Which metabolic pathway occurs in the mitochondria and contributes to ATP production via oxidative phosphorylation?

• A. Glycolysis and fermentation
• B. Photosynthesis
• C. Krebs cycle and oxidative phosphorylation
• D. Fermentation only

Answer: (C)

Oxidative phosphorylation is the main ATP-making process in the mitochondria, and it relies on the Krebs cycle to supply the needed electron carriers. In the mitochondrial matrix, the Krebs cycle generates NADH and FADH2. These carriers donate electrons to the electron transport chain in the inner mitochondrial membrane. As electrons move through the chain, protons are pumped across the membrane, creating a proton gradient. ATP synthase then uses this gradient to convert ADP and inorganic phosphate into ATP. This combination—Krebs cycle feeding the electron transport chain and oxidative phosphorylation producing most of the cell’s ATP—occurs in the mitochondria. Glycolysis happens in the cytoplasm, fermentation occurs without mitochondria, and photosynthesis takes place in chloroplasts, not mitochondria.