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CHAPTER 11

Lipids

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The fruit of a small tree native to the eastern areas of the Mediterranean, olives are unusual in that they are an inedible fruit unless processed. Olive oil, which has been used in cooking since antiquity, is rich in monounsaturated fatty acids as well as antioxidants. These antioxidants may be one of the reasons for the positive health effects of the Mediterranean diet.

Lipids are defined as water-insoluble molecules that are highly soluble in organic solvents. This important class of molecules has a variety of biochemical roles. For instance, lipids are widely used to store energy. They are also key components of membranes (Chapter 12) and play a variety of roles in signal-transduction pathways (Chapter 13). Unlike the three other classes of biomolecules (carbohydrates, amino acids, and nucleic acids), lipids do not form polymers. Their individual and collective properties—as noncovalent assemblies—make them extraordinarily important. We will examine five classes of lipids here:

Free Fatty Acids (Nonesterified Fatty Acids). This simplest type of lipid is most commonly used as a fuel. Fatty acids vary in hydrocarbon chain length, which has important ramifications when they are used as fuels and as components of membrane lipids.
Triacylglycerols. This class of lipid is the storage form of fatty acids.
Phospholipids. These membrane lipids consist of fatty acids attached to a scaffold that also bears a charged phosphoryl group, creating a macromolecule with a polar head and nonpolar tail.

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Glycolipids. These lipids are bound to carbohydrates and are important membrane constituents.
Steroids. These lipids differ from the other classes in that they are polycyclic hydrocarbons. Steroids function as hormones that control a variety of physiological functions. The most common steroid is cholesterol, another vital membrane component.