Chapter I

Lipids play a variety of roles in biological systems. They are molecules that are insoluble in water and soluble in organic solvents. The diversity of lipids can be very high due to (a) the complexity of the hydrophobic acyl chains (i.e. carbon number and degree of unsaturation) and (b) the nature of the hydrophilic polar head group. Thus, over a thousand different lipid molecular species can be theoretically devised which exhibit a wide polarity range. Lipids can be divided into non-polar lipids (e.g. triglycerides and cholesterol) and polar lipids (e.g. phospholipids, glycolipids, and sphingolipids) based on the number of primary products formed after hydrolysis, Figure 1. The structure of selected lipids and their primary hydrolysis products are shown in Figure 2. In general, non-polar lipids yield no more than two types of primary products after hydrolysis. For example, triglycerides only yield fatty acids and one mole of glycerol when hydrolyzed; while polar lipids could yield three or more types of primary products under the same reaction. For example, phospholipids which are discussed in this thesis yield glycerol, fatty acids, inorganic phosphate, and an organic base. Although the role of lipids in the cellular area is still not fully understood, they are generally believed to be (a) used for energy storage, (b) serve as structural components of cell membranes, and (c) constitute important signaling molecules. Many publications have reported that phospholipids are important constituents for the above functions in all living cell membranes due to their amphiphilic structure.