Lipids have a variety of functions in living organisms.
Fats and oils are a convenient and concentrated means of storing food energy in plants and animals. Waxes provide vital waterproofing for body surfaces. Steroids include the adrenal hormones, sex hormones, and bile acids. Lipids can combine with proteins to form lipoproteins , e.g. in cell membranes. In bacterial cell walls, lipids may associate with polysaccharides to form lipopolysaccharides .