Membranes and Membrane Model

Phase transition is an important property of membranes. Below the phase transition temperature, lipids are tilted and highly ordered. They are in their solid or "gel" state. Increasing the temperature leads to a pre-transition, characterized by periodic undulations and straightening of the hydrocarbon chain. Further increase of the temperature causes the main phase transition. Above the main phase transition temperature, lipids are fluid or "liquid crystalline." Figure 3 shows the phase diagram for the interaction of water with a lipid as well as its inferred arrangements in a model membrane (5). Phase transitions in membranes and membrane models have been extensively studied by spectroscopic techniques and by differential scanning calorimetry. 

Transport by facilitated diffusion A large number of molecules and ions were shown to permeate membranes considerably faster than expected from their lipid-water partitioning behavior. This led to the recognition of additional transport mechanisms. Systematic investigations of permeability rates in membranes, reconstituted membranes, and membrane models as functions of the temperature of the nature and concentration of the permeant in the absence and in the presence of additives, suggested three different facilitated passive transport mechanisms ... 

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