Figure 2. How electron transfer down a membrane-embedded respiratory chain might pump protons.  In order to account for this process, Mitchell postulated the existence of “loops” in the respiratory chain.  Translated into English, this means alternation of electron/proton carriers with electron carriers across the membrane.  More specifically, a flavoprotein hydrogenase on the inner surface of the membrane [(Fp); upon reduction, FAD or FMN accepts electrons and protons] transfers its electron to a non-heme iron sulfur protein (NHFeS) disposed towards the outer surface of the membrane (iron accepts electrons only), and the proton is released into the external medium.  The NHFeS then transfers electrons to ubiquinone (Q) on the inner surface of the membrane which picks up protons from the internal solvent.  QH₂ then transfers electrons to cytochrome b (CYT b) towards the outer surface which accepts only electrons, and 2 protons are released into the external medium.  Finally, CYT. B transfers electrons to the terminal oxidase (CYT ox) on the inner surface which reduces oxygen on the inside utilizing another 2 protons to make water.