What organelle is the site of ATP production?

• A. Nucleus
• B. Mitochondria
• C. Ribosomes
• D. Endoplasmic Reticulum

Answer: (B)

Mitochondria are the cell’s powerhouse, generating most of the ATP the cell uses for energy. They carry out aerobic cellular respiration: the Krebs cycle in the mitochondrial matrix oxidizes nutrients to produce NADH and FADH2, and the electron transport chain on the inner mitochondrial membrane uses those high-energy electrons to pump protons and create a gradient. ATP synthase then uses that gradient to convert ADP and inorganic phosphate into ATP. Oxygen serves as the final electron acceptor, linking fuel oxidation to energy production. The mitochondria’s double membrane and internal folds (cristae) maximize surface area for these reactions, boosting efficiency. In contrast, the nucleus stores genetic material, ribosomes build proteins, and the endoplasmic reticulum handles protein and lipid processing. (Some ATP is produced during glycolysis in the cytosol, but the bulk comes from mitochondria.)