Thylakoid Protein Complex Localization: What Is True?

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November 25, 2025
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38. Following are statements related to the organization of the four major protein complexes of thylakoid membrane,
A. Photosystem II is located predominantly in the stacked regions of the thylakoid membrane.
B. Photosystem I is found in the unstacked regions protruding into stroma
C. Cytochrome B₆f complex is confined to stroma only.
D. ATP synthase is located in the unstacked regions protruding into stroma.
Which one of the following combinations of above statements is correct?
(1) A, B and C (2) A, B and D
(3) B, C and D (4) C, D and A

The correct combination is (2) A, B and D:

A. Photosystem II is located predominantly in the stacked regions of the thylakoid membrane.
Correct. PSII is enriched in the appressed (stacked/granal) regions of the thylakoid membrane, where it participates in water splitting and initial electron transfer.
B. Photosystem I is found in the unstacked regions protruding into stroma.
Correct. PSI, along with its associated antenna complex, is predominantly in the unstacked (stromal/intergranal) regions, connecting to the stroma of the chloroplast.
C. Cytochrome b6f complex is confined to stroma only.
Incorrect. Cytochrome b6f is not limited to either grana or stroma; it is distributed in both appressed and non-appressed regions of the thylakoid membrane, facilitating electron flow between PSII and PSI.
D. ATP synthase is located in the unstacked regions protruding into stroma.
Correct. ATP synthase complexes are predominantly found at the non-appressed regions (stromal lamellae and grana margins) of the thylakoid membrane, facilitating the generation of ATP.

Introduction

Key phrase: thylakoid membrane protein complex localization PSII PSI cytochrome b6f ATP synthase stroma grana

Photosynthesis relies on the specialized localization of its protein complexes within the thylakoid membrane. Their distribution is crucial for the efficient flow of electrons and optimal energy conversion in plant cells.

Explanation of Each Statement

A. PSII is enriched in stacked grana; correct.
B. PSI is predominant in non-stacked stromal regions; correct.
C. Cytochrome b6f distributes across both grana and stroma, not confined; incorrect.
D. ATP synthase occurs mainly at non-appressed stromal membrane; correct.

Key Facts: Complex Distribution in Thylakoids

PSII is in grana (stacked) thylakoids.
PSI and ATP synthase are in stroma-facing, non-appressed regions.
Cytochrome b6f operates in both thylakoid domains for electron flow.