Cellulose (C6H10O5) is a long-chain polymer composed of a single monomer (glucose), classified as a polysaccharide or carbohydrate. It is one of the main constituents of plant cell walls (about 33% of the plant mass), in combination with lignin, hemicellulose and pectin and is not digestible by man, constituting a dietary fiber. Some animals, particularly ruminants, can digest cellulose with the help of symbiotic microorganisms. Others, such as humans, lack cellulase, the enzyme responsible for hydrolyzing cellulose into glucose, and therefore cannot use it as an energy source.