How is fuel turned into ATP?

• A. Only Aerobically
• B. By Photosynthesis
• C. Aerobically and Anaerobically
• D. Only Anaerobically

Answer: (C)

Fuel is turned into ATP through cellular respiration, which can operate with or without oxygen. In either case, glycolysis happens first in the cytoplasm, splitting glucose into pyruvate and yielding 2 ATP and NADH. If oxygen is present, pyruvate enters mitochondria and the full aerobic pathway proceeds—the pyruvate is oxidized, the citric acid cycle runs, and oxidative phosphorylation via the electron transport chain generates the bulk of ATP (roughly 28–32 ATP per glucose, depending on the cell). If oxygen is not available, the cell cannot complete the full aerobic pathway; glycolysis can continue to produce a small amount of ATP, with fermentation regenerating NAD+ to keep glycolysis going, but there’s no large ATP payoff from oxidative phosphorylation. Photosynthesis, on the other hand, is the process by which plants build glucose using light energy, not the method by which existing fuel is converted into ATP in cells. So ATP can be produced both aerobically and anaerobically.