Cellulose could provide an abundant and cheap form of glucose for humans. Why is cellulose not a source of nutrients for humans?
Cellulose digestion must occur in a basic environment, but the human gut is acidic. Cellulose is a structural carbohydrate, and therefore is not useful as an energy source. Humans, and most vertebrates, lack the enzyme cellulase. Humans cannot absorb the hydrolyzed form of cellulose. Vertebrate enzymes are unable to hydrolyze (β 1–4) linked glucose monomers.
Cellulose is one of the most important carbohydrate of the plant that provide structural framework to the plant cells. Cellulose is made of the monomer of the glucose that are linked through the beta glycosidic linkages.
Although cellulose contains large number of glucose but it cannot be used as the nutrient source for humans. Humans and other vertebrates lacks the enzyme cellulase that is required for the digestion of cellulose. Cellulose cannot be broken down in the human body.
