Oxidative phosphorylation yield — Approximately how many molecules of ATP are synthesized per mitochondrial NADH oxidized via the electron transport chain (classic P/O ratio)?

Introduction / Context:
The P/O ratio estimates ATP yield per atom of oxygen reduced (or per NADH/FADH2 oxidized). Although modern estimates often cite ~2.5 ATP per NADH due to leaky membranes and variable coupling, classical teaching in many exam settings uses 3 ATP per NADH. Knowing both values and the rationale helps reconcile textbooks and experimental data.

Given Data / Assumptions:

• NADH donates electrons to Complex I; electrons pass to coenzyme Q, Complex III, cytochrome c, and Complex IV.
• Proton pumping occurs at Complexes I, III, and IV.
• ATP synthase uses the proton gradient to make ATP from ADP + Pi.

Concept / Approach:
Using the classical model, the number of protons pumped per NADH is sufficient to generate approximately 3 ATP molecules. Contemporary bioenergetics refines this to ~2.5 ATP per NADH because the H+/ATP stoichiometry and proton leak decrease the effective yield. Given the options, the expected key is 3.

Step-by-Step Solution:

Verification / Alternative check:
Modern calculations often use ~10 H+ pumped per NADH and ~4 H+ per ATP synthesized and transported, giving ~2.5 ATP. However, exam tradition frequently expects “3” unless specified otherwise.

Why Other Options Are Wrong:

Common Pitfalls:
Failing to distinguish between classic teaching values and modern estimates; not reading the exam’s framing.

Final Answer:
3.