The arrangement of phospholipids in cell membranes has been
deduced by X-Ray diffraction data. The phospholipids are arranged
as a bilayer (two molecules thick). The phospholipids are stacked
with the non-polar hydrocarbon chains pointed inward while the
polar ends act as the external surface as shown in graphic on
the left. The structure of the bilayer is another application
of the solubility principle of "likes dissolve likes".
Most of the fatty acids in the membrane are unsaturated because
this allows the membrane to be more flexible (cis bonds are bent)
to allow certain molecules through the membrane. However, the
interaction of the hydrophobic inside of the layer acts as a
barrier for ionic and polar molecules from entering the inside
of the cell.
In animal cells cholestrol is inserted between the non-polar
chains, and makes up abot 20% of the molecules of the membrane.
This helps to make the memebrane more rigid and adds strength.
Lipid Bilayer Graphic - left:
Red/white sphers represent water molecules on the outside
surfaces of the bilayer which are hydrophilic (water loving).
The gray spheres represent the non-polar hydrocarbon chains,
which are hydrophobic or water hating. The purple spheres represent
individual phospholid molecules.
The graphic shows 200 phospholipids in a gel crystal structure.
Bilayer - Chime
in new window
Chime from: Dr. William McClure, Department of Biological Sciences
Carnegie Mellon University, Pittsburgh, PA 15213
The coordinates are from the work of H. Heller, M. Schaefer,
and K. Schulten (1993) "Molecular dynamics simulation of
a bilayer of 200 lipids in the gel and in the liquid-crystal
phases", J. Phys. Chem. 97, 8343-60.