Final answer:
Amylase cannot break down cellulose because it is specific to starch, which has different structural glycosidic bonds than cellulose. Cellulose has beta 1-4 bonds, which require beta-glucosidase enzymes to hydrolyze them, enzymes that are absent in most animals including humans.
Step-by-step explanation:
Amylase is an enzyme that specifically breaks down starch into simpler sugars like maltose and glucose, which can then be absorbed by the body. Starch is composed of glucose monomers linked by 1-4 or 1-6 glycosidic bonds. In particular, amylose, a component of starch, has unbranched chains of glucose monomers with 1-4 linkages, while amylopectin, another component, is branched with 1-6 linkages at the branch points. However, cellulose, another polysaccharide made of glucose units, has different structural glycosidic bonds, namely beta 1-4 bonds, which cannot be broken by amylase.
Animals, including humans, have alpha-glucosidase enzymes such as amylase that hydrolyze the alpha glycosidic bonds in starch and glycogen. However, they lack the necessary beta-glucosidase enzymes needed to break down the beta 1-4 bonds found in cellulose. This specificity is crucial because it allows plants to maintain cellulose as a structural component while using starch as an energy reservoir. If amylase could break down cellulose, it would undermine the structural integrity of plant cell walls. Some animals, like grazing animals and termites, do host bacteria with beta-glucosidase enzymes, enabling them to digest cellulose.