19.4Lipids

The idea

• Describe triglycerides and the meaning of saturated versus unsaturated fat.
• Identify the role of phospholipids.

Lipids are a solubility class as much as a structural one: biomolecules that dissolve in nonpolar solvents. The most familiar are triglycerides. Glycerol, a three-carbon triol, forms ester links with three fatty acids. A saturated fatty acid has no carbon–carbon double bonds and packs easily, so its triglycerides tend to be solid fats. Unsaturated fatty acids have one or more cis double bonds, the chain kinks, packing is poorer, and the triglyceride is more likely an oil. Hydrogenation straightens and saturates them; partial hydrogenation can also flip cis bonds to trans.

Phospholipids replace one fatty acid with a phosphate-containing head. That one change makes a molecule with a polar end and two nonpolar tails, the bilayer builder. Steroids, including cholesterol, are lipids with a fused-ring skeleton. They are not fats, and “cholesterol” is not a triglyceride, but they travel with fats because they share the insolubility in water. Dietary-fat language — saturated, unsaturated, trans — is this section’s structures.

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