Which of the following statements is incorrect ?

The correct option (the incorrect statement) 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 the biological concepts behind each statement to understand why they are correct or incorrect:

1. Role of oxygen in aerobic respiration:
During aerobic respiration, the chemical role of oxygen is limited to the terminal stage of the electron transport chain (ETC). Oxygen acts as the final electron acceptor, combining with protons and electrons to form water:
$H_{2}O$
Even though oxygen only acts at the very end of the pathway, its presence is vital because it continuously removes electrons from the system, keeping the entire transport chain functioning. Thus, this statement is correct.

2. ATP synthesis through Complex V:
The energy stored in the proton gradient is harvested by Complex V, also known as ATP Synthase. As protons flow back into the mitochondrial matrix through Complex V, the enzyme undergoes conformational changes that catalyze the synthesis of ATP from ADP and inorganic phosphate:
$\mathrm{ADP}+\mathrm{Pi}\to \mathrm{ATP}$
Thus, ATP is indeed synthesized through Complex V, making this statement correct.

3. Proton gradient generation via redox reactions:
As electrons travel down the electron transport chain through a series of oxidation-reduction (redox) reactions, energy is released. The complexes (specifically Complexes I, III, and IV) use this energy to pump protons:
$H^{+}$
from the mitochondrial matrix into the intermembrane space. This active transport creates the proton gradient (proton motive force) across the inner mitochondrial membrane. Thus, this statement is correct.

4. ATP yield of NADH vs. FADH₂:
During oxidative phosphorylation in the ETC:
- One molecule of NADH + H⁺ enters at Complex I and contributes to pumping enough protons to synthesize 3 molecules of ATP (more precisely, 2.5 ATP).
- One molecule of FADH₂ enters later at Complex II, bypassing the first proton-pumping complex. As a result, it pumps fewer protons and yields only 2 molecules of ATP (more precisely, 1.5 ATP).
Because the statement incorrectly claims that NADH + H⁺ gives 2 ATP and FADH₂ gives 3 ATP, it is incorrect.