2.3. Bridging the gap: Molecular devices[[Student version, December 8, 2002]] 49
Figure 2.21:(Schematic; electron micrograph.) (a)Model for how kinesin drags an organelle along a microtubule.
Chaper 10 will discuss the action of this single-molecule motor. (b)Micrograph appearing to show the situation
sketched in (a). The microtubule is labeled “MT” (lower left). Arrows show the attachment points. Neurons from
rat spinal cord were flash-frozen and deep-etched to create the sample. [Image kindly supplied by N. Hirokawa; see
(Hirokawaet al.,1989).] [Copyrighted figure; permission pending.]
studying their analogs in fruitflies gives another good example.
2.3.1 The plasma membrane
In order to maintain its identity (for example, to control its composition), every cell must be
surrounded by some sort of envelope. Similarly, every organelle and vesicle, too, must somehow be
packaged. Remarkably, all cells have met all of these challenges with asinglemolecular construction:
the “bilayer membrane” (Figure 2.24). Thus for example the plasma membrane of any cell looks
like a double layer under the electron microscope; all have roughly similar chemical composition,
electrical capacitance, and so on.
As its name implies, the bilayer membrane consists of two layers of molecules, primarily the
phospholipids shown in Figure 2.24. It’s only about 4nmthick, and yet covers the entire exterior
of a cell, often a billion or more square nanometers! To be effective, this fragile-looking structure
must not rip, and yet it must also be fluid enough to let the cell crawl, endocytose, divide, and
so on. We will study the remarkable properties of phospholipid molecules which reconcile these
constraints in Chapter 8.
Weget another surprise when we mix phospholipid molecules with water: Even without of any
cellular machinery,bilayer membranes self-assemble spontaneously.Chapter 8 will show that this
phenomenon is driven by the same interactions that cause salad dressing to separate spontaneously
into oil and water. Similarly, microtubules and F-actin can self-assemble from their subunits,
without the intervention of any special machinery (see Figure 10.4 on page 356).
Bilayer membranes do far more than just partition cells. The outer cell membrane is also studded
with a rich variety of other devices (Figure 2.25):
- “Integral membrane proteins” span the membrane, projecting on both the inner and outer