In eukaryotic cells, which organelle performs cellular respiration to generate ATP?

• A. Nucleus
• B. Endoplasmic reticulum
• C. Ribosome
• D. Mitochondria

Answer: (D)

Cellular respiration that yields the most ATP happens inside the mitochondria. The inner membrane hosts the electron transport chain, and the matrix houses the enzymes of the citric acid cycle, with cristae providing a large surface area to drive ATP production efficiently. Glucose is first broken down by glycolysis in the cytoplasm, giving a small amount of ATP and pyruvate, which then enters the mitochondria and is turned into acetyl-CoA. The citric acid cycle oxidizes acetyl-CoA to CO2 and generates NADH and FADH2, which feed electrons into the electron transport chain. As electrons move through the chain, protons are pumped across the membrane, creating a proton gradient that powers ATP synthase to make ATP. The other listed organelles have different roles—nucleus stores genetic material, endoplasmic reticulum handles protein and lipid synthesis, and ribosomes synthesize proteins. Therefore, the mitochondrion is the organelle responsible for cellular respiration and ATP production.