Lipids Lipids are a diverse family of molecules that includes fats, phospholipids, waxes, and steroids such as cholesterol. One feature that all lipids share is their low solubility in water, a characteristic that is important to the functions of many lipids. Cells often contain droplets containing fats that are large lipid molecules rich in stored energy. Adipose cells in animals are almost completely filled with fat. In addition to storing energy, adipose tissue also insulates the body and cushions organs. The membranes around and within cells are made mostly of phospholipids. Cholesterol, a type of lipid called a steroid, is also a component of membranes. Butter, lard, margarine, and salad oil are composed of lipids called fats. To make a fat molecule, three fatty acids bind to a molecule of glycerol. For this reason, fat molecules are technically called triglycerides. Fat molecules do not mix with water because they have three long nonpolar hydrocarbon tails. Cells use fats for energy storage because the tails hold more potential energy than other biological molecules. The fatty acid tails in a fat can vary in length. A very important characteristic of fats is the number of double bonds in the hydrocarbon tails. Fats that contain only single bonds are called saturated fats. Fats containing double bonds in one or more tails are called unsaturated fats. Unsaturated fats tend to be liquids at room temperature; vegetable oils are examples. Saturated fats, like butter and lard, are solid at room temperature. Saturated fats in the diet can lead to heart disease, while unsaturated fats are safer. Phospholipids are important components of cell membranes. Like fats, phospholipid molecules contain fatty acid tails linked to a glycerol portion, but in phospholipids a group containing phosphate replaces one of the tails. This makes the molecule ambivalent with regard to water: Part of it is polar and mixes with water, while part of it is nonpolar and excluded from water. Lipids also include a family of molecules called steroids. All steroids have the same carbon skeleton made of four linked rings; they differ in what is attached to the rings. Cholesterol acts as an important component of cell membranes. Estradiol is a primary female sex hormone produced in the ovaries. Testosterone on the other hand is the most abundant sex hormone produced in the testes. While Vitamin D aids in calcium and phosphate metabolism.