Lipids, small molecules that are either hydrophobic or amphipathic, include fats, waxes, sterols, and fat-soluble vitamins (A, D, E, and K), monoacylglycerides, diacylglycerides, and phospholipids. In biological physiology, lipids play crucial roles not only in energy storage and structural components of cellular membranes but also in facilitating signal transduction, membrane transporting, and morphogenesis.