10.1. Survey of molecular devices found in cells[[Student version, January 17, 2003]] 355
Figure 10.1: (Sketches.) Organization of skeletal muscle at successively higher magnifications. The ultimate
generators of force in a myofibril (muscle cell) are bundles of myosin molecules, interleaved with actin filaments (also
called “F-actin”). Upon activation, the myosins crawl along the actin fibers, pulling them toward the plane marked
“M” and thus shortening the muscle fiber. [After McMahon, 1984.]
microtubules (Figure 2.26 on page 55). Somewhere between the truck and the highwaythere must
bean “engine.”
One particularly important example of such an engine,kinesin,was discovered in 1985, in
the course of single-molecule motility assays inspired by the earlier work on myosin. Unlike the
actin/myosin system, kinesin molecules are designed to walk individually along microtubules (Fig-