ABSTRACT
This chapter describes the classification and structures of membrane phospholipids. It is important to realize that within each class of phospholipid, a wide variety of different molecules are present due to complex composition of fatty acyl chains. Most biochemical and biophysical studies on phospholipid bilayers have been concerned primarily with symmetric phospholipids whose acyl chains are of identical carbon numbers. According to J. N. Hawthorne’s classification, phospholipids are divided into two large groups: glycerophospholipids and sphingophospholipids. Since phospholipids are amphipathic molecules, they exhibit only a very limited solubility in water. When phospholipids are at a concentration above the critical micellar concentration, they tend to aggregate sponatenously into organized structures in water. Phospholipids exhibiting highest degree of chain length asymmetry such as lysophospholipid species and platelet activating factor can self-assemble, in excess water, into a fully interdigitated bilayer at temperatures below the main phase transition temperature. The phase behavior of phospholipid bilayers can be influenced by many external and internal factors.
