Figure 7.

Structure of lipid membrane and its components. (A) Cell membrane lipid bilayers where the constituent phospholipid molecules have their hydrophobic ends tuned toward the interior and hydrophilic ends facing outside. (B) Two phospholipid molecules facing each other. (C) Structures of these phospholipid molecules are shown as mirror images. Sn, side chain number. At the third position, x stands for choline, ethanolamine, inositol, or serine. *Location of action of phospholipase A2 enzyme. #Location of action of phospholipase D. Phospholipids are named for the presence of phosphate group. Their main structural stem is a molecule of glycerol, a 3-carbon alcohol. The fatty acid side chains may be saturated (contains no double bonds and a name that ends in -anoic acid) or unsaturated (contains double bonds and a name that ends in -enoic acid). Unsaturated fatty acid contributes to the fluidity of the membranes. Double bonds can be introduced at the Δ⁴, Δ⁵, Δ⁶, and Δ⁹ positions, but not beyond the Δ⁹ position (Δ denotes the position of double bonds from the carboxylic acid terminal; ω denotes the position of double bonds from the methyl terminal end). Usually the fatty acid chains with an even number of carbon atoms attach to the first (Sn-1; saturated fatty acids such as palmitic or stearic acid) and second (Sn-2; unsaturated fatty acids such as oleic or arachidonic acid) carbons of the glycerol molecule. Linoleic acid and alpha-linolenic acid are the only essential fatty acids (EFAs) that must be obtained from the diet. All other fatty acids can be synthesized within the body. Alternate steps of elongation (by the enzyme elongase) and desaturation (by Δ desaturase enzyme) enable the synthesis of polyunsaturated fatty acids (PUFAs) from EFAs. The Sn-3 position hydroxyl group bonds with phosphoric acid to form phosphatidic acid that in turn bonds with an alcohol such as serine, ethanolamine, choline, inositol, and ethanolamine. When the alcohol is sphingosine, the molecule is called sphingo-phospholipid. Fatty acids are transported in the blood either by binding to albumin or in the form of triglycerides associated with lipoproteins. From lipoprotein triglycerides, the fatty acids are released by the action of lipoprotein lipase in the endothelial cells of the capillaries (Spector and Yorek, 1985; Ben-Zeev et al., 1990). Lipoprotein lipase levels are found higher in the hippocampus and neocortex (Ben-Zeev et al., 1990). Choline-containing phospholipids are preferentially located in the outer layer and phosphatidyl serine and phosphatidyl ethanolamine-containing phospholipids are located in the inner layer of the membrane bilayer (Murray et al., 2006). The enzyme flippases transfer phospholipids to different layers. Membranes also have cholesterol, cholesterol esters, and protein molecules embedded within them.