What is the process by which an organism uses oxygen to release energy from food?

• A. Digestion
• B. Fermentation
• C. Respiration
• D. Photosynthesis

Answer: (C)

Cellular respiration is the process by which cells use oxygen to release energy stored in food. In most organisms, glucose is broken down through glycolysis in the cytoplasm, producing a small amount of ATP and forming pyruvate. The pyruvate then enters mitochondria, where the citric acid cycle and the electron transport chain extract more energy. Oxygen acts as the final electron acceptor in the chain, combining with electrons and protons to form water and allowing a large amount of ATP to be produced. This combination of steps is why oxygen is essential for efficient energy extraction from food. If oxygen isn’t present, cells can switch to fermentation, which allows glycolysis to continue but yields far less ATP and makes byproducts like lactic acid or ethanol. Photosynthesis is the process that plants use to store energy from light in glucose, not to release energy from existing food.