Biological Physics: Energy, Information, Life

8.6. Self-assembly in cells[[Student version, January 17, 2003]] 285

membrane

proteins

bilayer

membrane

detergent

micelles

membrane

fragments

concentration

above CMC

Figure 8.8: (Schematic.) Solubilization of integral membrane proteins (black blobs) by detergent (objects with
shaded heads and one tail).Topright:Ataconcentration higher than its critical micelle concentration, a detergent
solution can form micelles incorporating both phospholipids (objects with white heads and two tails) and membrane
proteins.Bottom right:Detergent can also stabilize larger membrane fragments (which would otherwise self-assemble
into closed vesicles), by sealing off their edges.

• Phospholipids are not particularly exotic or complex molecules. They are relatively easy for a
  cell to synthesize, and phospholipid-like molecules could even have arisen “abiotically” (from
  nonliving processes) as a step toward the origin of life. In fact, bilayer membranes are even
  formed by phospholipid-like molecules that fall to Earth in meteorites (see Figure 8.7)!


• The geometry of phospholipids limits the membrane thickness. This thickness in turn dictates
  the permeability of bilayer membranes (as we saw in Section 4.6.1 on page 121), their electrical
  capacitance (using Equation 7.26 on page 236), and even their basic mechanical properties
  (as we will see in a moment). Choosing the chain length that gives a membrane thickness of
  several nanometers turns out to give useful values for all these membrane properties; that’s
  the value Nature has in fact chosen. For example, the permeability to charged solutes (ions)
  is very low, since the partition coefficient of such molecules in oil is low (Section 4.6.1 on page
  121). Thus bilayer membranes are thin, tough partitions, scarcely permeable to ions.


• Unlike, say, a sandwich wrapper, bilayer membranes are fluid. No specific chemical bond
  connects any phospholipid molecule to any other, just the generic dislike of the tails for
  water. Thus the molecules are free to diffuse around each other in the plane of the membrane.
  This fluidity makes it possible for membrane-bound cells to change their shape, as for example
  when an amœba crawls, or when a red blood cell squeezes through a capillary.


• Again because of the nonspecific nature of the hydrophobic interaction, membranes readily
  accept embedded objects, and so can serve as the doorways to cells (see Figure??on page
  ??), and even as the factory floors inside them (see Chapter 11). An object intended to
  pokethrough the membrane simply needs to be designed with two hydrophilic ends and a
  hydrophobic waist; entropic forces then automatically take care of inserting it into a nearby
  membrane. Understanding this principle also immediately gives us a technological bonus: a
  simple technique to isolate membrane-bound proteins (see Figure 8.8).
  The physics of bilayer membranes is a vast subject. We will only introduce it, finding a simple
  estimate of one key mechanical property of membranes, their bending stiffness.