Cellulose is the most abundant organic molecule in nature. It is a polysaccharide assembled from glucose monomer units, and it (together with other materials such as hemicellulose and lignin) is the main constituent of plant cell walls. Along with several undigestible polysaccharides, cellulose constitutes the main part of dietary fiber. Specifically cellulose is one of the components of insoluble fiber.

The glucose units in cellulose are combined in a way that results in the formation of very linear, flat molecules that can, in turn, form sheets that possess extensive networks of hydrogen bonds. The hydrogen bonds are both within individual sheets and between successive sheets. As a result of these bonds, sheets of cellulose are particularly strong—a property critical to the function of plant cell walls. Cellulose shows a variable degree of polymerization, with anywhere from 1,000 to 14,000 glucose residues comprising a single cellulose polymer. Because of its high molecular weight and crystalline structure, cellulose is insoluble in water and has a poor ability to absorb water.

Human beings lack the enzyme cellulase and are therefore unable to break cellulose down to individual glucose molecules. Although many fungi are able to break down cellulose to glucose, only a few types of bacteria have this ability. In the rumina of cows, sheep, and goats, two different types of bacteria produce the enzyme that breaks down cellulose.

Cellulose and its derivatives are used in a number of food products to modify those foods in different ways (e.g., as a thickener, stabilizer, or texturizer). The fibrous form is a basic material that is used to make both textiles and paper. Cellulose is also used to make nitrocellulose (an ingredient in explosives and lacquers) and as a binder in the manufacture of medicinal tablets.