34 Light reactions of photosynthesis are carried out by four major protein complexes: Photosystem I (PSI), photosystem II (PSII), the cytochrome b6f complex and ATP synthase. The following are certain statements on PS I:
A. PSI reaction centre and PSII reaction centre are uniformly distributed in the granal lamellae and stromal lamellae.
B. The electron donor for the P700 of PSI is plastocyanin and electron acceptor of P700^* is a chlorophyll known as A_o.
C. The core antenna and P700 are bound to two key proteins PsaA and PsaB.
D. Cyclic electron flow occurs from the reducing side of PSI via plastohydroquinone and b₆f complex. This supports ATP synthesis but does not reduce NADP⁺.
Which one of the following combinations of the above statements is correct?
(1) A, B and C
(2) A, C and D
(3) A, B and D
(4) B, C and D

The correct answer is (4) B, C, and D:

• B. The electron donor for the P700 of PSI is plastocyanin, and electron acceptor of P700 is a chlorophyll known as Ao.*
  Correct. Plastocyanin directly donates electrons to P700 in PSI. After excitation (P700*), the primary electron acceptor is A0, a special chlorophyll molecule.​

• C. The core antenna and P700 are bound to two key proteins PsaA and PsaB.
  Correct. PsaA and PsaB are the major proteins forming the PSI core, binding the antenna chlorophylls and P700 reaction center.​

• D. Cyclic electron flow occurs from the reducing side of PSI via plastohydroquinone and b6f complex. This supports ATP synthesis but does not reduce NADP+.
  Correct. In cyclic electron flow, electrons move from PSI, via ferredoxin, through the b6f complex, promoting ATP synthesis without NADP+ reduction.​

• A. PSI reaction center and PSII reaction center are uniformly distributed in the granal lamellae and stromal lamellae.
  Incorrect. PSI is mainly located in stroma lamellae and non-appressed thylakoid regions, while PSII is concentrated in grana (appressed membranes); distribution is not uniform.​

Photosystem I: Structure, Electron Flow, and Function

Introduction

Key phrase: photosystem I PSI structure electron donor PsaA PsaB cyclic electron flow ATP synthesis

Photosystem I is a critical complex in oxygenic photosynthesis, mediating efficient electron transfer and supporting both linear and cyclic electron flows. Its structure and function rely on specific protein subunits, electron carriers, and specialized arrangement within thylakoid membranes.​

Explanation of Each Statement

• A. Uniform distribution of PSI and PSII in thylakoid membranes

  • Incorrect. PSI and PSII are differentially localized: PSI in stroma lamellae and PSII in grana.​

• B. Plastocyanin is the electron donor for P700; A0 is the primary acceptor after excitation

  • Correct. This reflects canonical electron transfer in PSI.​

• C. PSI core antenna and P700 are bound by PsaA and PsaB proteins

  • Correct. PsaA and PsaB anchor core antenna pigments and the P700 dimer.​

• D. Cyclic electron flow via b6f produces ATP but not NADPH

  • Correct. This flow supports ATP synthesis and protects against photoinhibition but does not reduce NADP+.​

Key Facts: PSI Subunits and Electron Flow

• PSI accepts electrons from plastocyanin, excites P700, and passes electrons to A0.

• PsaA and PsaB form the structural and functional core of PSI.

• Cyclic electron flow serves the plant’s ATP needs but bypasses NADP+ reduction.​