160. Upon application of the inhibitor, DBMIB (Dibromothymoquinone) which one of the following events in the
chloroplast electron transport chain will NOT take place?
1. Reduction of reaction centre QA
2. Reduction of Plastocyanine (PC)
3. Reduction of Cytb6f
4. PQ will remain oxidized

Understanding the Impact of DBMIB on Chloroplast Electron Transport Chain

Photosynthesis in chloroplasts relies on a complex electron transport chain (ETC), where electrons move through a series of carriers, facilitating ATP and NADPH production. DBMIB (Dibromothymoquinone) is a well-known inhibitor used to study photosynthetic mechanisms because of its specific action on this chain.

What is DBMIB and How Does It Work?

DBMIB is a competitive inhibitor that blocks the plastoquinol (PQH₂) binding site on the cytochrome b6f complex, a key protein complex in the photosynthetic ETC. When DBMIB is applied, it prevents the transfer of electrons from plastoquinol (PQH₂) to cytochrome b6f, effectively halting downstream processes.

Chloroplast Electron Transport Chain Overview

To better understand what DBMIB inhibits, here’s a simplified flow of the photosynthetic electron transport:

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⁺ → NADPH

Effect of DBMIB on Key Components

Let’s now evaluate the options provided based on what DBMIB affects:

1. Reduction of Reaction Centre QA

• Occurs: QA is the first plastoquinone electron acceptor in PSII, upstream of DBMIB’s action site. So this step still occurs.

2. Reduction of Plastocyanin (PC)

• Does NOT Occur: Since DBMIB blocks electron flow beyond PQ, cytochrome b6f cannot transfer electrons to PC. Thus, PC is not reduced.

3. Reduction of Cytochrome b6f

• Occurs: The b6f complex can still accept electrons from plastoquinol; the inhibition occurs at electron release, not acceptance. So it can be reduced.

4. PQ Will Remain Oxidized

• False: PQ still gets reduced by PSII to PQH₂; DBMIB blocks the next step, but not PQ’s reduction.

Correct Answer: 2. Reduction of Plastocyanin (PC)

Because DBMIB blocks electron flow at the cytochrome b6f complex, plastocyanin (PC) will not be reduced, as it is downstream of the blockade.

Conclusion

When DBMIB is applied to chloroplasts, it inhibits electron transfer from plastoquinol to cytochrome b6f, preventing subsequent reduction of plastocyanin (PC). Other upstream components like PSII and PQ can still function normally, but downstream events are halted. Understanding this mode of action is essential for studying photosynthetic regulation and inhibition mechanisms.