Introduction
The cell membrane is a continuous barrier around all cells, built on a bilayer of phospholipids, each with a hydrophilic head and two hydrophobic tails. About 50% of the membrane mass is lipid, and the lipid composition differs between cells (examples: phosphatidic acid, glycolipids, cholesterol, sphingomyelin); lipids are distributed inhomogeneously in the bilayer. This topic covers the phase behaviour of the bilayer and the mobility of lipids and proteins: lateral and transversal diffusion.
Phase behaviour of the bilayer
A lipid bilayer can exist in a gel (solid-like) phase or a liquid crystalline (fluid) phase. Each lipid has a characteristic transition temperature at which it melts from gel to liquid; a gel-like bilayer becomes fluid on heating. In the gel phase the lipids are tightly packed and almost immobile.
The transition temperature depends on the lipid tails. Longer tails have stronger van der Waals attractions, so the lipid packs more tightly and melts at a higher temperature. A double bond puts a kink in an unsaturated tail, which disturbs the packing and lowers the transition temperature. This is why butter, rich in saturated fat, is solid at room temperature, while vegetable oil, mostly unsaturated, is liquid.
Natural membranes are mixtures of many lipids, so liquid and gel regions can exist side by side. This phase separation matters biologically, because membrane proteins can gather in one phase and become concentrated or activated there. Cholesterol is a key component of these mixed systems and affects permeability, mechanical strength and protein function.
Lamellar and nonlamellar phases
In a lamellar phase, lipids form flat, stacked bilayers, the usual arrangement in membranes. Some lipids, such as phosphatidylethanolamine, have small head groups and tend to form nonlamellar phases, such as hexagonal or cubic phases, which curve the membrane. These structures are thought to matter in membrane fusion.