Biological Membranes - A. N. Ohurtsov 2012

Structure and Functions of Biomembranes
Structure of Lipid Membranes
Effect of Lipid Composition on the Thickness and Curvature of Biomembranes

The Lipid Composition of a given bilayer patch directly affects its thickness, which in turn determines precisely which Membrane Proteins will localize to that region. Experimental evidence shows that sphingomyelin forms a more gel-like and thicker membrane than Phospholipids do (Figure 26).

Similarly, Cholesterol and other molecules that increase membrane viscosity simultaneously increase membrane thickness.

However, in the case of sphingomyelin, its fatty acid tails are already optimally stabilized, so The addition of cholesterol does not affect the thickness of the sphingomyelin bilayer.

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Figure 26 - Effect of lipid composition on biomembrane thickness

Another biomembrane property significantly influenced by local lipid composition is local membrane curvature, which depends on the relative sizes of the polar HEAD group and nonpolar tails of the constituent phospholipids. Lipids with long tails and large head groups have a cylindrical shape—or more precisely, that of a flattened cylinder (e.g., PC)—whereas those with long tails and small head groups are conical in shape (e.g., PE) (Figure 27(a)). Consequently, bilayers composed predominantly of cylindrical lipids form relatively flat membrane patches, whereas areas rich in conical lipids form highly curved bilayers (Figure 27(b)).

Figure 27 - Effect of lipid molecule shape: a - on local membrane curvature for phosphatidylcholine (PC) and phosphatidylethanolamine (PE), b - on overall membrane shape

This influence of membrane lipid composition on bilayer curvature can play a crucial role in The formation of highly curved pits and invaginations on the membrane surface, intracellular membrane vesicles, and specialized membrane structures such as microvilli.