Q.12 Determine the correctness or otherwise of the following Assertion [a] and Reason [r].

Assertion: In the process of ATP synthesis in oxidative phosphorylation, ATP synthase is not a part of electron transport chain on inner mitochondrial membrane.

Reason: ATP synthase is coupled to electron transport chain through proton motive force.

Both the assertion and reason are true, but the reason does not explain the assertion. The correct answer is (B).

ATP synthase, known as Complex V, resides in the inner mitochondrial membrane but functions separately from the electron transport chain (ETC), which consists of Complexes I-IV responsible for electron transfer and proton pumping. The proton motive force generated by the ETC indirectly couples to ATP synthase to drive ATP synthesis during oxidative phosphorylation.​

Assertion Analysis

The assertion states that ATP synthase is not part of the ETC on the inner mitochondrial membrane during ATP synthesis in oxidative phosphorylation. This holds true because the ETC specifically refers to the redox protein complexes (I: NADH dehydrogenase, II: succinate dehydrogenase, III: cytochrome bc1, IV: cytochrome c oxidase) that transfer electrons from NADH/FADH₂ to oxygen, creating the proton gradient. ATP synthase (Complex V) uses this gradient for ATP production but does not participate in electron transport itself.​

Reason Analysis

The reason claims ATP synthase couples to the ETC via proton motive force. This is accurate: electron flow through ETC complexes pumps protons into the intermembrane space, establishing a proton motive force (electrochemical gradient) that protons flow back through ATP synthase’s F₀ subunit, rotating the F₁ subunit to synthesize ATP from ADP + Pᵢ.​

Option Breakdown

• (A) Both true, [r] correct reason for [a]: Incorrect. While both are true, the reason describes the coupling mechanism but does not justify why ATP synthase is excluded from the ETC definition.​

• (B) Both true, [r] not correct reason for [a]: Correct. The assertion addresses ATP synthase’s distinct role, unrelated to the reason’s explanation of energy coupling.​

• (C) Both false: Incorrect. Standard biochemistry confirms both statements.​

• (D) [a] false, [r] true: Incorrect. ATP synthase is not part of the ETC.​

Introduction to ATP Synthase and Electron Transport Chain

In oxidative phosphorylation, the electron transport chain (ETC) on the inner mitochondrial membrane drives ATP production, but ATP synthase stands apart. This CSIR NET-style assertion-reason question tests understanding of their distinct roles: ETC (Complexes I-IV) pumps protons, while ATP synthase harnesses the proton motive force for ATP synthesis.​

Mechanism of Oxidative Phosphorylation

Electrons from NADH and FADH₂ flow through ETC complexes, releasing energy to pump H⁺ into the intermembrane space. This creates a proton gradient (proton motive force). Protons re-enter the matrix via ATP synthase’s F₀ channel, spinning its rotor to phosphorylate ADP in the F₁ head—yielding ~30-32 ATP per glucose.​

• Complex I-IV: Electron carriers (FMN, Fe-S, cytochromes, Cu).​

• ATP synthase (Complex V): Proton-driven motor, not electron-transferring.​

Why ATP Synthase Is Not ETC

Textbooks define ETC as Complexes I-IV; ATP synthase is oxidative phosphorylation’s ATP-generating enzyme, coupled indirectly. It’s embedded in the membrane but lacks redox centers for electron shuttling.​

CSIR NET Exam Insights

Such questions emphasize chemiosmotic theory: ETC builds gradient, ATP synthase uses it. Inhibitors like oligomycin block ATP synthase, not ETC. Master for competitive exams.​

Clinical Relevance

Defects in ATP synthase link to mitochondrial diseases; ETC disruptions (e.g., cyanide on Complex IV) halt proton motive force.​