In mitochondrial electron transport, what is the direct role of \(\mathrm{O_2}\)?

• A. It acts as the final electron acceptor, forming water.
• B. It donates electrons to Complex I.
• C. It breaks down glucose into pyruvate.
• D. It pumps protons across the inner mitochondrial membrane.

Answer: (A)

Verified step by step guidance

1. Understand the context: The mitochondrial electron transport chain (ETC) is a series of protein complexes located in the inner mitochondrial membrane. Its primary function is to transfer electrons from electron carriers (NADH and FADH2) to oxygen, generating a proton gradient that drives ATP synthesis.
2. Identify the role of oxygen (\(\mathrm{O_2}\)): Oxygen is crucial in the ETC because it serves as the final electron acceptor. Without oxygen, the chain would back up, and no further electron transfer would occur, halting ATP production.
3. Clarify the reaction involving oxygen: At Complex IV (cytochrome c oxidase), oxygen accepts electrons and combines with protons (\(\mathrm{H^+}\)) to form water (\(\mathrm{H_2O}\)). This reaction is essential for maintaining the flow of electrons through the ETC.
4. Eliminate incorrect options: Oxygen does not donate electrons to Complex I (this is the role of NADH), it does not break down glucose into pyruvate (this occurs in glycolysis), and it does not directly pump protons across the membrane (this is done by Complexes I, III, and IV).
5. Conclude the correct answer: The direct role of oxygen in the ETC is to act as the final electron acceptor, forming water. This ensures the continuation of the electron transport process and the generation of the proton gradient necessary for ATP synthesis.