Question

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

• A. i, ii and iv
• B. ii, iii and iv
• C. i, iii and iv
• D. i, ii and iii

Hint:

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.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
The development of a proton gradient ($\Delta$pH) across the thylakoid membrane is essential for ATP synthesis via chemiosmosis.
The objective is to create a high concentration of protons ($H^{+}$) in the thylakoid lumen (acidic) and a low concentration in the stroma (basic).
Key Formula or Approach:
The gradient is developed through three main actions:
1. Adding protons to the lumen.
2. Removing protons from the stroma.

Step 2: Detailed Explanation:
Let's analyze the statements:
$\bullet$ Statement i (Correct): Plastoquinone (PQ) is a mobile electron carrier. It picks up $H^{+}$ from the stroma and, upon transferring electrons to the cytochrome $b_{6}f$ complex, releases those protons into the lumen.
$\bullet$ Statement ii (Correct): Protons are consumed in the stroma when NADP$^{+}$ is reduced to NADPH by NADP reductase. This lowers the stromal $H^{+}$ concentration.
$\bullet$ Statement iii (Incorrect): ATP synthase does not release protons into the lumen; instead, it provides a channel for protons to flow out of the lumen into the stroma, which dissipates the gradient.
$\bullet$ Statement iv (Correct): The photolysis of water (\( 2H_{2}O \rightarrow O_{2} + 4H^{+} + 4e^{-} \)) occurs on the inner side of the thylakoid membrane, releasing protons directly into the lumen.

Step 3: Final Answer:
The correct processes are i, ii, and iv.
This matches option (A).