A complex carbohydrate that forms the tough carbon latticework of cell walls in most vegetation and is important in the manufacture of numerous products; such as, paper, textiles, pharmaceuticals, and even explosives: Plant cellulose is indigestible to animals and serves as the plants' armor against predatory pests.

Research in biofuels is striving to refine plant cellulose in the form of corn husks instead of corn kernels, bagasse (dry dusty pulp that remains after juice is extracted from sugar cane) instead of cane sugar, and agricultural and industrial wastes of all kinds; such as, wood chips, the remains of paper mills, and fallen tree leaves.

Many creatures, including termites and cattle, use gut microbes to break down plant cellulose in their digestive systems where microbes do it by secreting enzymes called cellulases.