33. Given below are a few statements regarding light reaction of photosynthesis
A. Photo system II oxidizes water to 0₂ and in the process releases protons into lumen.
B. Photosystem II oxidizes water to O₂ and in the process releases protons into stroma.
C. ATP synthase, produces ATP while protons go back through it from lumen to stroma.
D. ATP synthase produces ATP while protons go back through it from stroma to lumen.
Which one of the following combinations of above statements is correct?
(1) B and D                 (2) A and D
(3) B and C                   (4) A and C

The correct answer is (4) A and C:

• A. Photosystem II oxidizes water to O2 and in the process releases protons into lumen

  • Correct. The oxidation of water by PSII occurs at the oxygen-evolving complex (OEC) located at the luminal side of the thylakoid membrane. Protons generated in water splitting are released into the thylakoid lumen, increasing proton concentration inside the lumen.​

• C. ATP synthase produces ATP while protons go back through it from lumen to stroma

  • Correct. The proton gradient created by proton accumulation in the lumen powers ATP synthase. Protons flow back into the stroma through ATP synthase, driving ATP formation.​

• B. PSII oxidizes water to O2 and in the process releases protons into stroma

  • Incorrect. Protons are not released into the stroma but into the thylakoid lumen during water oxidation by PSII.​

• D. ATP synthase produces ATP while protons go back through it from stroma to lumen

  • Incorrect. Protons flow in the opposite direction through ATP synthase—from the lumen to the stroma—not from stroma to lumen.​

Light Reaction: Water Oxidation, Proton Movement, ATP Synthesis

Introduction

Key phrase: photosystem II water oxidation protons thylakoid lumen ATP synthase stroma

Coordination of water oxidation, proton separation, and ATP synthesis in photosynthetic light reactions is essential for efficient energy conversion in chloroplasts.​

Explanation of Each Statement

• A. Water oxidation by PSII releases protons into the thylakoid lumen, increasing the proton gradient across the membrane.​

• B. Incorrect. Protons generated from water oxidation do not enter the stroma.​

• C. ATP synthase uses the proton gradient to drive protons from lumen to stroma, resulting in ATP formation.​

• D. Incorrect. Protons do not move from stroma to lumen during ATP synthesis.​

Key Facts: Light Reaction Proton Flow

• Water is split at OEC of PSII on the luminal membrane side.

• Protons released into the lumen boost the proton gradient.

• ATP synthase exploits lumen-to-stroma proton flow for ATP generation.​