Cellulose

Cellulose is a polysaccharide built from a linear chain of D-glucose units linked by β(1→4) bonds, with the formula (C₆H₁₀O₅)ₙ. It serves as a key structural component in the cell walls of green plants, many algae, oomycetes, and the tests of sea squirts, while some bacteria produce it to create biofilms. As the most abundant organic polymer on Earth, its content varies: cotton fiber is about 90% cellulose, dried hemp around 57%, and wood 40–50%.

Industrially, cellulose is primarily used to make paperboard and paper, with smaller amounts converted into derivatives like cellophane and rayon. It is also being explored as a renewable fuel source through the conversion of energy crops into cellulosic ethanol. Most industrial cellulose comes from wood pulp and cotton. It interacts strongly with certain organic liquids such as formamide and DMSO, and short-chain amines like methylamine and ethylamine act as effective swelling agents.

In nature, ruminants and termites digest cellulose with the help of gut-dwelling symbiotic microorganisms like *Trichonympha*. For humans, cellulose is an insoluble dietary fiber that acts as a hydrophilic bulking agent in feces, potentially aiding defecation.

The compound was first isolated in 1838 by French chemist Anselme Payen, who determined its formula. Cellulose was used to create the first successful thermoplastic, celluloid, in 1870 by the Hyatt Manufacturing Company. Rayon production began in the 1890s, and cellophane was invented in 1912. Hermann Staudinger identified its polymer structure in 1920, and the first chemical synthesis without biological enzymes was achieved by Kobayashi and Shoda in 1992.

Cellulose is tasteless, odorless, hydrophilic (with a contact angle of 20–30°), insoluble in water and most organic solvents, chiral, and biodegradable. Pulse tests in 2016 showed it melts at 467 °C. It can be broken down into glucose units by concentrated mineral acids at high temperatures. Its structure consists of D-glucose units joined by β(1→4)-glycosidic bonds, contrasting with the α(1→4) bonds in starch and glycogen. Unlike starch, cellulose is a straight-chain polymer with no coiling or branching, adopting an extended, stiff rod-like shape due to the equatorial conformation of glucose residues. Hydroxyl groups on one chain form hydrogen bonds with oxygen atoms on the same or neighboring chains, holding them together side-by-side into microfibrils with high tensile strength. This strength is key in plant cell walls, where microfibrils are embedded in a polysaccharide matrix, and in wood, where cellulose fibers are distributed within a lignin matrix—analogous to reinforcement bars in concrete, with lignin acting as the glue. The mechanical properties of cellulose in primary cell walls are linked to plant cell growth and expansion, with live fluorescence microscopy aiding research.

Cellulose is more crystalline than starch. While starch transitions from crystalline to amorphous above 60–70 °C in water, cellulose requires 320 °C and 25 MPa to become amorphous in water. Several forms exist, distinguished by hydrogen bond locations between and within strands. Natural cellulose (cellulose I) has subtypes Iα (enriched in bacteria and algae) and Iβ (dominant in higher plants). Regenerated cellulose fibers form cellulose II, and the conversion from cellulose I to II is irreversible, indicating cellulose I is metastable. Chemical treatments can also produce cellulose III and IV.

Properties depend on chain length, or degree of polymerization (n in the formula). Wood pulp cellulose has chain lengths of 300–1700 glucose units; cotton, other plant fibers, and bacterial cellulose range from 800 to 10,000 units. Short-chain breakdown products, called cellodextrins, are soluble in water and organic solvents, unlike long-chain cellulose. Plant-derived cellulose is typically mixed with hemicellulose, lignin, pectin, and other substances, while bacterial cellulose is purer, with higher water content and tensile strength due to longer chains. Cellulose consists of fibrils with both crystalline and amorphous regions, which can be individualized through mechanical treatment of pulp, often aided by chemical oxidation or enzymatic action.