Correct Answer: Both (A) and (R) are correct and (R) is the correct explanation of (A).

Oxidative phosphorylation is a key ATP-producing process in cellular respiration. It couples the electron transport chain (ETC) with ATP synthesis, relying on energy from a proton gradient.

Assertion (A) Analysis

Assertion (A) states that oxidative phosphorylation requires energy for ATP synthesis from ADP and Pi. This is correct because ATP formation is endergonic, needing about 30.5 kJ/mol under cellular conditions to drive the phosphorylation reaction.

Reason (R) Analysis

Reason (R) states that the proton gradient from the ETC provides this energy. This is accurate: ETC complexes (I, III, IV) pump protons into the intermembrane space, creating an electrochemical gradient (proton-motive force) that powers ATP synthase.

Option Breakdown

• Both (A) and (R) correct, (R) explains (A): Selected, as the proton gradient directly supplies the energy for ATP synthesis via chemiosmosis.

• Both correct, but (R) not explanation: Incorrect, since the gradient is the precise mechanism linking ETC to phosphorylation.​

• (A) correct, (R) incorrect: Wrong, as (R) accurately describes the energy source.​

• (A) incorrect, (R) correct: Invalid, because energy input is essential for the non-spontaneous reaction.​

Introduction to Oxidative Phosphorylation

Oxidative phosphorylation ATP synthesis energy requirement is met by the proton gradient generated during the electron transport chain in mitochondria. This process yields ~30 ATP per glucose, far exceeding glycolysis.

Mechanism of Energy Coupling

Electrons from NADH/FADH2 flow through ETC complexes, pumping H+ across the inner mitochondrial membrane. The resulting proton gradient (ΔpH and Δψ) drives H+ back through ATP synthase, rotating it to phosphorylate ADP.

Assertion-Reason in Exam Context

In competitive exams, such questions test chemiosmotic theory. Here, (R) explains (A) because without the gradient, no ATP forms—uncouplers like DNP dissipate it, halting synthesis.

This ties into your molecular biology focus, emphasizing ETC efficiency for plant/animal respiration studies.