2.12–2.14: Lipids store energy for a rainy day.

Protected from the cold: lipids help insulate seals.

Lipids are a second group of macromolecules important to all living organisms. Lipids, just like carbohydrates, are made primarily from atoms of carbon, hydrogen, and oxygen, but the atoms are in different proportions. The lipids in your diet, for example, tend to have significantly more energy-rich carbon-hydrogen bonds than carbohydrates, and so contain significantly more stored energy (FIGURE 2-28).

Figure 2.28: Lipids serve many roles in the body.

What exactly is a lipid? That’s not as easy to answer as you might expect. Lipids come in a wide variety of structures. They don’t have any unique subunits (such as the simple sugars that make up disaccharides and polysaccharides) or particular ratios of atoms that serve as defining features. Consequently, lipids are defined based on their physical characteristics. Most notably, lipids do not dissolve in water and are greasy to the touch—think of salad dressings.

Lipids are insoluble in water because they tend to have long chains consisting only of carbon and hydrogen atoms. In contrast to water, these chains of carbon and hydrogen atoms are nonpolar—there are no regions of positive or negative charge. Thus they do not form weak bonds with water and cannot dissolve in water. Nonpolar molecules (or parts of molecules) tend to minimize their contact with water and are considered hydrophobic (“water-fearing”). Lipids cluster together when mixed with water, never fully dissolving. Molecules that readily form hydrogen bonds with water, on the other hand, are considered hydrophilic (“water-loving”).

One familiar type of lipid is fat, the type most important in long-term energy storage and insulation. (Penguins and walruses can maintain relatively high body temperatures, despite living in very cold habitats, due to their thick layer of insulating fat.) Lipids also include sterols, which include cholesterol and many of the sex hormones that play regulatory roles in animals, and phospholipids, which form the membranes that enclose cells.