Cellulose is a polysaccharide carbohydrate. Cellulose is the main structural chemical of plants. It is tough - insoluble in water, almost unaffected by normal cooking and indigestible by humans and most animals. However, cellulose is a key part of the human diet, providing much of the fibre.
Herbivores like cows, however, can digest cellulose through a symbiotic relationship between cows and bacteria in their rumen, an extra stomach, and the bacterial enzyme, cellulase. In herbivores, e.g. cows and deer, these symbiotic bacteria live in special extra stomachs "upstream", e.g. the rumen, of the true stomach. Other herbivores have other methods but all their processes to break down cellulose require a multi-stage digestion helped by symbiotic bacteria.
As above, cellulose is important to humans because it can only be slightly digested, i.e. it is the fibre or roughage that we are often being told to include in our diets. Roughage helps move digestible food through the human gut. Without roughage, the digestive system slows down and constipation and other bowel disorders result. These bowel disorders are typical of those with Western diets containing too much refined food and not enough cellulose or fibre.
- McCarthy, E.M. (undated) The ruminant stomach, Online Biology Dictionary, accessed 8 June 2016 
- Stobart, T. (1981) The Cook's Encyclopedia: ingredients & processes, Harper & Row. ISBN 0060141271.