Cellulose and starch are both polymers of glucose units, but most animals can only use starch as a source of carbohydrate in their diets. Why are most animals incapable of digesting cellulose?

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Most animals do not produce enzymes which are capable of hydrolyzing β linkages between glycoses. Cellulose is made up of β-glucose monomers linked by β-1,4 glycosidic bonds, and animals lack the enzyme cellulase which is necessary to break down these bonds. In contrast, starch is made up of α-glucose monomers linked by α-1,4 glycosidic bonds, which animals can break down with the enzyme amylase.

Cellulose and Starch Are Both Polymers of Glucose Units, but Most