Final answer:
The polysaccharide bond that cannot be broken down by mammalian enzymes is the β (1—4) glycosidic linkage, which is present in cellulose and some other forms of dietary fiber.
Step-by-step explanation:
The polysaccharide bond that cannot be broken by mammalian enzymes which normally digest polysaccharides is the β (1—4) glycosidic linkages. Mammalian digestive systems lack the enzymes necessary to break these β-glycosidic bonds found in substances like cellulose, making them indigestible as dietary fiber.
Monosaccharides can link together to form polysaccharides through dehydration reactions that create glycosidic bonds. In the digestive system, enzymes like amylase can readily hydrolyze the α (1—4) glycosidic linkages found in starch components like amylose, but not the β (1—4) glycosidic linkages found in cellulose.
Furthermore, other linkages such as α (1—6) are also digestible by mammalian enzymes. This process is seen in the breakdown of glycogen, a storage form of glucose in animals, which contains α (1—6) linkages at its branching points. Unlike cellulose, glycogen and starches allow for easy access and digestion due to their α-linkages.