Starch and cellulose are both glucose, joined by a bond that changes everything
Potatoes and paper are made of the same sugar. Starch links its glucose one way and coils into something we digest easily; cellulose links it the other way into straight, stiff rods that bundle into fibres strong enough to hold up trees. It is the most abundant organic polymer on Earth.
Cellulose is a chain of several hundred to many thousands of glucose units joined by what chemists call beta 1-4 bonds. In starch and glycogen, the same sugar is joined by alpha bonds, which let the chain coil and branch. Cellulose instead stays straight and rigid, and hydroxyl groups along each chain form hydrogen bonds with neighbouring chains, locking them side by side into microfibrils of great tensile strength. The difference shows in the kitchen: starch loses its crystalline order in water at 60 to 70 degrees Celsius, while cellulose needs 320 degrees and enormous pressure.
It frames the cell walls of green plants and many algae and even forms the tough outer tunic of sea squirts. Cotton fibre is about 90 percent cellulose and wood 40 to 50 percent. Plants spin it at the cell membrane using rosette-shaped protein complexes about 25 nanometres across, each extruding a microfibril into the wall. Intriguingly, plants appear to have inherited the key cellulose synthase gene from the ancient bacterium that became the chloroplast.
Few animals can digest it. Ruminants and termites rely on gut microbes, such as Trichonympha, to break it down, and in human diets it passes through as insoluble fibre. Industry turns most cellulose into paper and paperboard, and smaller amounts into cellophane and rayon; researchers are also working to convert it into cellulosic ethanol fuel.
The French chemist Anselme Payen isolated and named it in 1838. It went on to yield celluloid in 1870, the first successful thermoplastic, and rayon, sold as artificial silk, from the 1890s. Hermann Staudinger worked out its polymer structure in 1920, and in 1992 Kobayashi and Shoda made it chemically for the first time without enzymes.
Source: Cellulose
