATP synthase' reverses the direction of spontaneous proton flow, which passes from lumen to stroma through CF0 to drive ATP synthesis; 'Protons diffuse spontaneously from the stroma into the lumen...' is thermodynamically unfavorable because the lumen already has a much higher proton concentration; 'Protons are generated in the stroma by RuBisCO...' confuses the dark reaction RuBisCO with the photochemical electron transport chain.
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