Properties and phase behavior of membranes
Phase transition in the lipid bilayers. Lamellar and nonlamellar liquid crystal phases. Mobility of the membrane components - lateral and transversal diffusion.
A lipid bilayer can exist in a gel (solid-like) phase or a liquid crystalline phase. Each lipid has a characteristic transition temperature at which it melts from gel to liquid. In the gel phase the lipids are tightly packed and almost immobile.
In the liquid phase, a lipid swaps places with its neighbors millions of times per second. This random movement is lateral diffusion, and it lets lipids travel across the membrane surface. Transversal diffusion, or flip-flop, moves a lipid from one leaflet to the other. It is very slow unless enzymes such as flippases help, which is why the two leaflets can keep different compositions.
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, which is 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, which is 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.