Cellulose is a complex carbohydrate composed of repeating glucose residues (molecules) joined by chemical bonds called glycosidic links or bonds. Cellulose is the main constituent of higher plants and gives adds structural strength to the plant. Cellulose is also the main diet of herbivores. Animals classified as herbivores produce the necessary enzymes to digest cellulose. Or do they!! Actually, they must rely on bacteria, which have the necessary enzyme "cellulase" to digest cellulose. Humans, on the other hand, cannot fully digest cellulose, thereofer we cannot rely on it as an energy source. However, we can use it as fibre, since it passes through our guts, only partially digested. Bacteria in our intestines also acts on it to a degree. Herbivores, in comparison, can fully digest cellulose and therefore, obtain energy from it. Cellulose is strong and can be used for textiles (eg., cotton), plastics )eg., celluloid), ropes, and many other useful products. What other substances can bacteria help creatures digest? See lignin.