Final answer:
Humans cannot digest cellulose because we lack cellulase, the enzyme needed to break down cellulose's β-glycosidic linkages, making cellulose a form of indigestible dietary fiber.
Step-by-step explanation:
It is impossible for humans to digest food containing cellulose because humans lack the necessary enzyme, cellulase, to break down the ß-glycosidic linkages in cellulose. While cellulose is a source of D-glucose after acid hydrolysis, the inability of human digestive enzymes to process these linkages means that we can consume plant-based foods like potatoes, which contain digestible starches, but not grass, which is rich in cellulose. Instead, cellulose remains undigested and serves as dietary fiber, adding bulk to the stool and helping to facilitate bowel movements. Certain microorganisms in the digestive systems of herbivores and termites produce cellulase, enabling them to use cellulose as an energy source.