Cellular Respiration—Major Source of ATP During aerobic cellular respiration in eukaryotic cells, the majority of adenosine triphosphate (ATP) molecules are generated by which process?

Introduction / Context:
Aerobic cellular respiration extracts energy from glucose and other fuels in several stages: glycolysis, pyruvate oxidation, the tricarboxylic acid (TCA) cycle, and the electron transport chain coupled to oxidative phosphorylation. Understanding where most ATP is made is essential for biochemistry and physiology.

Given Data / Assumptions:

• Glycolysis yields a net of 2 ATP per glucose by substrate-level phosphorylation.
• TCA cycle directly yields only small amounts of ATP (or GTP) via substrate-level phosphorylation.
• Reduced cofactors (NADH, FADH2) donate electrons to the respiratory chain to drive ATP synthesis.

Concept / Approach:
Oxidative phosphorylation couples electron transport to ATP formation via chemiosmosis. Electron flow through complexes I–IV pumps protons, creating a proton-motive force that powers ATP synthase. This step accounts for the largest fraction of ATP produced per glucose under aerobic conditions.

Step-by-Step Solution:

Verification / Alternative check:
Pharmacologic inhibition (e.g., oligomycin or cyanide) collapses ATP output despite intact glycolysis/TCA, confirming oxidative phosphorylation as the principal ATP source in aerobiosis.

Why Other Options Are Wrong:

• Photophosphorylation: occurs in chloroplasts during photosynthesis, not cellular respiration.
• Glycolysis and substrate-level phosphorylation produce only a small portion of total ATP.
• Fermentation does not maximize ATP yield and occurs without the electron transport chain.

Common Pitfalls:
Assuming “most ATP” comes from the TCA cycle itself; the cycle mainly provides NADH/FADH2 for oxidative phosphorylation.

Final Answer:
Oxidative phosphorylation