14. Thylakoid membrane has lateral asymmetrical positioning of photosystem in chloroplast. Which statement is correct for distribution of PS-I and PS-II?
(1) PS-I in no appressed portion and PS-II in appressed portion
(2) PS-II in non appressed portion and PS-I in appressed portion
(3) Both PS-I and PS-II in appressed portion
(4) Both PS-I and PS-II in non appressed portion of thylakoid

The correct statement is (1): PS-I in non-appressed portion and PS-II in appressed portion. In chloroplast thylakoid membranes, PSII is predominantly located in the appressed (stacked) grana regions, while PSI is found chiefly in the non-appressed (unstacked) stroma lamellae. This lateral asymmetry is crucial for optimal energy transfer and electron transport.​

Photosystem Distribution in Thylakoid Membranes

Introduction

Key phrase: lateral asymmetrical distribution of PS-I and PS-II in thylakoid membranes

The spatial arrangement of Photosystem I and Photosystem II within thylakoid membranes enables highly efficient light harvesting and electron transport in plants. Their distinctive locations—PSII in stacked grana and PSI in non-stacked stroma lamellae—play critical roles in the architecture of energy conversion and photoprotection.​

Explanation of Each Option

• (1) PS-I in non-appressed portion and PS-II in appressed portion

  • Correct. PSII is concentrated in the appressed (grana) regions, tightly packed to maximize light harvesting and water splitting, while PSI resides in the non-appressed stroma lamellae, aiding in linear and cyclic electron flow.​

• (2) PS-II in non-appressed portion and PS-I in appressed portion

  • Incorrect. This reverses the accurate spatial allocation; PSII does not dominate the non-appressed stroma regions.​

• (3) Both PS-I and PS-II in appressed portion

  • Incorrect. While both may be present at grana margins, PSII is dominant in appressed grana stacks, and PSI is largely excluded due to steric and functional reasons.​

• (4) Both PS-I and PS-II in non appressed portion of thylakoid

  • Incorrect. PSI is prevalent in non-appressed regions, PSII is not; their functional segregation is key to chloroplast efficiency.​

Key Facts About Thylakoid Photosystem Organization

• Appressed/Grana stacks: Rich in PSII for water splitting and photolysis.

• Non-appressed/stroma lamellae: Rich in PSI for NADPH formation and cyclic electron flow.​

• This asymmetry allows rapid adaptation and prevents energy losses during photosynthesis.​