What process converts molecular oxygen into energy stored in ATP?

• A. Photosynthesis
• B. Chemiosmosis
• C. Cellular respiration
• D. Fermentation

Answer: (C)

The process that converts molecular oxygen into energy stored in ATP is cellular respiration. This biochemical pathway takes place in several stages: glycolysis, the citric acid cycle (Krebs cycle), and oxidative phosphorylation. During cellular respiration, glucose and other organic molecules are metabolized, and molecular oxygen serves as the final electron acceptor in the electron transport chain. This results in the production of ATP through oxidative phosphorylation, where the energy from electrons is used to pump protons across the inner mitochondrial membrane, creating a proton gradient that drives ATP synthase to generate ATP.

In contrast, photosynthesis primarily involves the conversion of light energy into chemical energy in the form of glucose, which is later used during cellular respiration. Chemiosmosis, while a key component of ATP production in cellular respiration, does not itself convert oxygen into energy; instead, it describes the process of ATP synthesis linked to the proton gradient established during electron transport. Fermentation occurs in anaerobic conditions where oxygen is not present, producing ATP without using molecular oxygen. Thus, the correct answer reflects the complete and aerobic process of energy conversion involving oxygen.