Such fluidity is a critical property and is determined by temperature and lipid composition. Fluidity can be determined by chain length of the fatty acid of the phospholipid. The shorter the fatty acyl chains of a phospholipid, the lower its melting temperature, the more fluid at 37oC, compared to a saturated lipid at 37oC. Fluidity can be determined by the number of unsaturated bonds within the fatty acid of the phospholipid. Unsaturated bonds prevent phospholipids from packing together. Therefore, the greater the degree of unsaturation of phospholipid fatty acid chains, the lower the melting point.
Cholesterol also affects fluidity.
Cholesterol molecules, which align themselves with the fatty acyl chains of the phospholipids in a bilayer, affect membrane fluidity by preventing the tight packing of the long hydrocarbon tails. As a result, cholesterol tends to abolish sharp transition temperatures and may also increase the stability and decrease the permeability of the membrane .