As I look out my window, I see a world made up of cellulose. Cellulose is the building block trees, plants, grass, micro-organisms, and a host of other living creatures.
As the mass of plant-based life outweighs the rest of the creatures in the world, cellulose is the dominant biopolymer on the planet. Even some single celled organisms employ cellulose within their cell walls. Many types of algae are seen as potential sources of cellulose for papermaking or as potential feedstock for bio-energy.
Cellulose is a major and important constituent of the living world.
It was first isolated in 1838 by the French chemist Anselme Payen although his initial report did not use the word. Rather, he was examining the composition of the tissues of plants and woody materials.
It was a year later when the word cellulose was first used as chemists noted the similarities in composition between cellulose and starch.
Both molecules are polysaccharides. They are made up of glucose units strung together like beads on a chain. In essence, both are complex carbohydrates composed of sugar molecules joined through ether linkages generated by condensation - or, at least, that is how a chemist might describe the molecules.
The difference between cellulose and starch is in how the individual units are linked. A glucose molecule is a six-membered ring with hydroxy or -OH groups hanging off it. Whether starch or cellulose is produced depends upon how the hydroxy groups of the sugar molecule form the linkages.
In cellulose, the individual glucose molecules have a "beta 1,4" linkage. (In starch, it is an "alpha 1,4" linkage.) Individual cellulose molecules are then composed of thousands of monomeric sugars linked together in a linear fashion.
The linkage and structure of cellulose are important. The "beta" linkage means it forms a long straight chain and is a linear polymer - a long thread of molecular components.
With starch, the "alpha" linkage generates twists, turns, and branches. As a consequence, starch is digestible by organisms such as you and me. It forms an integral part of our diet as a good source of complex carbohydrates.
Cellulose, with its "beta" linkage and linear structure, is not digestible. Cows, horses, and other ruminants consume grasses, shrubs, wood, and other plants but symbiotic microorganisms in their guts, such as trichonympha, actually do all of the work of digestion. This is why cows chew their cud - they swallow the grass, inoculate it with microorganisms, regurgitate it and grind the resulting mass to make sure everything is well mixed, and then swallow the whole mash.
Cellulose is a tough material to break down or consume. This is a good thing, otherwise our houses and other wooden structures wouldn't last very long. Trees wouldn't be able to stand and plants wouldn't grow. Having very few organisms capable of digesting cellulose has allowed plant life to flourish across the face of the planet.
Of course, chemists have found non-biological ways to break down cellulose. Concentrated sulfuric acid does the job but it tends to be very destructive. Still, the remaining sugars can be used to make ethanol.
Other chemists have been interested in modifying cellulose to generate new materials. The first thermoplastic polymer, celluloid, was synthesized from cellulose in 1870. Rayon was developed in the 1880s and 1890s from nitrocellulose or gun cotton. Rayon was the first synthetic fibre and was sold as synthetic silk. Cellophane followed suit in 1912.
However, it took until 1992 before chemists were able to artificially synthesize the polymer in the laboratory. In part, it took so long to complete the synthesis because there is no shortage of cellulose in our natural environment and no urgency to create a synthetic source. Most cellulose is obtained through simple extraction processes from plants.
More recently, scientists have been exploring the use of nano-cellulose fibres. First produced in the late 1970s at the ITT Rayonier labs, this material is a gel-type substance generated by passing wood pulp through a milk homogenizer at high pressures and temperatures.
Under some conditions, nano-cellulose fibres are a gel and under others they flow like a liquid. The material is hydrophilic, allowing it to swell and absorb large quantities of water. It is used in a wide variety of applications such as food, providing bulk or fibre, hygiene products as an adsorbent, and as the backing for flexible solar cells.
One of the more interesting uses for nano-cellulose is generated by acidification of the native fibre. The resulting nano-cellulose forms rigid rods which can be used in the construction of carbon-based automobile body parts.
These parts are stronger than steel but lighter than aluminum. We might soon be driving cars with lightweight body panels made from trees rather than metal.
When we reach that day, as I look out my window, I will see the feedstock for our modern age.
