​Identify the correct site of action of DBMIB (2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone), an inhibitor of the chloroplast electron transport chain.​

Correct option is C

Explanation-

In the light reactions of photosynthesis (in the thylakoid membrane), electrons are transported from water to NADP⁺, and ATP is also synthesized. The sequence of major components is:
1. Photosystem II (PSII)
2. Plastoquinone (PQ)
3. Cytochrome b₆f complex (Cyt b₆f)
4. Plastocyanin (PC)
5. Photosystem I (PSI)
6. Ferredoxin (Fd)
7. NADP⁺ reductase

Electron flow ( Z scheme)

​$$H₂O → PSII → PQ → Cyt b₆f → PC → PSI → Fd → NADP⁺$$

​DBMIB (2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone) is a herbicide and an artificial quinone analog that mimics plastoquinone (PQ), binds competitively at the PQ binding site on the cytochrome b₆f complex and prevents electron transfer from reduced PQ (PQH₂) to Cyt b₆f.

DBMIB blocks the step- 

​$$PQH₂ (reduced plastoquinone) → Cytochrome b₆f complex$$

This step is essential for proton pumping into the thylakoid lumen, which drives ATP synthesis. Blocking this step halts the flow of electrons downstream toward Photosystem I (PSI) and prevents NADPH formation.

So, the correct answer is option C - P​_Q → Cyt b₆f

​
Incorrect options-

​Option A -  QA → QB
This is the internal transfer of electrons within PSII (between quinones QA and QB). DBMIB does not act here.
Option B - QB → PQ
This is the transfer from QB (bound plastoquinone in PSII) to the plastoquinone pool. Again, DBMIB does not interfere here.
Option D-  Cyt b₆f → PC
This is the step after Cyt b₆f, where electrons move to plastocyanin. DBMIB acts before this step.