Final answer:
Cellulose is not a nutrient source for humans due to the absence of cellulase, making it indigestible and serving only as beneficial dietary fiber.
Step-by-step explanation:
Humans cannot digest cellulose mainly because the human digestive system lacks the enzymes, specifically cellulase, required to break down the β-glycosidic linkages present in cellulose. These linkages keep the glucose units tightly bound together, giving cellulose its strength and preventing its breakdown in the human digestive system.
While herbivores, such as cows and horses, have specific microorganisms in their digestive tracts that produce cellulase, humans do not, rendering cellulose as indigestible dietary fiber in our diet. Although cellulose is not a source of nutrients for humans, it is still included as dietary fiber in our diet for its health benefits, such as promoting a healthy digestive tract and reducing the risk of certain diseases.