Chapter 8 - Lipid Dynamics in Membranes

Abstract

The lipid bilayer, both as simply a lipid bilayer and as a bilayer in a biological membrane, has unique properties that are particularly well defined by dynamics measurements. In their active state, bilayers are in the liquid crystal state. Highly asymmetric dynamics are observed in this liquid crystalline state that are best characterized by motional order parameters. The interior of the membrane is disordered, but the region of the membrane near the headgroups is highly ordered. The timescales of the dynamics exhibited by the lipids in the bilayer are independent from the motional order. While this is not necessarily intuitive, it is essential to an adequate understanding of the unique properties of membranes. Membrane components can diffuse laterally in the plane of the membrane. Lipids can exhibit lateral phase separation in the plane of the membrane under narrowly defined conditions, forming large domains with specific composition. Transmembrane movement of lipids (flip-flop) is mostly limited and controlled by membrane proteins. Plasma membranes have an asymmetric lipid composition, which is also seen to a lesser extent in intracellular membranes.