ETC has ______ protein complexes and ______ is the final electron acceptor

• A. 3, NAD+
• B. 4, oxygen
• C. 5, NAD+
• D. 4, water

Answer: (B)

The electrons transport chain uses four protein complexes embedded in the inner mitochondrial membrane. Electrons flow from NADH (and FADH2) through Complex I (and II) to ubiquinone, then Complex III, cytochrome c, and finally Complex IV, where oxygen accepts the electrons and combines with protons to form water. This transfer of electrons creates the proton gradient that powers ATP synthase to make ATP. NAD+ is not the final electron acceptor—it’s the oxidized form that accepts electrons earlier in metabolism, while water is the product formed at the end. So four complexes and oxygen as the final electron acceptor is the correct pairing.