BioPHYSICAL chemistry

from a detergent solution will depend
upon the thermodynamic parameters that
are effectively summarized with the phase
diagram. For example, whether a detergent
exists in solution as a monomer or micelle
is dependent upon the detergent concen-
tration and temperature (Figure 4.6). As
the detergent concentration increases the
micelle is more likely to form. However, the
specific concentrations at which this occurs
is strongly dependent upon temperature
and there is usually a critical temperature
below which the formation does not occur.
Also, detergents and lipids can form much
more complex phases under specific condi-
tions. One such phase, termed the cubic lipid
phase, is discussed in Chapter 17.

Research direction: lipid rafts

Cell membranes are bilayers composed of a number of different lipids, with
the most common shown below. These lipids are not uniformly distributed
but rather are found in specific regions of the bilayers. The classic picture
of the cell membrane, as developed by S. Jonathan Singer and Garth
Nicholson (1972), is thefluid mosiac concept.In this model, the bilayer serves
as a two-dimensional neutral environment for the membrane proteins,
lipids, and proteins that are free to move within the bilayers (see Figure 1.5).
In model lipid bilayers, the behavior is more complex, as moving lipids
can be highly restricted to motion in gel states that are influenced by the
lipid composition.
In cell membranes, the complex composition leads to non-uniform dis-
tributions of the membrane components. A well-established example of such
heterogeneity is the asymmetry in the lipid composition for the two sides
of the bilayer. Another more controversial example is a lipid raft (Simons
& Ikonen 1997; Jacobson & Dietrich 1999; Parton & Hancock 2004). The
basic concept of a lipid raft is that lipid microenvironments exist in the cell
membrane, with the microenvironments having a composition enriched
in certain lipids and proteins. The raft concept remains unproven despite
the large amount of experimental evidence in its favor (Brown & London
1998; Edidin 2001). As one example of the difficulties of unambiguously
establishing their presence in cells, consider the straightforward measure-
ment of the density of membrane fractions that first indicated the presence
of lipid rafts. Fractionation studies of cell membranes showed both low-
density and high-density components (see Figure 4.7). The low-density

CHAPTER 4 PHASE DIAGRAMS AND MIXTURES 77

40

80

60

20

0

Temperature (

C)

Detergent concentration

Micellular

Monomeric

Figure 4.6A phase

diagram for a

detergent showing

the monomeric and

micellular regions.

As the detergent

concentration is

increased at 65°C,

the solution

undergoes a phase

separation.