The molecular arrangement within a lamellar structure composed of human erythrocyte lipids is determined. The 45 A thick lipid layer, in water, is filled in the interior with a liquid-like configuration of the hydrocarbon chains of phospholipid molecules and is covered on both sides by their hydrophilic polar groups. Cholesterol is located so that part of its steroid nucleus is between the polar groups of the phospholipid molecules while the rest of the molecule extends into the inner hydrocarbon layer. This lipid leaflet would be expected to have the mechanical properties of a purely liquid surface, as other authors have shown for the "black" lipid membranes. Data are presented which demonstrate that the intact erythrocyte membrane is a tough viscoelastic substance with a Young's modulus of 106–108 dynes/cm2 and a viscosity of 107–1010 poises. The parameters and the kinetics of membrane breakdown are incompatible with the model system of pure lipid. Caution must be exercised in applying various data on the model systems to intact membranes.

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