There are even more similarities between lecithin bilayers, and
biological membranes. Bilayers show many membrane-like properties.
They are easily permeable to water molecules, but not to ions such
as Na+, K+, and Cl--. They have
a high electrical resistance across the membrane because of the
insulating layer of hydrocarbon.
Several small antibiotics, such as valinomycin, are known to function
as "carriers" and to make natural membranes permeable
to K+ and other ions that ordinarily cannot penetrate
them. These same carrier molecules also transport ions across lecithin
bilayers. Such evidence has led us to believe that lipid bilayers
are at the heart of membrane structure.
A cross section through the cell membrane of a red blood cell
is shown at the far right. It appears in the electron micrograph
as two parallel dark lines where the osmium stain has been taken
up most, with a 25-Å unstained space between. The entire membrane
structure is 90 Å thick. Such a membrane is 60% protein and
40% Lipid, with the lipid part being half cholesterol (see Page
13) and half lecithin, cephalin, and similar molecules.