Cellulose is the major structural polysaccharide in woody and fibrous plants and is the most abundant single polymer in the biosphere. Like amylose, cellulose is a linear polymer of D-Glucose (and hence is also a glucan), but in cellulose the sugar residues are connected by (1->4) linkages. This seemingly small difference from starch has remarkable structural consequences. Cellulose can exist as fully extended chains, with each glucose residue flipped by 180° with respect to its neighbor. In this extended form, the chains can form ribbons that pack side by side with a network of hydrogen bonds within and between them. This arrangement is reminiscent of the -sheet structure in silk fibroin, and as in fibroin, the fibrils of cellulose have great mechanical strength but limited extensibility.

The same small difference between cellulose and starch has another important consequence: Animal enzymes that are able to catalyze the cleavage of the (1->4) link in starch cannot cleave cellulose. For this reason, humans, even if starving, are unable to utilize the enormous quantities of glucose all around them in the form of cellulose. Ruminants such as cows can digest cellulose only because their digestive tracts contain symbiotic bacteria that produce the necessary cellulases. Termites manage to eat woody substances in a similar fashion-their guts harbor protozoans capable of cellulose digestion. Many fungi also produce such enzymes, which is why some mushrooms can live on wood as a carbon source. Cellulose is not made strictly in the plant kingdom. Marine invertebrates called tunicates produce cellulose in the hard outer mantle. There may be even small amounts of cellulose in human connective tissue.