Cells are more than just passive objects responding to external stresses: they can actively change shape or move with respect to their environment. A very familiar example of cellular shape change is the contraction of our muscle cells. Less familiar, but very important to our health, is the locomotion of cells such as macrophages, which work their way through our tissues to capture and remove hostile cells and material. Another example of cell movement is the rotation of flagella (Latin plural for the noun whip) which extend from some cells and provide them with propulsion in a fluid medium. Flagella can be seen at both ends of the bacterium pictured in Fig. 9.1. Structurally related to flagella are cilia (Latin plural for eyelash), which occur on the surfaces of some cells and wave in synchrony like tall grass in the wind, creating currents in their fluid environment. What microscopic mechanisms underlie a cell's movement or motility?

In Section 9.1, we review several aspects of cell movement and trace their origin to the molecular level. Dynamic biofilaments alone can be responsible for certain shape changes because they can generate a force by increasing their length, as described in Section 9.2. However, contractile forces in muscles, or transport along a microtubule, arise from specialized motor proteins capable of crawling along a biofilament. A number of models have been proposed for the operation of such molecular motors, and these are explained in Section 9.3.