Role of oxygen in cellular metabolism Molecular oxygen (O₂) is used in which direct role within aerobic energy metabolism?

Introduction / Context:
Understanding precisely where oxygen participates clarifies why aerobic organisms gain more ATP per molecule of glucose than anaerobes. Oxygen’s role is tightly linked to the electron transport chain (ETC) and oxidative phosphorylation in mitochondria or bacterial membranes.

Given Data / Assumptions:

• Glycolysis does not require O₂ directly.
• The Krebs cycle does not use O₂ directly as a substrate.
• Photophosphorylation occurs in phototrophs using light-excited electrons.

Concept / Approach:
In aerobic respiration, electrons from NADH and FADH₂ flow through ETC complexes to oxygen. Oxygen serves as the terminal electron acceptor, being reduced to water (H₂O). This electron flow drives proton pumping and establishes a proton motive force used by ATP synthase to form ATP. Neither glycolysis nor the Krebs cycle consumes molecular oxygen directly; their dependence on oxygen is indirect via the need to reoxidize NADH/FADH₂ through the ETC.

Step-by-Step Solution:
Identify the pathway that requires O₂ directly: the ETC.Recognize O₂ is reduced to H₂O at cytochrome c oxidase (Complex IV in mitochondria).Link this reduction to proton pumping and ATP synthesis by ATP synthase.Choose “terminal electron acceptor” as the correct role.

Verification / Alternative check:
Inhibitors of Complex IV (e.g., cyanide) block O₂ reduction, collapse proton motive force, and halt ATP production, confirming oxygen’s role at the chain’s terminus.

Why Other Options Are Wrong:

• Photophosphorylation: driven by light, not oxygen as an electron donor.
• Glycolysis/Krebs: no direct O₂ substrates in their reactions.
• Substrate-level phosphorylation: occurs in glycolysis and Krebs independently of O₂.

Common Pitfalls:
Assuming “aerobic” means oxygen is used in every pathway step; it is specifically the ETC that requires O₂.

Final Answer:
As the terminal electron acceptor in the electron transport chain.