There are only trace amounts of phospholipid in dietary fats and oils. However, because the fatty acids in fats and oils provide substrate for the synthesis of phospholipid in the body, it is important to discuss this subtype of fat. Phospholipid is a critical structural component of all cells, both plant and animal. It is composed of two fatty acids on the sn-1 and sn-2 positions and a moiety frequently referred to as a polar head group on the sn-3 position of glycerol, the latter via a phosphate bond (Figure 5). Phospho-lipid molecules are amphipathic—that is, there are both hydrophobic and hydrophilic domains in the molecule. The two fatty acids confer hydrophobic properties and the polar head group hydrophilic properties. The specific fatty acids esterified to the glycerol backbone tend to be unsaturated fatty acids. The different polar head groups, most commonly phosphorylcholine, phosphorylserine, phos-phorylinositol, or phosphorylethanolamine, result in phospholipids that vary in size and charge. Due to their amphipathic nature, phospholipids serve as the major structural component of cellular membranes by forming bilayers and in so doing also serve as a reservoir for metabolically active unsaturated fatty acids. Due to their amphipathic properties, in the
small intestine they play an important role in the emulsification and absorption of dietary fat and fat-soluble vitamins. On the surface of lipoprotein particles, they provide a critical component in the packaging and transport of lipid in circulation.
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