P/O ratio (classic value) – NADH to ATP For each pair of electrons transferred from one NADH (generated inside mitochondria) to oxygen via the electron transport chain, how many ATP molecules are formed under classical textbook assumptions?

Introduction / Context:
The phosphorylation-to-oxygen (P/O) ratio links electron transport to ATP synthesis. While modern bioenergetics often cites approximate values (about 2.5 ATP per mitochondrial NADH), many foundational exam keys use the classic value. Knowing the convention being tested avoids avoidable mistakes.

Given Data / Assumptions:

• Classic textbooks: 3 ATP per NADH, 2 ATP per FADH₂.
• Modern estimates: ~2.5 ATP per NADH, ~1.5 ATP per FADH₂.
• The question references “inside the mitochondria,” implying no shuttle losses.

Concept / Approach:
Using the traditional convention, one NADH donates a pair of electrons to the ETC, driving proton pumping at Complexes I, III, and IV. The resulting proton motive force is sufficient (in the classical model) to generate approximately 3 ATP per NADH oxidized. Many standardized tests retain this rounded value for simplicity.

Step-by-Step Solution:
Apply the classic P/O ratio for NADH: 3 ATP per NADH.Confirm context: mitochondrial NADH avoids cytosolic shuttle penalties.Select “3” as the answer.

Verification / Alternative check:
Calculations of total ATP per glucose in older schemas (36 ATP for eukaryotes) derive from 3 ATP per NADH and 2 per FADH₂, validating the testing convention.

Why Other Options Are Wrong:

• 1 or 2: underestimate classical yield.
• 4: exceeds typical theoretical yields.
• 2.5: closer to modern estimates but not the traditional exam key.

Common Pitfalls:
Mixing modern mechanistic values with legacy exam conventions; always follow the convention indicated by companion questions (e.g., total ATP per glucose of 36 in eukaryotes).

Final Answer:
3.