Figure 2.

Electron transport paths. Electrons from NADH can go through one of 4 paths. The first branch (shown in red) is the partially coupled branch involving NDHII and Cyt bd, where electron transport is only coupled to proton translocation in Cyt bd. The fully coupled branch (shown in blue) utilizes the typical electron transport chain machinery to pump protons out. The other two paths involve Fix and Rnf (shown in black), which generate low-potential electrons using electron bifurcation or proton motive force, respectively.