The rate at which this two-dimensional diffusion occurs is extremely temperature dependent. At high temperatures, the thermal energy of any given lipid molecule is greater than the interaction energy that would tend to hold adjacent lipid molecules together. Under these con- ditions, lateral diffusion can proceed rapidly, and the lipid is said to be in the sol state. At lower temperatures, interac- tion energies exceed the thermal energies of most individual molecules. Thus, phospholipids diffuse slowly because they lack the energy to free themselves from the embraces of their neighbors. This behavior is characteristic of the gel state