Which process releases a large amount of energy in cells by breaking down food substances in the presence of oxygen?

• A. Aerobic respiration
• B. Anaerobic respiration
• C. Fermentation
• D. Photosynthesis

Answer: (A)

This is about producing a lot of energy by breaking down food with oxygen, a process that happens in the mitochondria called aerobic respiration. In this pathway glucose is fully oxidized through a series of steps. First, a small amount of energy is captured in glycolysis in the cytoplasm. The resulting pyruvate then enters the mitochondria, where it’s further processed in the Krebs cycle and, crucially, the electron transport chain. Oxygen acts as the final electron acceptor in that chain, allowing electrons to flow and protons to be pumped across membranes. The energy from this flow drives the synthesis of a large amount of ATP through oxidative phosphorylation. The complete breakdown of glucose in the presence of oxygen also produces carbon dioxide and water as waste products. Because oxygen lets the full amount of energy be extracted, aerobic respiration releases much more energy than pathways that occur without oxygen, like fermentation.