6.3.1 Stoichiometry of proton ejection
It is commonly stated that about 10 protons are ejected for each pair of electrons abstracted from NADH, such that 4 protons are ejected at each of complexes I and IV, and 2 at complex III1. Complex II does not eject any protons but just abstracts them from FAD and passes them on to ubiquinone. If you look at table 6.2.2-1, you will notice that the difference in the redox potentials of FAD and ubiquinone is rather small. Consequently, the amount of free energy associated with the transfer of electrons from FAD to ubiquinone is too small to permit the performance of work against the proton gradient.
1: Note that this number is at variance with the model of the coenzyme Q cycle given above. Generally speaking, the figures given for the numbers of protons ejected at each stage vary quite a bit between various books.