Large amounts of ATP are generated when

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

Answer: (A)

ATP production is maximized when cells have enough oxygen to power aerobic respiration. With oxygen present, glucose is fully broken down in the mitochondria through glycolysis, the citric acid cycle, and the electron transport chain. The electron transport chain uses the energy from electrons to pump protons and drive oxidative phosphorylation, yielding about 36–38 ATP per glucose molecule. In contrast, without oxygen, cells rely on glycolysis alone (anaerobic), which makes only about 2 ATP per glucose and is far less efficient. CO2 buildup or falling blood glucose don’t directly boost ATP yield; they’re not the driving cause of the high ATP output seen with adequate oxygen.