The following are certain statements regarding the PSII electron carrier during the light reaction of photosynthesis:

A. The first electron released from reaction centre P680 is transferred to Q_A to produce a plastosemiquinone.

B. Q_A is the mobile plastoquinone.

C. he first electron transferred from Q_A to Q_B converts Q_B into plastosemiquinone.

D. Q_B is tightly bound to the complex and is not mobile.

Which one of the following options represents the correct statement(s)?

Correct option is C

Photosystem II (PSII) is the first protein-pigment complex in the Z-scheme of light-dependent reactions of oxygenic photosynthesis in plants, algae, and cyanobacteria. It plays a central role in capturing light energy and initiating the electron transport chain (ETC).​

1.      Statement A: The first electron released from reaction centre P680 is transferred to QA to produce a plastosemiquinone.

• In PSII, the excited electron from P680 (P680) is first transferred to pheophytin*, but shortly afterward, it is transferred to QA, the first plastoquinone acceptor.
• QA receives one electron and becomes reduced to semiquinone (QA⁻).
• Although semantically pheophytin is the immediate acceptor before QA, QA is indeed the first quinone acceptor, which is what the question seems to focus on.
• Hence, the intent of Statement A is biologically accurate and acceptable in this context.

2.      Statement B: QA is the mobile plastoquinone.

• Incorrect. QA is tightly bound to the D2 protein in PSII and is immobile.
• It functions as a one-electron carrier, transferring electrons to QB, which is loosely bound and mobile.
• QB, not QA, carries the electrons to the plastoquinone pool (PQ pool).
• So, this statement is incorrect.

3.      Statement C: The first electron transferred from QA to QB converts QB into plastosemiquinone.

• QB accepts one electron and one proton to form a semiquinone (QB⁻), and after a second round of electron and proton addition, it becomes fully reduced to plastoquinol (PQH₂).
• So technically, this statement is partially true, but it misses the role of protons in forming plastosemiquinone.
• Given the precision required in photosynthetic electron transport, this is not entirely accurate, hence considered incorrect in strict biochemical terms.

4.      Statement D: QB is tightly bound to the complex and is not mobile.

• This is incorrect.
• QB is loosely bound and is mobile, functioning to shuttle electrons from PSII to the cytochrome b6f complex after forming PQH₂.
• The statement reverses the properties of QA and QB, and thus is not correct.