Motion Constraints
In contrast to this large in-plane mobility, it is very difficult for lipid molecules to flip-flop from one side of the bilayer to the other. In a phosphatidylcholine-based bilayer this process typically occurs over a timescale of weeks. This discrepancy can be understood in terms of the basic structure of the bilayer. For a lipid to flip from one leaflet to the other, its hydrated headgroup must cross the hydrophobic core of the bilayer, an energetically unfavorable process. Unlike liquid phase bilayers, the lipids in a gel phase bilayer are locked in place and exhibit neither flip-flop nor lateral mobility. Due to this limited mobility, gel bilayers lack an important property of liquid bilayers: the ability to reseal small holes. Liquid phase bilayers can spontaneously heal small voids, much the same way a film of oil on water could flow in to fill a gap. This functionality is one of the reasons that cell membranes are usually composed of fluid phase bilayers.
Read more about this topic: Lipid Bilayer Phase Behavior
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