21. In chloroplast, the site of coupled oxidation-reduction reactions is the
(1) outer membrane              (2) inner membrane
(3) thylakoid membrane    (4) stromal space

The correct answer is (3) thylakoid membrane. The thylakoid membrane is the site of coupled oxidation-reduction (redox) reactions in the chloroplast, where the light-dependent electron transport chain operates, driving both water oxidation and reduction of electron carriers during photosynthesis.​

Thylakoid Membrane: The Site of Redox Reactions in Chloroplast

Introduction

Key phrase: thylakoid membrane oxidation reduction reactions photosynthesis

In chloroplasts, the vital coupled oxidation and reduction reactions—which are core to the light-dependent phase of photosynthesis—occur within the thylakoid membrane. This specialized photosynthetic membrane is organized with protein complexes (PSII, PSI, cytochrome b6f, ATP synthase) that orchestrate electron transport and facilitate energy conversion.​

Explanation of Each Option

• (1) Outer membrane

  • Incorrect. The outer membrane surrounds the chloroplast but does not participate in active electron transfer or oxidation-reduction reactions involved in photosynthesis.​

• (2) Inner membrane

  • Incorrect. The inner membrane contains transporters for metabolites but is not the primary site for photosynthetic electron transport or coupled redox reactions.​

• (3) Thylakoid membrane

  • Correct. This is where all major light-driven redox reactions—electron transfers from water to NADP+, formation of ATP, and oxygen evolution—occur in protein complexes embedded in the membrane.​

• (4) Stromal space

  • Incorrect. The stroma is the soluble phase within the chloroplast where carbon fixation (Calvin cycle) reactions occur, predominantly involving reduction but not the light-driven coupled oxidation-reduction electron transport.​

Key Facts About Thylakoid Membrane Redox Function

• Hosts PSII (water oxidation), PSI (NADP+ reduction), cytochrome b6f, and ATP synthase.

• Drives the rapid conversion of light into chemical energy through electron flow.

• Main site for oxygen evolution and ATP/NADPH formation in chloroplasts.​