The gel–liquid crystalline phase transition in a synthetic lipid bilayer

a. A schematic view of the change at the transition temperature. Below this temperature the hydrocarbon tails are packed together in a nearly crystalline gel state (left). Above this temperature the movement of the chains becomes more dynamic, and the interior of the membrane resembles a liquid hydrocarbon (right).

b.Detection of the transition by calorimetry. Measurement of the heat absorbed by a membrane as the temperature is raised each degree shows a sharp spike at the transition temperature (Tm) for a pure dipalmitoylphosphatidyl choline bilayer. This well-defined transition from gel to liquid is called melting of the membrane. When 20 mol % cholesterol is mixed into the bilayer, the Tm is not changed, but the transition is broadened.