Which of the following statements is incorrect ?

The correct answer is: In ETC (Electron Transport Chain), one molecule of NADH+H⁺ gives rise to 2 ATP molecules, and one FADH₂ gives rise to 3 ATP molecules.

Let us analyze each statement step-by-step to understand why this statement is incorrect and the others are correct:

1. Role of oxygen in aerobic respiration:
During aerobic respiration, oxygen acts as the final hydrogen acceptor. Its role is indeed limited to the terminal stage of the electron transport chain (ETC), where it combines with electrons and protons to form water. However, the presence of oxygen is vital because it drives the entire process by removing electrons from the system. Therefore, the statement "During aerobic respiration, role of oxygen is limited to the terminal stage" is correct.

2. ATP yield from NADH+H⁺ and FADH₂ in the ETC:
In the respiratory electron transport chain, the oxidation of one molecule of NADH + H⁺ leads to the pumping of protons across the inner mitochondrial membrane, which generates enough proton-motive force to synthesize
$3$
molecules of ATP (not 2). On the other hand, the oxidation of one molecule of FADH₂ yields
$2$
molecules of ATP (not 3). Because the values are reversed in the statement, "In ETC, one molecule of NADH+H⁺ gives rise to 2 ATP molecules, and one FADH₂ gives rise to 3 ATP molecules" is incorrect.

3. Synthesis of ATP through Complex V:
Complex V of the electron transport chain is ATP Synthase. This enzyme complex utilizes the proton gradient established across the inner mitochondrial membrane to synthesize ATP from ADP and inorganic phosphate (Pi). Therefore, the statement "ATP is synthesized through complex V" is correct.

4. Production of proton gradient:
As electrons pass through the electron transport carriers (Complexes I to IV) via sequential oxidation-reduction reactions, energy is released. This energy is coupled to pump protons (H⁺) from the mitochondrial matrix into the intermembrane space, creating a proton gradient. Therefore, the statement "Oxidation-reduction reactions produce proton gradient in respiration" is correct.