FIG. 1.

(color). Schematic model of the bacterial flagellar motor. Left: The passage of a proton, or possibly a group of protons, through a torque-generating unit (a MotA/B stator—only one stator of about 10 is shown) causes a protein spring to stretch to its next attachment site, represented by circles, on the rotor. Right: To translocate, a proton must pass through an external gate, over a barrier, and finally through an internal gate, with all the steps assumed to be reversible. The net energy difference driving proton translocation is the electrical potential energy, −eΔψ, minus the work, τδθ, necessary to stretch the protein spring by δθ, where τ is the torque applied by the spring to the rotor.