Oxidative phosphorylation – required conditions Organisms can synthesize ATP by oxidative phosphorylation under which of the following conditions?

Introduction / Context:
Oxidative phosphorylation couples the energy released by electron transfer through a respiratory chain to ATP synthesis by ATP synthase. It is distinct from substrate-level phosphorylation and photophosphorylation. Identifying the condition that enables oxidative phosphorylation is critical for understanding energy yields in cells.

Given Data / Assumptions:

• Presence of an electron transport chain (ETC) is essential.
• A suitable terminal electron acceptor (commonly O₂ in aerobic respiration) is required.
• Proton motive force across a membrane drives ATP synthase.

Concept / Approach:
During aerobic respiration, electrons from NADH/FADH₂ flow through ETC complexes (including cytochromes) to O₂. Proton pumping establishes a gradient used by ATP synthase to form ATP—this is oxidative phosphorylation. Fermentation lacks an ETC; glycolysis alone does not constitute oxidative phosphorylation. Photophosphorylation is a light-driven, not oxidative, process in phototrophs.

Step-by-Step Solution:
Identify options that include an ETC and terminal acceptor.Recognize that electron flow to O₂ via cytochromes defines aerobic oxidative phosphorylation.Select the corresponding option.

Verification / Alternative check:
Inhibitors of the ETC (e.g., cyanide, antimycin A) or uncouplers (e.g., DNP) block ATP synthesis via oxidative phosphorylation, confirming the dependence on electron transfer to O₂ and proton motive force.

Why Other Options Are Wrong:

• Fermentation/glycolysis: rely on substrate-level phosphorylation; no ETC involvement.
• Photophosphorylation: light-driven in chloroplasts or photosynthetic membranes, not oxidative phosphorylation.
• Substrate-level phosphorylation in Krebs: generates GTP/ATP without an ETC.

Common Pitfalls:
Equating any ATP formation with oxidative phosphorylation; only ETC-driven ATP synthesis qualifies.

Final Answer:
When electrons pass to oxygen via an electron transport chain containing cytochromes.