Methanogens are bacteria that produce methane during the breaking down of substrates like cellulose. Plants are the major food consumed by ruminant animals. Cellulose is a polymer of glucose found in the cell wall of plants. The gut of the ruminant animals have methanogens that help in the digestion of cellulose and produce methane. Methanogens are absent in the case of human beings. Because of this reason, humans are unable to digest cellulose.