Insoluble fiber is a subclass of dietary fiber. Like its soluble cousin, insoluble fiber differs from starch because the chemical bonds that join individual sugar units can not be digested by enzymes in the human gastrointestinal tract. Insoluble fiber is considered a “noncarbohydrate carbohydrate” since the components that make up insoluble fiber are lignins, cellulose, and hemicelluloses. All of these compounds form the structural parts of plants and do not readily dissolve in water and are not metabolized by intestinal bacteria. Bran fiber is rich in hemicelluloses, while a cotton ball is pure cellulose.