Large amounts of adenosine triphosphate (ATP) are generated when:

• A. Circulating blood glucose levels fall.
• B. The cells function with adequate oxygen.
• C. Carbon dioxide levels in the blood are high.
• D. The cells function without oxygen.

Answer: (B)

Aerobic respiration produces the most ATP because oxygen serves as the final electron acceptor in mitochondria, allowing the cell to harvest energy from glucose through glycolysis, pyruvate oxidation, the citric acid cycle, and oxidative phosphorylation. When oxygen is available, glucose is fully oxidized, yielding about 30–32 ATP per molecule, which is much more than what anaerobic pathways can produce. If oxygen is scarce, cells rely on anaerobic glycolysis, which only makes about 2 ATP per glucose and uses lactate, so total ATP is much lower. Substrate availability or high carbon dioxide levels don’t drive large ATP production; they don’t enable the same efficient energy harvest as oxygen-dependent oxidative phosphorylation.