GATE 2025 BT Q.37: solution and answer

Official answer: ASource: Official answer key, GATE 2025 BT (IIT Roorkee)

1. FCCP is a protonophore (uncoupler). It carries protons across the inner mitochondrial membrane, so it makes the membrane permeable to protons and collapses the proton gradient. P matches 2. This removes option D, which gives P-3.

2. Cyanide binds the heme iron of cytochrome $a_3$ in Complex IV (cytochrome c oxidase) and blocks electron transfer to ${\text{O}}_2$. Q matches 1. This removes options B (Q-3) and C (Q-4).

3. Oligomycin A binds the $F_0$ part of $F_0{F}_1$ ATP synthase and blocks its proton channel, so it inhibits ATP synthase. R matches 4.

4. Butyl malonate is a malonate analogue that inhibits the dicarboxylate carrier of the inner membrane. It blocks mitochondrial uptake of succinate (and malate). S matches 3. Plain malonate would instead inhibit succinate dehydrogenase directly, but butyl malonate is the transport blocker.

5. The matching is P-2; Q-1; R-4; S-3, which is option A. Options B, C and D were already removed by steps 1 and 2.