10. Which ion gradient is formed across the thylakoid membrane, if a light is incident on isolated thylakoid?
(1) H⁺                               (2) Na⁺
(3) Both Na⁺ and H⁺          (4) Cl^–

The correct answer is (1) H+ (protons): A proton gradient (H+ ion gradient) is formed across the thylakoid membrane when light is incident on isolated thylakoids. This proton (H+) buildup in the thylakoid lumen is essential for driving ATP synthesis in photosynthesis.​

Ion Gradient Across Thylakoid Membrane

Introduction

Key phrase: proton gradient across thylakoid membrane photosynthesis

In the light-dependent reactions of photosynthesis, thylakoid membranes develop a strong proton gradient due to the movement of H+ ions. This proton gradient powers ATP synthesis, supporting energy conversion essential for plant growth and metabolism.​

Explanation of Each Option

• (1) H+

  • Correct. Light-driven electron transport causes protons (H+) to be pumped from the stroma into the thylakoid lumen. This creates a steep proton gradient across the thylakoid membrane, which is crucial for chemiosmotic ATP production.​

• (2) Na+

  • Incorrect. Sodium ions (Na+) do not play a significant role in photosynthetic electron transport or in establishing a trans-thylakoid ion gradient during light reactions.​

• (3) Both Na+ and H+

  • Incorrect. Only H+ ions are actively pumped during the light reactions; a Na+ gradient is not formed.​

• (4) Cl-

  • Incorrect. Chloride ions can move to balance charge in some minor processes, but the main and functionally crucial gradient is for protons (H+).​

Quick Facts About the Thylakoid Proton Gradient

• Protons accumulate inside the thylakoid lumen, lowering its pH relative to the stroma.

• The proton gradient is harnessed by ATP synthase to generate ATP, a key energy currency for the cell.​

• This gradient is the principal driver of energy storage during light-dependent photosynthesis.​