21. Lipids and Carbohydrates   Previous PageNext Page
       Membrane Structure

If lecithin is suspended in hot water, it forms a monolayer on the surface that is similar to that found with soaps and long-chain alcohols. If a loop of wire or a thin metal plate with a hole is pushed down through this film, a lipid bilayer will be formed within the loop or hole, as diagrammed on the right. In the original surface film, the polar heads of the molecules were underneath, in contact with water. The hydrocarbon tails were above, exposed to the air.

The lecithin bilayer formed at the opening in the plate will have the structure shown right, with polar heads exposed on either face of the bilayer and hydrophobic chains meeting at the center. Such a bilayer made with chains the length of stearic acid will be 70 Å thick, similar to biological membranes. Lecithin bilayers are like soap films turned inside out. Both are bilayers of half hydrophobic and half polar molecules.

Soap films, in contact with air on either side, have their hydrophobic chains exposed and their polar heads immersed in a layer of water in the center of the film. Lipid bilayers in aqueous solution have their molecules in the reversed position-hydrophobic portion in the center and polar heads out.

  Page 05 of 25 HomeGlossary