In organic chemistry, saturation refers to a compound has no double or triple bonds. In saturated linear hydrocarbons, every carbon atom is attached to two hydrogen atoms, except those at the ends of the chain, which bear three hydrogen atoms. In the case of saturated methane, four hydrogen atoms are attached to the single, central carbon atom. Of simple hydrocarbons, alkanes are saturated, and alkenes are unsaturated. The degree of unsaturation specifies the amount of hydrogen that a compound can bind. The term is applied similarly to the fatty acid constituents of lipids, where the fat is described as saturated or unsaturated, depending on the constituent fatty acids contain carbon-carbon double bonds. Unsaturated is used when any carbon structure contains double or occasionally triple bonds. Many vegetable oils contain fatty acids with one (monounsaturated) or more (polyunsaturated) double bonds in them. The bromine number is an index of unsaturation.