Cellulose is a complex long chain of molecules and very few animals have the capacity to digest them. Humans cannot digest cellulose because we lack the enzymes that are required to break the bonds that hold the glucose monomers together.
Cellulose is a complex long chain of molecules and very few animals have the capacity to digest them. Digestion is the breakdown of food into smaller and simpler molecules. In order to breakdown cellulose we would have to have much larger digestive systems. That is one reason why the dinosaurs grew so big. They ate vegetation that contained a lot of cellulose, so in order to gain the nutrition it had to offer they needed huge gizzards (dinosaur stomachs).