One property of a lipid bilayer is the relative mobility (fluidity) of the individual lipid molecules and how this mobility changes with temperature. This response is known as the phase behavior of the bilayer. Broadly, at a given temperature a lipid bilayer can exist in either a liquid or a solid phase. The solid phase is commonly referred to as a “gel” phase. All lipids have a characteristic temperature at which they undergo a transition (melt) from the gel to liquid phase. In both phases the lipid molecules are constrained to the two dimensional plane of the membrane, but in liquid phase bilayers the molecules diffuse freely within this plane. Thus, in a liquid bilayer a given lipid will rapidly exchange locations with its neighbor millions of times a second and will, through the process of a random walk, migrate over long distances. 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 lipid 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. Motion constraints on lipids in lipid bilayers are also imposed by presence of proteins in biological membranes, especially so in the annular lipid shell 'attached' to surface of integral membrane proteins. Physical origins The phase behavior of lipid bilayers is largely determined by the strength of the attractive Van der Waals interactions between adjacent lipid molecules. The extent of this interaction is in turn governed by how long the lipid tails are and how well they can pack together. Longer tailed lipids have more area over which to interact, increasing the strength