Correct matching is: P–iv, Q–i, R–ii, S–iii, so the right option is (B).​​

Introduction

This article explains how to match four classic bioenergetic inhibitors—Oligomycin, Antimycin A, DCMU and Valinomycin—with their specific targets in oxidative phosphorylation and photosynthesis, a concept frequently asked in competitive life‑science exams. Understanding these inhibitor–target pairs helps in solving assertion–reason questions, interpreting respiratory and photosynthetic electron‑transport experiments, and mastering CSIR‑NET level problems.​

Step‑by‑step matching of Group 1 and Group 2

Group 1 contains four inhibitors:

• (P) Oligomycin

• (Q) Antimycin A

• (R) DCMU

• (S) Valinomycin

Group 2 lists their possible targets:

• (i) Cytochrome bc1 complex

• (ii) Photosystem II

• (iii) K⁺ ionophore

• (iv) Fo ATP synthase

Correct matches:

• P – iv: Oligomycin → Fo ATP synthase

• Q – i: Antimycin A → Cytochrome bc1 complex

• R – ii: DCMU → Photosystem II

• S – iii: Valinomycin → K⁺ ionophore

Thus the correct option is (B) P–iv, Q–i, R–ii, S–iii.​

Why each match is correct

P – Oligomycin → Fo ATP synthase (iv)

• Oligomycin is a macrolide antibiotic that specifically binds to the Fo (membrane) sector of the mitochondrial F₁F₀ ATP synthase and blocks its proton channel.​

• By preventing proton flow through Fo, it stops ATP synthesis and also inhibits ATP hydrolysis, making Fo ATP synthase the precise target.​

So P must match iv.

Q – Antimycin A → Cytochrome bc1 complex (i)

• Antimycin A is a potent inhibitor of the mitochondrial electron‑transport chain at Complex III (cytochrome bc1 complex).​

• It binds at the Qi site on cytochrome b, blocking electron transfer from ubiquinol to cytochrome c₁ and thereby arresting proton pumping by Complex III.​

So Q must match i.

R – DCMU → Photosystem II (ii)

• DCMU [3‑(3,4‑dichlorophenyl)‑1,1‑dimethylurea] is a herbicide that specifically blocks the QBB plastoquinone binding site on Photosystem II (PSII).​

• This inhibition stops electron flow from PSII to plastoquinone, thereby blocking photosynthetic electron transport and oxygen evolution at PSII.​

So R must match ii.

S – Valinomycin → K⁺ ionophore (iii)

• Valinomycin is a cyclic depsipeptide antibiotic that acts as a highly selective potassium ionophore, transporting K⁺ across biological and model lipid membranes.​

• It forms a complex with K⁺, carries it through the hydrophobic core of the membrane, and collapses K⁺ gradients, classifying it as a K⁺ ionophore.​

So S must match iii.

Explanation of each option in the MCQ

The options given:

• (A) P–i, Q–ii, R–iii, S–iv – Incorrect because:

  • Oligomycin does not inhibit the cytochrome bc1 complex; it targets Fo ATP synthase.​

  • DCMU is not a K⁺ ionophore; it blocks Photosystem II.​

  • Valinomycin is not part of ATP synthase; it transports K⁺ ions.​

• (B) P–iv, Q–i, R–ii, S–iii – Correct because each inhibitor matches its experimentally established molecular target as detailed above.​

• (C) P–iii, Q–I, R–iv, S–ii – Incorrect because:

  • Oligomycin is not a K⁺ ionophore; that role belongs to valinomycin.​

  • DCMU does not affect ATP synthase directly; it acts on Photosystem II.​

  • Valinomycin has no direct action on Photosystem II.​

• (D) P–iv, Q–ii, R–I, S–iii – Incorrect because:

  • While Oligomycin → Fo ATP synthase and Valinomycin → K⁺ ionophore are right, Antimycin A does not inhibit Photosystem II and DCMU does not target cytochrome bc1 complex.​

Quick revision table for exams

This structured understanding of how to match the inhibitors in Group 1 with their respective targets in Group 2 will strengthen conceptual clarity for CSIR‑NET, GATE and other life‑science examinations.​