Final answer:
Complex I, or NADH dehydrogenase, moves four hydrogen ions (protons) across the mitochondrial inner membrane for each pair of electrons it receives from NADH.
Step-by-step explanation:
Complex I, which is also called NADH dehydrogenase, is the first enzyme complex in the electron transport chain (ETC) of mitochondrial oxidative phosphorylation. When two electrons are transferred from NADH to Complex I, this complex uses the energy released to pump four hydrogen ions (protons, H+) across the inner mitochondrial membrane from the matrix into the intermembrane space.
As electrons pass through Complex I, they move from flavin mononucleotide (FMN) through a series of iron-sulfur (Fe-S) clusters to coenzyme Q (ubiquinone), which facilitates the establishment and maintenance of a hydrogen ion gradient that is essential for ATP synthesis in cells.
Complex I can pump four protons across the membrane per pair of electrons received from NADH, contributing to the proton motive force used by ATP synthase to generate ATP. The efficiency of ATP production is directly proportional to the number of protons pumped across the inner mitochondrial membrane; therefore, the function of Complex I is crucial in cellular energy metabolism.