Chemistry Reference
In-Depth Information
glycoprotiens, will need to be retaken, with a better appreciation of the active role
of lipids and other membrane-bound agents.
5.6.2. Membrane Surfactants and Lipids
The surface-active components of biological membranes are referred to as lipids,
with the majority consisting of double-chain phospholipids or glycolipids. The
hydrophobic tails normally contain chains of 16-18 carbons, one of which is gene-
rally unsaturated. Those structural features immediately indicate that such amphi-
philes will have values of P
c
1. Those factors guarantee that the lipids will have
significant surface activity and will spontaneously form self-assembled bilayer
membranes that can encapsulate or isolate different regions and functions in biolo-
gical systems (e.g., as vesicles), or influence the curvature and conformation of
membranes when incorporated into the overall structure. In addition, the long
chain lengths ensure that such lipids will have relatively low solubility in water
(as the monomer) and a low cmc, and therefore their assemblies will be stable
and remain intact while contacting surrounding fluids. The presence of unsaturation
in the hydrocarbon chains also helps guarantee that the structures they form will
remain relatively fluid and flexible over a wide, biologically relevant temperature
range. In that way, their chemical composition helps ensure the functional viabi-
lity of the biological structure and the organism of which it is a part under varied
environmental conditions.
Size, structure, and fluidity of membrane lipids are also important characteristics
because those aspects of the amphiphilic molecules make it possible for them to
efficiently pack into a variety of bilayer membrane structures with various degrees
of curvature and flexibility. That flexibility makes possible the inclusion of other
important components of the cell wall,
including proteins, glycoproteins, and
cholesterol.
In terms of molecular geometry, one can visualize a mixed amphiphilic system
in which one class of lipid having a P
c
<
1 that will produce a truncated cone shape
(Figure 5.8a), while another will have P
c
>
1 for an inverted truncated cone
(Figure 5.8b). Combinations of the two can then accommodate the inclusion of,
for example, proteins and cholesterol, while maintaining an overall planar structure
(or a given degree of curvature), or increase curvature to produce a smaller asso-
ciated unit. The situation is shown schematically in Figure 5.8c.
Biological membranes, like micelles and vesicles, are theoretically dynamic
structures in which the component lipids and proteins can move about and undulate
relatively freely. Nevertheless, the exchange of individual molecules with the sur-
rounding solution will be significantly slower than in micelles, so that the structure
as a whole remains intact. It simply wouldn't do to have biological cells falling
apart too often. To carry out its biological function, the cell membrane will also
have heterogeneous regions of lipids, proteins, or other materials, which may serve
as specific binding sites, transport ''channels,'' and similar structures. The compo-
nents of the entire structure, however, must all have one thing in common—they
must be able to associate spontaneously to form the necessary stable assembly of
