Starch is a mixture of amylose and amylopectin, both of which are composed of α−D− glucose units linked by glycosidic linkage. Thus, amylose is not a component of cellulose. In proteins, there are nearly 20 different types of amino acids. The furanose structure of fructose has four carbon atoms and one oxygen atom in the ring while its pyranose form contains five carbon atoms and one oxygen atom in the ring.