In the mitochondrial electron transport chain, what is the role of cytochrome c?

• A. Donates protons to the matrix
• B. Transfer electrons between complexes III and IV
• C. Catalyze ATP synthesis
• D. Repair mitochondrial DNA

Answer: (B)

Cytochrome c is a mobile electron carrier in the inner mitochondrial membrane. It shuttles a single electron from Complex III (the cytochrome bc1 complex) to Complex IV (cytochrome c oxidase) by cycling its heme iron between Fe3+ and Fe2+. This transfer keeps the electron flow going toward oxygen, supporting proton pumping by the other complexes and the resulting proton gradient used by ATP synthase to make ATP. It doesn’t move protons itself, nor does it synthesize ATP directly, and it isn’t involved in repairing mitochondrial DNA.