Final answer:
Starch and cellulose are both polysaccharides made of glucose but differ in their glycosidic linkages, with starch being digestible by humans and cellulose indigestible due to its structural rigidity.
Step-by-step explanation:
The key difference between starch and cellulose lies in their structural composition and digestibility. Both starch and cellulose are polysaccharides composed of glucose units, but they have different glycosidic linkages. Starch consists of both amylose and amylopectin with α-1.4 and α-1.6 glycosidic linkages, making it digestible by humans because we have enzymes that can break these linkages down. In contrast, cellulose has β-1.4 glycosidic linkages, which human digestive enzymes cannot hydrolyze, rendering it indigestible. Furthermore, cellulose serves as a structural material in plant cell walls due to its rigidity caused by β-1.4 glycosidic linkages and additional hydrogen bonding between chains, while starch acts as an energy reserve and is more flexible and accessible for digestion.