Chapter 15

After studying this chapter, you should be able to:

• Define simple and complex lipids and identify the lipid classes in each group.
• Indicate the structure of saturated and unsaturated fatty acids, explain how the chain length and degree of unsaturation influence their melting point, give examples, and explain the nomenclature.
• Understand the difference between cis and trans carbon-carbon double bonds. Describe how eicosanoids are formed by modification of the structure of unsaturated fatty acids; identify the various eicosanoid classes and indicate their functions.
• Outline the general structure of triacylglycerols and indicate their function.
• Outline the general structure of phospholipids and glycosphingolipids and indicate the functions of the different classes.
• Appreciate the importance of cholesterol as the precursor of many biologically important steroids, including steroid hormones, bile acids, and vitamins D.
• Recognize the cyclic nucleus common to all steroids and explain the difference between the “chair” and “boat” forms of the six-carbon rings and that the rings may be either cis or trans in relation to each other, making many stereoisomers possible.
• Explain why free radicals are damaging to tissues and identify the three stages in the chain reaction of lipid peroxidation that produces them continuously.
• Understand how antioxidants protect lipids from peroxidation by either inhibiting chain initiation or breaking the chain and give physiological and nonphysiological examples.
• Understand that many lipid molecules are amphipathic, having both hydrophobic and hydrophilic groups in their structure, and explain how this influences their behavior in an aqueous environment and enables certain classes, including phospholipids, sphingolipids, and cholesterol, to form the basic structure of biologic membranes.

The lipids are a heterogeneous group of compounds, including fats, oils, steroids, waxes, and related compounds, that are related more by their physical than by their chemical properties. They have the common property of being (1) relatively insoluble in water and (2) soluble in nonpolar solvents such as ether and chloroform. They are important dietary constituents not only because of their high energy value, but also because fat-soluble vitamins and essential fatty acids are contained in the fat of natural foods. Fat is stored in adipose tissue, where it also serves as a thermal insulator in the subcutaneous tissues and around certain organs. Nonpolar lipids act as electrical insulators, allowing rapid propagation of depolarization waves along myelinated nerves. Combinations of lipid and protein (lipoproteins) serve as the means of transporting lipids in the blood. Lipids have essential roles in nutrition and health and knowledge of lipid biochemistry is necessary for the understanding of many important biomedical conditions, including obesity, diabetes mellitus,and atherosclerosis.

1. Simple lipids: Esters of fatty acids with various alcohols.

1. Fats: Esters of fatty acids with glycerol. Oils are fats in the liquid state.

1. Waxes: Esters of fatty acids with higher molecular weight monohydric alcohols.

2. Complex lipids: Esters of fatty acids containing groups in addition to an alcohol and a fatty acid.

1. Phospholipids: Lipids containing, in addition to fatty acids and an alcohol, a ...

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