Across all of cellular metabolism, during which process is the greatest quantity of ATP manufactured per molecule of glucose under aerobic conditions?

Introduction / Context:
Energy yield from glucose comes from multiple processes: substrate-level phosphorylation in glycolysis and the TCA cycle, and oxidative phosphorylation driven by the electron transport chain. The question asks which process contributes the most ATP under aerobic conditions in respiring cells.

Given Data / Assumptions:

• Glycolysis produces a small ATP yield directly.
• The TCA cycle generates GTP/ATP directly but mainly provides NADH/FADH2.
• Oxidative phosphorylation converts the energy in NADH/FADH2 into ATP using a proton gradient.

Concept / Approach:
Most ATP per glucose is made as electrons from NADH and FADH2 move through complexes I–IV of the electron transport chain, pumping protons that drive ATP synthase. Depending on P/O ratios assumed, the bulk—often >80%—of ATP comes from oxidative phosphorylation, far exceeding the small contributions from substrate-level steps.

Step-by-Step Solution:

Verification / Alternative check:
Inhibitors (e.g., oligomycin) that block ATP synthase abolish the major ATP output; uncouplers increase O2 consumption but collapse ATP production, confirming where most ATP arises.

Why Other Options Are Wrong:

• Fermentation/glycolysis: low ATP yield per glucose.
• Light reactions: pertain to photosynthetic organisms and generate ATP for the chloroplast, not general cellular ATP from glucose oxidation.
• Substrate-level steps alone can never match oxidative phosphorylation output.

Common Pitfalls:
Assuming glycolysis is the main ATP source because it happens first; in aerobic cells, oxidative phosphorylation dominates energy yield.

Final Answer:
oxidative phosphorylation (electron transport chain coupled to ATP synthase).