358 Chapter 10. Enzymes and molecular machines[[Student version, January 17, 2003]]
Figure 10.4:(Photomicrograph.) Microtubule polymerization distending an artificial bilayer membrane. Several
microtubules gradually distort an initially spherical vesicle by growing inside it at about 2μmperminute. [Digital
image kindly supplied by D. K. Fygenson. See Fygenson et al., 1997. ]
Figure 10.5:(Photomicrograph.) Polymerization from one end of an actin bundle provides the force that propels
aListeriabacterium (black lozenge) through the cell surface. The long tail behind the bacterium is the network of
actin filaments whose assembly it stimulated. [From Tilney & Portnoy, 1989.]
scary process at work.
Force generation by the polymerization of actin filaments or microtubules is another example of
amotor, in the sense that the chemical binding energy of monomers turns into a mechanical force
capable of doing useful work against the cell membrane (or invading bacterium). The motor is of
the one-shot variety, because the growing filament is different (it’s longer) after every step.^4
10.2 Purely mechanical machines
Tounderstand the unfamiliar we begin with the familiar. Accordingly, this section will exam-
ine some everyday macroscopic machines, show how to interpret them in the language of energy
landscapes, and develop some terminology.