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.