Two types of molecular motors. (a) Linear motor: Two linear molecular motors, kinesin and dynein, are shown. Both walk along a microtubule, but in opposite directions. In this schematic the motors are shown attached to vesicles for intracellular transport along a neuronal process (reproduced with permission from Vale85). (b) Rotary motor: Schematic of a bacterial flagellar rotary motor. Rotational speed is over 100,000 rpm and it measures just 50 nm across (Image provided courtesy of A. Ishijima, Tohoku University, Japan).