The graphic, below, illustrates a dissected kernel of wheat. Burkitt noticed in 1983 in Africa that people eating high fiber diets had reduced incidences of GI disease.
Cellulose comes from and is a constituent of the main cell wall.
It is insoluble fiber and holds water. It acts as a laxative and reduces colonic intraluminal pressure. It also binds minerals. It is insoluble, it is an anti-oxidant and it binds bile acids and metals. The absorption of water leads to a bulkier stool and gives a laxative effect.