Load Dependence of F1 Rotation
Rotation of F1 is slower when a longer rod (actin filament) is attached to the rotor subunit. This is because the rod is rotating in water and the viscous friction imposed on the rod is proportional to the cube of the rod length. Precise analysis indicates that F1 produces a constant torque (rotary force) of about 40 pN nm, irrespective of the viscous load. To appreciate this value, suppose you were an F1 molecule. You would be standing at the bottom of a large swimming pool, holding a rod some 500 m long above you. You are to rotate it in water at the speed shown in the movie at the middle. The power you would need is equivalent to the 40 pN nm of torque for an F1 molecule.