Fatty acids are a variant of lipids that provides an important source of energy for many organisms. Triglycerides yield more than twice as much energy for the same mass as do carbohydrates or proteins. All cell membranes are built up of phospholipids, each of which contains two fatty acids. Fatty acids are also used for protein modification. The metabolism of fatty acids, therefore, consists of catabolic processes which generate energy and primary metabolites from fatty acids, and anabolic processes which create biologically important molecules from fatty acids and other dietary carbon sources.