Final answer:
Starch and cellulose are both made up of glucose units, but starch contains α-1,4 and α-1,6 glycosidic linkages, making it digestible by humans, while cellulose contains β-1,4 glycosidic linkages, making it indigestible due to its rigid structure.
Step-by-step explanation:
Starch and cellulose are both polysaccharides comprised of glucose units, but their structures differ significantly. Starch consists mostly of α-1,4 glycosidic linkages, which is found in its two components, amylose and amylopectin. Amylose is a linear molecule, while amylopectin has branches due to α-1,6 glycosidic linkages. These linkages in starch allow it to be more easily digestible by humans. On the other hand, cellulose has β-1,4 glycosidic linkages, which form tight crosslinks creating a rigid and linear structure that contributes to its role as a plant's structural component, such as in cell walls.
The β-1,4 glycosidic linkages in cellulose result in a rigid structure that human digestive enzymes cannot break down, hence we cannot digest cellulose. Ruminants like cows have special adaptations, including a multi-chambered stomach and symbiotic microorganisms, which allow them to break down cellulose. For humans, cellulose acts as dietary fiber, which aids in digestion but is not broken down for energy.
While both starch and cellulose provide structural roles and energy storage in plants, the slight difference between the β and α forms of their glycosidic linkages results in humans being able to extract energy from starch but not from cellulose.