Answer: Oxygen supply is insufficient (anaerobic conditions/oxygen debt).
- A Mitochondrial electron transport chains are saturated with surplus oxygen
- B Oxygen supply is insufficient (anaerobic conditions/oxygen debt)
- C Glycogen stores are fully depleted after prolonged starvation
- D Cytoplasmic ATP concentration exceeds the cell's regulatory threshold
Correct answer: B. Oxygen supply is insufficient (anaerobic conditions/oxygen debt)
Explanation: Under oxygen debt (vigorous exercise), muscle cells convert pyruvate to lactic acid to regenerate NAD+ for glycolysis to continue, without needing O<sub>2</sub>.
Aerobic respiration unfolds in three locations: glycolysis in the cytoplasm yields a small amount of ATP directly, the Krebs cycle in the mitochondrial matrix harvests electron carriers (NADH, FADH2), and the electron transport chain in the inner mitochondrial membrane uses those carriers to generate the bulk of the ATP, with O2 needed only at this final stage.
Concept context
Glycolysis, the Krebs cycle, the electron transport chain, fermentation, and the respiratory quotient. The cellular energy-release counterpart to photosynthesis.