50 Chapter 2. What’s inside cells[[Student version, December 8, 2002]]
sides. Some examples include thechannels,which allow the passage of specified molecules
under specified conditions, “receptors,” which sense exterior conditions, and “pumps, which
actively pull material across the membrane (see Figure 2.30).- Receptors can in turn connect to “peripheral membrane proteins,” which communicate infor-
mation to the interior of the cell. - Still other integral membrane proteins anchor the cell’s membrane to its underlying actin cor-
tex, helping the cell to maintain its optimal shape. A related example concerns the membrane
of the human red blood cell. A network of elastic protein strands (in this case spectrin) is
anchored to the membrane by integral membrane proteins. This network deforms as the red
cell squeezes through the body’s capillaries, then pops the cell back to its normal shape after
its passage.
2.3.2 Molecular motors
As mentioned earlier, actin filaments form the “tracks” along which tiny motors walk, generating
muscle contraction (Chapter 10). Many other examples of walking motors are known in cells.
Figure 2.21 shows a vesicle being dragged along a microtubule to its destination at an axon terminal.
This “axonal transport” brings needed proteins to the axon terminal, as well as the ingredients from
which synaptic vesicles will be built. A family of single-molecule motors called “kinesins” supply
the motive force for this and other motions, for example the dragging of chromosomes to the two
halves of a dividing cell. Indeed, selectively staining both the microtubules and the kinesin (by
attaching fluorescent markers to each) shows that they are generally found together in the cell
(Figure 2.26). It is even possible to follow the progress of individual kinesin molecules as they walk
along individual microtubules (Figure 2.27). In such experiments, the kinesin molecules begin to
walk as soon as a supply of ATP molecules is added; they stop when the ATP is used up or washed
away.
The cilia mentioned in Section 2.1.2 are also powered by walking motors. Each cilium contains
abundle of microtubules. A motor molecule called dynein attaches to one microtubule and walks
along its neighbor, inducing a relative motion. Coordinated waves of dynein activity create traveling
wavesofbending in the cilium, making it beat rhythmically.
Other motors generaterotarymotion. Examples include the motor that drives the bacterial
flagellum (Figure 2.3b; see Chapters 5 and 11), and the one that drives the synthesis of ATP in
mitochondria (Chapter 11). Rather than being driven directly by ATP, both of these motors use
as their “fuel” a chemical imbalance between the sides of the membrane they span. Ultimately the
imbalance comes from the cell’s metabolic activity.