Final answer:
Complex II in the electron transport chain transfers electrons but does not pump protons across the inner mitochondrial membrane, unlike Complex I, III, and IV.
Step-by-step explanation:
The electron transport chain (ETC) consists of four complexes (I-IV) that are involved in the transfer of electrons and movement of protons across the inner mitochondrial membrane, which is vital for ATP production. While Complex I, Complex III, and Complex IV contribute to creating the proton gradient by pumping protons into the intermembrane space, Complex II is unique as it does not translocate protons. Instead, Complex II helps in transferring electrons to Coenzyme-Q (Q) but bypasses proton pumping, therefore it does not contribute directly to the generation of the proton motive force used for ATP synthesis.
FADH2 donates electrons to the ETC via Complex II, leading to fewer ATP molecules being produced compared to when NADH donates electrons via Complex I, since Complex II does not pump protons. This differential impact on ATP synthesis highlights the distinct roles of different complexes in the ETC. Overall, it is Complex II that does not translocate protons to the inner membrane space.