why can't humans digest cellulose?

Humans cannot digest cellulose because the appropriate enzymes to breakdown the molecules, are lacking in our body. Some animals cannot digest cellulose directly but can digest with the help of symbiotic bacteria present in their body.

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Humans can 't digest grass because thay contain cellulose, and our stomach does not contain cellulose-digesting bacteria. The biggest part of the cow 's stomach is called the "rumen" and it contains billions and billions of friendly germs. These germs are microorganisms that we can see only with a microscope. These germs - there are many different types in the rumen - are not harmful, in fact, they are badly needed by the cow so that it can use the nutrients in grass and other plants. The nutrients in grass are in the form of complicated big molecules that must be broken down into smaller pieces that can then be used by the cow 's digestive tract and so used by the cow for growth of the body and to maintain life. The germs can do that job of breaking down those plant molecules, and that is why they are so important.

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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). 
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