Lipids are not covalently anchored to other lipids
Non-covalent interactions among lipids allows individual lipids to migrate.
Membranes are highly flexible.
Gel Phase is the formation of a semisolid lipid bilayer which constrains the motion of individual lipid molecules. This is due to lack of energy from temperature to allow motions to occur.
Liquid-Disordered state, the interior of the bilayer is fluid and allows for rotation of lipid molecules and free migration, this occurs when there is enough energy from temperature to overcome intermolecular interactions.
Liquid-Ordered State is intermediate between liquid disordered and gel phase. It requires the presence of cholesterol. Acyl chains are ordered, but there is allowance for lateral movement of lipids.
Lipid Characteristics Which Influence Membrane Fluidity: Saturation
Saturated Fatty acids can pack more tightly together producing higher melting points and lower membrane fluidity. Unsaturated fatty acids have kinks in their hydrocarbon chains that disallow tight packing, increasing membrane fluidity
Lipid Characteristics Which Influence Membrane Fluidity: Hydrocarbon Length
Hydrocarbon chain length influences amount of hydrophobic interactions. Longer chains provide more interactions decreasing membrane fluidity and shorter chains have less interactions increasing membrane fluidity.