Fig. 1

Comparison between a macroscopic task and a microscopic information ratchet. Comparison of a moving a brick up a staircase in the macroscopic world, in which losses through friction are inevitable, and b an information ratchet appropriate for description of a molecular machine in the microscopic world where diffusion provides a mechanism for motion without loss owing to friction but in which the second law forbids net directed motion without the input of energy, and c how this can be described in terms of the four state mechanism for a minimal Brownian machine. Each clockwise cycle through the four states describes the movement of the “brick” one step to the right, where W = white, B = black, S = substrate, and P = product, and subscript L denotes the “bound” form of the motor as S and P lose their individual identity once bound to the motor, and \({K}_{\mathrm{b}}\,{\mathrm{and}}\,{K}_{\mathrm{f}}\) are equilibrium constants between the black and white stairs, respectively. Directionality of cycling in c is controlled by the position dependence of the rates for the binding/dissociation processes of S and P