Abstract
This review discusses the structural role of phospholipids in biological membranes. Phospholipids in an aqueous environment exhibit both thermotropic and lyotropic polymorphism. Most important is the smectic mesophase or the lamellar phase consisting of bimolecular layers of lipid molecules separated by layers of water. The bilayers of the lamellar phase exhibit a crystalline-to-liquid crystalline phase transition caused by changes in temperature, pH or ionic environment, as detected by X-ray diffraction, calorimetry and dilatometry. The lipid phase transition is a "melting" transition of the lipid hydrocarbon chains without disruption of two-dimensional networks formed by the hydrophilic group of the lipid molecules. The physiological relevance of the liquid crystalline state of lipids in cell membranes is pointed out.
