Mammals can digest starch but not cellulose primarily due to the structural differences in these carbohydrates. Starch consists of glucose units linked by alpha 1→4 glycosidic bonds, which enzymes in the mammalian digestive system, such as amylase, can easily break down. In contrast, cellulose is composed of glucose units linked by beta 1→4 glycosidic bonds, which are resistant to digestion by these enzymes. This structural difference makes cellulose indigestible for mammals, as they lack the necessary enzymes to break the beta linkages.
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