Chemistry and properties of lipids and phospholipids

Lipids are referred to as a large and diverse group of naturally occurring compounds. The term “lipid” is restricted to long-chain fatty acids, their derivatives, and compounds that are related structurally and functionally to them [1,2]. They are highly soluble in organic solvents (e.g., hexane, diethyl ether, and chloroform) and slightly soluble/nonsoluble in water [3]. Lipids are found ubiquitously in plants, animals, and microorganisms, as well as the human body. They are the major constituents of the cell membrane, which is responsible for holding the cell together [4]. Lipids can be classified as polar and neutral lipids [5]. The International Lipid Classification and Nomenclature Committee developed a more detailed classification scheme (Lipid Classification System LIPID Metabolites and Pathways Strategy; http://www.lipidmaps.org) based on the concept that lipids may result from complete or partial carbanion-based condensations of ketoacyl thioesters or carbocation-based condensations of isoprene units [6]. Accordingly, lipids can be classified into eight groups, namely, fatty acyls, glycerolipids, glycerophospholipids, sphingolipids, sterol lipids, prenol lipids, saccharolipids, and polyketides (Figure 2.1) [7,8]. This classification scheme is applicable to archaea and synthetic lipids, covers eukaryotic and prokaryotic sources, and allows the subdivision of the main categories into subclasses to include the new lipids structures.