Figure 13.

Anatomy of a “power stroke”. In clockwise cycling, the motor moves through the transition B → A. Seemingly, the energy ΔG_AB provides the power for the power stroke and according to Howard^[82] allows the motor to do work against an applied force or applied torque provided ΔG_AB X_θ, with a maximum efficiency^[51] of η_max = ΔG_AB/X_ξ. This analysis is correct for an optically driven machine, but is totally wrong for a chemically driven motor. In the thermodynamic control limit, irrespective of whether ΔG_AB is positive, negative, or zero, a chemically driven motor can do work against an applied force or applies torque provided X_ξ X_θ, and the maximum eddiciency is bounded only by unity.