**1. Introduction**

The concept of fatty acid was first introduced by the French chemist Michel Eugène Chevreul as *graisse acide* (acidic fat) [1]. Fatty acids are chemically defined as carboxylic acids with either saturated or unsaturated aliphatic chains and are derived after hydrolysis of fats or oils. A fatty acid has, therefore, an acid group at one end of its molecule and a methyl group at the other end [2, 3]. Fatty acids are essential structural components of the cell; they also play important roles in energy requirements and signaling cascades in the cell. Both plant and animal cells can synthesize fatty acids. Animal cells, however, cannot synthesize some of the fatty acids; they must take them from plant sources. These fatty acids are called essential fatty acids (EFAs) in the animal body. Some fatty acids are also synthesized by lower organisms such as phytoplanktons, which act as primary members of the food chain. On the basis of the location of the double bonds from the methyl terminal position of the unsaturated fatty acids (UFAs), they are named as ω-3 and ω-6 UFAs. Biologically, fatty acids are esterified with glycerol, phosphoglycerol, and cholesterol and are referred to as triacylglycerol, phospholipids, and cholesterol esters, respectively. Esterified fatty acids can constitute the structural components

or dietary fuels for cells and organisms; they can also form complex liposomal structures (including lipoproteins) for transporting lipid components from the hepatic tissues to extrahepatic tissues and vice versa.

### **1.1 Saturated versus unsaturated fatty acids**

Fatty acids whose aliphatic carbon chains are fully saturated with hydrogen atoms or contain only C-C single bond and/or contain no C=C double bonds are simply referred to as saturated fatty acids (SFAs). Fatty acids containing C=C double bonds are referred to as unsaturated fatty acids (UFAs). UFAs are again classified as monounsaturated fatty acids (MUFAs) and polyunsaturated fatty acids (PUFAs): if they contain only one C=C double bond, they are MUFAs; if they contain more than one C=C double bond, they are then called PUFAs (see **Figure 1** for detail). Because of the presence of C-C single bonds or C=C double bonds, they have characteristic structural features and differences in physical as well as chemical properties and have significant roles in the constitution of cellular membranes.
