48 Chapter 2. What’s inside cells[[Student version, December 8, 2002]]
Figure 2.20:(Electron micrographs; sketch.) Microtubules and their structure. (a)The main figure shows several
microtubules (130 000×magnification). The arrangement of the wall subunits into parallel rows is clearly visible
(brackets). The inset shows a single microtubule in cross-section (830 000×magnification). Each microtubule is
constructed from a circle of 13 protofilaments surrounding a central channel (C). (b)The sketch shows how the
subunits line up to form the microtubule wall. Tubulin monomers, calledαandβ,first link inαβpairs to form the
dumbbell-shaped subunits shown in the diagram; the dumbbells then line end-to-end to form the microtubule. The
dashed lines are separated by 8nm,the distance between adjacentβsubunits. [From (Wolfe, 1985).] [Copyrighted
figure; permission pending.]
Actin filaments(also called “filamentous” actin, orF-actin)form a second class of cytoskeletal
elements. F-actin fibers are only 7nmin diameter; they can be several micrometers long (Fig-
ure 2.4k). A thin meshwork of these filaments underlies the surface of the cell, forming the cell’s
actin cortex. Filopodia, lamellipodia, and microvilli are all full of actin fibers, which cross-link
to each other, forming stiff bundles that help to push these projections out of the cell. Finally,
actin filaments furnish the “tracks” along which single-molecule motors walk to generate muscle
contraction (Chapter 10).
Examples of even more elaborate protein assemblies include the shells surrounding viruses, and
the whiplike bacterial flagellum (see Figure 2.3 on page 32).
2.3 Bridging the gap: Molecular devices
As projected (Roadmap, page 32), we now have a catalog of beautifulstructuresin cells, but little
has been said about how they form from the molecules in Section 2.2, nor indeed about how cells
carry out the many otheractivitiescharacteristic of life. To begin bridging this gap, this section will
sketch a few of the molecular devices cells use. The unity of living things becomes very apparent
when we study molecular devices: While all cells are somewhat similar at the level of physiology,
they areverysimilar at the molecular level. Today’s routine use of bacteria as factories for the
expression of human genes testifies to this unity. The discovery of the function of human genes by