Krebs cycle, oxidative phosphorylation, and substrate-level phosphorylation occur in which cellular compartment?

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

Answer: (B)

Energy-yielding steps for carbohydrate oxidation are housed in mitochondria. The Krebs cycle runs in the mitochondrial matrix, oxidizing acetyl-CoA to CO2 and generating NADH and FADH2. Oxidative phosphorylation—the production of ATP via the electron transport chain and ATP synthase—happens across the inner mitochondrial membrane, using the proton gradient created by those electron transfers. Substrate-level phosphorylation occurs during the Krebs cycle itself when succinyl-CoA is converted to succinate, yielding GTP (which can be converted to ATP) within the mitochondrial matrix. Because these three steps are coordinated inside the mitochondria, this is the correct compartment. The cytosol houses glycolysis (which also includes substrate-level phosphorylation) but not the Krebs cycle or oxidative phosphorylation; the nucleus and endoplasmic reticulum are not sites of oxidative metabolism.