Lipids include fats, oils, waxes, and two other important molecules, phospholipids, which form the membranes around cells, and steroids, which include cholesterol and the sex hormones. Fats and oils are also called triglycerides because they are composed of 3 fatty acids joined to a molecule of glycerol. Fatty acids are long chains of carbons linked to each other and to hydrogens. Fats and oils are used by animals and plants respectively to store energy. They are not soluble in water. Waxes are used as a waterproofing substance in plants and have several uses in animals.