Which of the following processes result in development of a proton gradient across the thylakoid membrane during photosynthesis?
i. Release of protons into the lumen of the thylakoid by plastoquinone
ii. Consumption of protons in the stroma during the reduction of NADP$^+$
iii. Release of protons into the lumen of the thylakoid by ATP synthase
iv. Release of protons into the lumen of the thylakoid by water splitting reaction
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Remember that the thylakoid lumen becomes highly acidic (high $H^+$ concentration) while the stroma remains basic (low $H^+$ concentration).
Water splitting (internal) and plastoquinone pumping (inward) add protons to the lumen, while NADP$^+$ reduction (external) removes protons from the stroma.
ATP synthase acts as the "drain" that lets protons escape from the lumen to make ATP.
Step 1: Understanding the Question:
This question asks us to identify the physiological processes during the light-dependent reactions of photosynthesis that contribute to the generation of a proton gradient ($\Delta \text{pH}$) across the thylakoid membrane of chloroplasts.
The proton gradient is essential for driving ATP synthesis via chemiosmosis. Step 2: Key Formula or Approach:
The chemiosmotic hypothesis states that a high concentration of protons ($H^+$) must accumulate inside the thylakoid lumen relative to the stroma.
Any process that either increases the proton concentration inside the lumen or decreases the proton concentration in the stroma will help develop or maintain this gradient. Step 3: Detailed Explanation:
• Statement i (Correct): Plastoquinone (PQ) is a mobile electron carrier in the thylakoid membrane.
When PQ receives electrons from Photosystem II, it also picks up protons from the stroma to form plastoquinol ($\text{PQH}_2$).
As $\text{PQH}_2$ transfers its electrons to the Cytochrome $b_6f$ complex, it releases these protons directly into the thylakoid lumen, increasing the lumenal proton concentration.
• Statement ii (Correct): On the stromal side of the thylakoid membrane, the enzyme NADP$^+$ reductase catalyzes the reduction of $\text{NADP}^+$ to $\text{NADPH} + \text{H}^+$.
This reaction consumes free protons from the stroma, which directly decreases the proton concentration in the stroma and enhances the gradient.
• Statement iii (Incorrect): ATP synthase does not release protons into the lumen.
Instead, ATP synthase acts as a channel that allows protons to flow down their electrochemical gradient out of the thylakoid lumen and back into the stroma.
This passive transport of protons dissipates the gradient rather than developing it.
• Statement iv (Correct): The water-splitting complex (oxygen-evolving complex) is associated with Photosystem II on the inner (lumenal) side of the thylakoid membrane.
The photolysis of water molecule splits water into oxygen, electrons, and protons:
\[ 2\text{H}_2\text{O} \rightarrow \text{O}_2 + 4\text{H}^+ + 4e^- \]
These protons are directly released into the thylakoid lumen, significantly contributing to the high proton concentration there. Step 4: Final Answer:
Since processes i, ii, and iv contribute to generating the proton gradient, the correct combination is i, ii, and iv.
