43. 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.
(1)Q_A —àQ_B                (2) Q_B —àP_Q’
(3) P_Q —àCytb₆ f         (4) Q_A —àQ_B’

The correct site of action of DBMIB (2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone) is (3) PQ → Cytb6f.

DBMIB competitively inhibits the electron transport chain by binding to the plastoquinol (PQH₂) binding site on the cytochrome b6f complex. This prevents the transfer of electrons from reduced plastoquinone (PQH₂) to the cytochrome b6f complex, halting electron flow downstream to plastocyanin and PSI.​

DBMIB Action: Blocking Plastoquinone to Cytochrome b6f Step

Introduction

Key phrase: DBMIB inhibitor electron transport PQ Cyt b6f chloroplast site of action mechanism

DBMIB is a classic inhibitor used in photosynthesis research to dissect the electron transport chain. By mimicking plastoquinone, it binds specifically at the site where PQ should transfer its electrons to cytochrome b6f, interrupting the linear electron flow and preventing production of ATP and NADPH.​

Explanation of Each Option

• (1) QA → QB

  • Incorrect. This is the transfer between the primary and secondary quinone electron acceptors in PSII, upstream of DBMIB’s inhibition site.​

• (2) QB → PQ’

  • Incorrect. This is the transfer from the secondary bound quinone in PSII to the mobile PQ pool, not the site targeted by DBMIB.​

• (3) PQ → Cytb6f

  • Correct. DBMIB acts here, blocking the transfer of electrons from the reduced mobile plastoquinone pool to the cytochrome b6f complex, effectively halting all downstream electron transport including reduction of plastocyanin and PSI.​

• (4) QA → QB’

  • Incorrect. This is not a standard step in mainstream ETC nomenclature; DBMIB does not act on quinone movement within PSII.​

Key Facts: DBMIB and Electron Transport in Chloroplasts

• DBMIB inhibits PQH₂ oxidation at cytochrome b6f complex.

• Stops reduction of plastocyanin and terminal electron flow to PSI.

• Widely used in research to study energy conversion and transport regulation.​