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The purpose of this chapter is to give some of the basics regarding biochemical properties of lipids and the terminology used to describe them. It is intended to introduce readers to several classes of lipids and the subtle distinctions that make these classes different from one another. Lipid refers to oily or fatty biological substances that are not soluble in water but dissolve in other solvents, such as chloroform, alcohol, or hexane. The term lipid encompasses fatty acids, triglycerides, phospholipids, the fat-soluble vitamins (A, D, E, and K), steroids (cholesterol and many of its metabolic products), and a variety of less commonly discussed biological materials such as terpenes. Fat is often thought to be synonymous with lipid, but fats are technically considered to be the triglycerides and, in some circles, only those triglycerides that solidify at room temperature. Triglycerides that remain liquid at room temperature are considered to be oils, although a substance need not be a triglyceride to be classified as oil. These distinctions are often overlooked today. The fact that lipids do not dissolve in water is one of their most notable features. Such chemical incompatibility is due to the fact that lipids are nonpolar molecules, while water is polar. Water molecules behave like tiny magnets, sticking to one another through positive and negative poles (like the north and south poles of a magnet). This is what causes water to bead up on a freshly waxed surface. Lipids lack these polar properties, behaving more like globs of grease and fat—they don’t have magnetic personalities. In other words, “like dissolves like,” or “oil and water don’t mix.” The idea of polar versus nonpolar substances will be used frequently in this book, along with terms such as hydrophilic (water loving), hydrophobic (water fearing), and lipophilic (lipid loving). For most biological substances, “fearing” water and “loving” lipids amounts to the same biochemical behavior. Appendix A reviews the fundamentals of chemical bonding and polarity. 2 Lipid Terminology, Structure, and Function 15 NUTRITIONAL, CHEMICAL, AND PHYSIOLOGICAL PROPERTIES 16 Fatty Acids The simplest lipids are the fatty acids. Fatty acids are typically components of other classes of lipids, particularly triglycerides, phospholipids, and cholesteryl esters. As the simplest unit, fatty acids can be transferred between or removed from these other lipids, a process that is fundamental to a wide range of metabolic actions and physiological functions. The fatty acids themselves, particularly the polyunsaturated fatty acids, are the major focus of this book, so it is important to get to know several of these by name. It will also be important to associate the names with some of their chemical or structural characteristics, such as saturated versus unsaturated, and omega-3 versus omega-6 polyunsaturated fatty acids. There are four different fatty acids illustrated in figure 2.1. Each of them contains eighteen carbon atoms (as well as hydrogen and oxygen), but they differ in whether or not they have carbon-to-carbon double bonds, the number of double bonds, and the configuration of the double bond (cis or trans). When all the carbon atoms are connected by single bonds, the fatty acid is saturated (it has the maximum number of hydrogen atoms, hence it is saturated with hydrogen). In order to form a carbon-to-carbon double bond, a hydrogen atom must be removed from each carbon forming the double bond, making the molecule unsaturated with hydrogen. When there is one carbon-to-carbon double bond, the molecule is monounsaturated, whereas polyunsaturated fatty acids have more than one double bond. Stearic acid (far left side in figure 2.1) is a common saturated fatty acid found in foods, but palmitic acid (with sixteen carbon atoms) is the predominant saturated fatty acid in most foods, as well as in the human body. Oleic acid (left-center in figure 2.1) is a major monounsaturated fatty acid that has the cis configuration for the double bond, which gives rise to a bend in the carbon chain. The significance of the bend in cis unsaturated fatty acids will be discussed below. Oleic acid is the major fatty acid found in olive oil, palm oil, and canola oil, which are promoted as monounsaturated oils. Elaidic acid (far right side in figure 2.1) has one carbon-to-carbon double bond but has a trans configuration that imparts a slight kink in the carbon chain. However, the carbon chain of elaidic acid is essentially straight, like that of a saturated fatty...

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