Lipids are a large and diverse group of naturally-occurring organic compounds that are primarily made of carbon, hydrogen and oxygen. Some lipid molecules may also contain phosphorus, nitrogen, and sulfur. Although there is great structural variety among lipids, they are all related by their general insolubility in water and solubility in nonpolar organic solvents such as benzene, ether, and chloroform. This is because of the presence of nonpolar carbon-hydrogen bonds. The 4 biologically important lipids are fats, phospholipids, waxes and steroids.