Biochemistry - The Chemical Reactions of Living Cells, Volume 1 - D. Metzler 1980

Membranes and Cell Walls
Membrane Structure
Artificial Membranes

The most reliable evidence supporting Cell/29.html">The Lipid Bilayer model of The Cell membrane was obtained from studies of very thin artificial membranes, typically prepared from a solution of Phospholipids (such as phosphatidylcholine or a mixture of phospholipids and Cholesterol) in a hydrocarbon solvent. A drop of this solution is placed across a small aperture in a polymer partition separating two aqueous compartments. The solution in the aperture rapidly thins out, much like a soap bubble, until the iridescent Interference colors characteristic of such thin films disappear, yielding a so-called "black membrane" [12]. Membranes of this type, but lacking any residual hydrocarbon solvent, have also been formed by bringing together two lipid monolayers previously spread at an air—Water interface [13]. The thickness of these membranes appears to be no more than 6—9 nm. They are elastic and capable of self-healing, which allows them to be stained with osmium tetroxide to reveal the characteristic trilaminar ultrastructure seen in natural Introduction/36.html">Biological Membranes.

Biological membranes act as a barrier to polar molecules and ions, a property attributable to their high electrical resistance and substantial electrical capacitance. For instance, the electrical resistance of biological membranes is typically around 10³ Ом∙см^-2, with a capacitance of 0.5—1.5 мкФ∙см^-2. The corresponding values for artificial bilayer membranes are approximately 10⁷ Ом∙см^-2 and 0.6—0.9 мкФ∙см^-2, respectively. The lower electrical resistance of biological membranes compared to artificial ones can be explained by the presence of Proteins and other ion carriers, or perhaps simply by the existence of aqueous "pores." Notably, the capacitance values for both types of membranes are remarkably close to those predicted by the lipid bilayer model [14—16].