Animal fats, in contrast, are often saturated and occur as hard fats. Human diets with large amounts of saturated fats may contribute to clogged arteries and raise the risk of developing diseases of the circulatory system. Waxes, which are used by land plants and some animals as a waterproofing material, differ from fats and oils by having a chemical backbone slightly different from glycerol. Phospholipids also differ from oils in that one of their fatty acids is replaced by a phosphate group attached to a nitrogen-containing group. Phospholipids play a key role in the structure of cell membranes.
Membranes often contain steroids, a lipid having a structure very different from oils.
Steroids are composed of four carbon rings. Most of your cell membranes contain the steroid cholesterol. Male and female sex hormones are also steroids. Figure 14 Saturated and polyunsaturated fats.