Answer is: The monosaccharide monomers in cellulose are bonded together differently than in starch.
The glucose monomers (C₆H₁₂O₆) in cellulose are bonded in a beta glycosidic linkage (β-1,4 bond), whereas those in starch have an alpha glycosidic linkage (α-1,4 bond). The enzyme amylase is specific for the alpha glycosidic linkage.
