Figure 8.

a) Schematic picture of how a chemical process can drive directed mechanical motion. The mechanism illustrated with the solid arrows involves two “power strokes”, downhill “slides” on the slopes of U₁ and U₂. The mechanism with the dotted blue arrows looks impossible from the point of view of macroscopic physics, but when the activation energies for the chemical processes are equal, ε = ε*, the mechanism with dotted blue arrows in which the motor steps left is just as likely as the mechanism involving the solid blue arrows in which the motor steps right. b) and c) Potential energy surfaces for the ratchet mechanism in Figure 7a for the cases ε* < ε and ε < ε*, respectively, where the most probable trajectories are shown by the white dashed and solid curves.