Question

Why human beings can not digest cellulose

Answer 1

Humans, unlike cows for ex. don't have the necessary bacteria in their intestines that make the enzymes(cellulases) necessary to breakdown the 1-4 beta glucose bonds. So we cannot break down cellulose into its constituent glucose molecules as we do with starch. Therefore this is secreted as unbroken down chains of glucose called fiber.

Answer 2

human cannot digest cellulose because they do not have long small intestine as the herbivorous animals have