Figure 1

Directed vs. random motion. Traveled distance (directed movement) and root mean square displacement (random movement) of directed and random motion plotted as functions of time (left) and square root of time (right). On the left hand side, directed motion yields a straight line, with a slope given by its speed. On the right hand side, random motion yields a straight line, with a slope given by its diffusion coefficient (also called motility coefficient in this paper).