Lipids are a diverse class of water-insoluble, colorless organic compounds present in the tissues of plants, animals, and microorganisms, from which organic solvents such as ether, naphtha, or chloroform can extract them. The main types of lipids are the neutral lipids (or triglycerides, i.e., fatty-acid esters of the alcohol glycerol), the phospholipids (or phosphoglycerides, i.e., fatty-acid esters of glycerol and phosphoric acid), the sphingolipids (complex lipids derived from alcohols such as sphingosine), the sterols (such as cholesterol), carotenoids, and prostaglandins. Greasy to the touch, lipids comprise one of the three large classes of foods and, with proteins and carbohydrates, are components of all living cells. The structure of lipids varies from simple chainlike molecules consisting of hydrogen, carbon, and oxygen to complex ring, or cyclic, structures with side chains of varying composition and complexity. Many naturally occurring lipids are associated with proteins, in combinations called lipoproteins.