Where do NADH molecules produced by the Krebs cycle donate their electrons?

• A. Cytoplasm
• B. Intermembrane space
• C. Inner mitochondrial membrane
• D. Outer mitochondrial membrane

Answer: (C)

NADH produced by the Krebs cycle donates its electrons at the inner mitochondrial membrane, specifically to Complex I of the electron transport chain. Since the Krebs cycle runs in the mitochondrial matrix, the NADH is already inside the mitochondrion and hands off electrons to NADH dehydrogenase located on the inner membrane. From there, electrons move through the chain to eventually reduce oxygen, while protons are pumped across the inner membrane to drive ATP synthesis. The other membranes and the cytoplasm aren’t where this donation occurs, so the inner mitochondrial membrane is the correct site.