Final answer:
Polysaccharide fibers such as cellulose and chitin feature beta-glycosidic bonds, forming linear, crystalline structures that make them strong and indigestible, contrasting with the soluble and digestible alpha-linked polysaccharides like starch and glycogen.
Step-by-step explanation:
Polysaccharide fibers such as cellulose and chitin differ from other polysaccharides primarily in their glycosidic bonds and their structural role. While polysaccharides like starch and glycogen contain alpha-glycosidic bonds and are branched, serving as energy-storage molecules, fibers have beta-glycosidic bonds which create linear structures. These linear structures allow cellulose and chitin to form strong, crystalline structures that are not easily digestible by human enzymes and do not dissolve in water.
Moreover, cellulose and chitin are part of a larger matrix in plant and fungal cell walls, interacting with other polysaccharides like hemicellulose and pectin, which can have variable compositions and linkages. This variability and complexity contribute to the strength and integrity of cell walls. The insolubility and indigestibility of fibers like cellulose and chitin are what make them functionally different from starch and glycogen which are easily broken down and soluble in water.
Fibers are also important because they interact with each other through multiple polymer molecules, providing additional strength to the structures they form. While enzymes break down most polysaccharides into glucose units, the beta-glycosidic linkages in fibers resist this enzymatic action, which is part of the reason dietary fiber is crucial for digestive health.