Elastic and flexoelectic aspects of out-of-plane fluctuations in biological and model membranes

Abstract

Out-of-plane fluctuations of biological membranes representing an important manifestation of cell surface dynamics were discovered about one hundred years ago, but their purely physical (Brownian) character has been recognized only recently. This has been a demonstration of the successful application of the liquid crystal approach to the structure and functions of biomembranes. The present article concentrates on the liquid crystal (elastic and flexoelectric) aspects of these fluctuations of membrane curvature. The theoretical basis of curvature elasticity and curvature-induced (flexoelectric) polarization of a membrane is provided by liquid crystal physics. Special attention is paid to experimental and theoretical studies of curvature elasticity and flexoelectricity of a pure lipid bilayer in a liquid crystal state. Optical and electrical (noise) investigations of out-of-plane fluctuations in model and living membrane systems are reviewed. The influence of out-of-plane fluctuations on surface interactions and cell-cell contact of flexoelectric membranes is considered. The article concludes by underlining the unique character of information about the mechano-electrical and other liquid crystal properties of membranes that may be gained from future extensive studies of membrane fluctuations.