Which process in mitochondria generates ATP through oxidative phosphorylation and the citric acid cycle?

• A. Photosynthesis.
• B. Fermentation.
• C. Protein synthesis.
• D. Cellular respiration.

Answer: (D)

Cellular respiration in mitochondria generates most of the cell’s ATP by oxidative phosphorylation coupled to the citric acid cycle. In this pathway, glucose is broken down stepwise, and the citric acid cycle oxidizes acetyl-CoA to carbon dioxide, producing energized carriers that feed electrons into the electron transport chain in the inner mitochondrial membrane. As electrons move through this chain, a proton gradient is created across the membrane, and ATP synthase uses that gradient to phosphorylate ADP into ATP—this linking of electron transport to ATP production is oxidative phosphorylation. Photosynthesis happens in chloroplasts and stores energy as sugars. Fermentation can yield some ATP without the mitochondria but omits the citric acid cycle and full oxidative phosphorylation. Protein synthesis, while energy-requiring, is the assembly of amino acids into proteins and not the primary ATP-generating pathway through mitochondria.