Cellulose

Cellulose is the main structural material of plants. It is tough, insoluble in water, little affected by ordinary cooking, and wholly indigestible by humans and most animals. Nevertheless, it is still an essential part of human diet.

Herbivorous animals such as cows can digest and be nourished by cellulose. Cellulose is a carbohydrate of the polysaccharide type; its molecules consist of long chains of simple sugar molecules. To digest cellulose, a digestive system must produce enzymes which can split the big molecule into individual sugar units. Herbivorous animals act as host to bacteria which produce such enzymes. In the ruminants (cows, deer, and others) these bacteria live in a series of auxiliary stomachs ‘upstream’ of the true stomach. Other herbivorous animals have different arrangements, but digestion is always a slow, multi-stage process aided by bacteria.