In the Overview, we talked about the large size of macromolecules. Fats are much smaller than polysaccharides or proteins, and sometimes they're not even classified as macromolecules. Yet fats share many features with polysaccharides and proteins.

Let's take a look at their structure first. Triglycerides are an important class of fats. Blood tests often include a measurement of serum triglycerides. High levels of serum triglycerides are associated with an increased risk of heart disease.

Triglycerides are produced in condensation reactions between glycerol and three fatty acid molecules. That's why they name it "tri", get it? A water molecule is released each time a fatty acid molecule reacts with glycerol to produce an ester. The three fatty acids can be the same or they can be different. Regardless of the exact fatty acid, all triglycerides are nonpolar. The fatty acid part of the molecule is nonpolar, and the glycerol part of the molecule is also nonpolar once the OH groups are part of ester linkages.

The structure of triglycerides makes them ideal for storing fat in the adipose tissue of animals. The nonpolar fatty acid chains tend to pack together very tightly, so a lot of energy can be stored in relatively little space.

OK, let's look at phospholipids as the other group of fats. Phospholipids are diglycerides containing fatty acids at carbons one and two of the glycerol molecule. Carbon three contains a phosphate group. Sometimes this group is just a phosphate ion, but there can also be a polar organic group on the phosphate, shown here as R. The polar phosphate group has a huge influence on the molecule's function.

Phospholipids usually aggregate, or come together, in bilayers as seen here. The nonpolar fatty acid chains point to the center, and the phosphate groups point to the outside. Because the fatty acids are oriented to the interior, where they're inaccessible to the enzymes that convert them to energy, phospholipids are usually used for structure. As we'll explore in detail when we study cells, cell membranes are made of phospholipid bilayers.

Once again, the structure of fats influences their function. Triglycerides are used for energy storage. But when one of the fatty acids is replaced by a phosphate group, the resulting phospholipids are used to form the structure of a cell membrane.

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