Final answer:
Starch is digestible because it has a-1,4-glycosidic linkages easily broken down by enzymes in our digestive system, whereas cellulose is not digestible due to its ß-1,4-glycosidic linkages and its rigid, tightly packed structure resistant to human digestive enzymes.
Step-by-step explanation:
Why Starch Is Digestible and Cellulose Is Not
Both starch (amylose) and cellulose are long polymers of glucose. However, the digestibility of starch is due to its a-1,4-glycosidic linkages, which are easily broken down by enzymes in the human digestive system. Conversely, cellulose contains ß-1,4-glycosidic linkages, creating a more extended and rigid structure that is resistant to the enzymatic activity present in humans. This difference in linkages affects the overall three-dimensional structure, making cellulose indigestible to humans, who lack the necessary enzymes to break these bonds. While starch serves as an energy store in plants and is accessible to us for digestion, cellulose provides structural support in plant cell walls, which is why its resistance to digestion is beneficial to plants.
Starch, which includes the polymers amylose and amylopectin, can be branched or unbranched. Amylopectin is a branched molecule that, along with amylose, gets easily hydrolyzed by our digestive enzymes. Cellulose, on the other hand, has a linear, unbranched form that through extensive hydrogen bonding forms tightly packed fibers, contributing to its rigidity and resistance to digestion.