Mechanical properties of model membranes studied from shape transformations of giant vesicles

Membrane deformations occur frequently in cell functioning. From the physical point of view, the understanding of such shape changes requires the introduction of mechanical parameters like bending elasticity. In this article it is shown how this physical property can be obtained from the analysis of small or large shape transformations from giant vesicles. Then it is demonstrated that the bending modulus is strongly dependent on the membrane composition and environmental conditions. This is the case for one-component bilayers (dilauroylphosphatidylcholine (DLPC), dimyristoylphosphatidylcholine (DMPC), dipalmitoylphosphatidylcholine and stearoyloleoyl-phosphatidylcholine (SOPC) and for two-component lipid mixtures (DMPC/cholesterol, DLPC/dilauroylphosphatidic acid). Further it is shown that the bending elasticity of natural lipid extracts (egg phosphatidylcholine, digalactosyl diglyceride and red blood cell lipid extracts) is generally smaller than that of comparable synthetic model membranes. The role of transmembrane proteins is examined by measuring the bending elasticity of SOPC/gramicidin mixtures. Finally, larger scale shape transformations of giant vesicles under an alternative electric field are discussed.