Which organelle would be most affected by a toxin that inhibits ATP production through oxidative phosphorylation?

• A. Nucleus
• B. Chloroplast
• C. Mitochondrion
• D. Lysosome

Answer: (C)

Oxidative phosphorylation is the main way cells produce ATP during cellular respiration, and it occurs in the inner membrane of the mitochondrion. A toxin that blocks this step stops the production of ATP, so the organelle whose job is to generate ATP is the most directly and severely affected—the mitochondrion. Without functional oxidative phosphorylation, the cell cannot meet energy demands, leading to widespread energy failure.

Chloroplasts in plant cells can generate ATP through photophosphorylation during the light reactions, so they would be less affected by a toxin that specifically inhibits oxidative phosphorylation. The nucleus uses ATP for processes like transcription, but it does not generate ATP itself. Lysosomes rely on ATP to power proton pumps, but their energy supply would also drop as a downstream effect, not because their primary function is to produce ATP.