Lipid Bilayers

Abstract Lipid bilayer is the most simple model of the cell membrane which retains many of its properties even in the absence of proteins. Its formation, structure and properties rely on a subtle combination of different physical forces as well as on the unique chemical nature of the lipid molecules constituted by a polar head covalently linked to a long hydrophobic hydrocarbon chain. Here we present a brief summary of the chemical structure and related geometry of the most common lipids together with a short description of the relevant collective properties of the resulting supramolecular aggregate. The key role played by the solvent is emphasised. Among the key features of the lipid bilayer, we mention the order–disorder phase transition of the hydrophobic tails, the lateral phase separation of the bilayer's components into patches of different composition, the associated variations of the geometrical and transport properties and lastly the intricate interplay of different attractive and repulsive forces between bilayers brought at close distance. Key Concepts: Lipid bilayers provide the fundamental architecture of biological membranes. Lipid molecules contain a polar head linked to a hydrophobic tail. Formation of the lipid bilayer arises from the synergistic hydrophobic effect and maximum exposure of the polar heads to water. The bilayer forms a pseudo two‐dimensional fluid that may undergo solid–liquid transitions similar to those of common fluids. The lipid (or lipid–protein) components are not ideally mixed. Often domains with different composition spontaneously appear. Compositional heterogeneity along the membrane plane or perpendicular to it may induce profound modification of the bilayer's shape. Adjacent bilayers interact through a combination of attractive and repulsive forces.