Wednesday, 9 January 2013

Secondary Chemistry - Superstuffs: Cellulose (Part 1)

Since pre-historic times one substance has provided mankind with fuel, shelter, clothing and many other useful materials.  That substance is cellulose.
Cellulose is a carbohydrate like sugar and starch.  Plants make cellulose for their cell walls.  It gives them rigidity and the strength to support their roots, stems and leaves.  The first stage in producing cellulose is photosynthesis.  Light energy is used to react carbon dioxide with water to make glucose (C6H12O6) and release oxygen as waste by-product.  As well as using glucose as a source of energy, plant cells use glucose to make fats and proteins and other carbohydrates.  Cellulose is manufactured in cell membranes where special enzymes link glucose molecules together to form a polymer chain.
n C6H12O6   ->   (C6H10O5)n  + nH2O
glucose                 cellulose
The length of the polymer chain varies with n being 300 to 1700 in woody plants to up to 10,000 in cotton.
Each glucose link in the cellulose chain has a flat, hexagonal ring.  This means that the molecules can lie close to each other, held by forces called hydrogen bonds.  This makes the polymer strong as well as flexible.
Plant cell walls act like a strong but floppy bag unless the cell is full of water.  That is why plants wilt without water.   Woody plants have an extra substance called lignin which acts like a glue and makes the cellulose chains stiffer.  Wood will keep its shape even when it has dried out.
Cellulose is flammable and burns to release the energy, carbon dioxide and water that were originally bound together by photosynthesis.  Until the Industrial Revolution wood was the main fuel available.  It is still the most used fuel in developing countries and as a renewable source of energy it promises to be an important fuel for the future.   Wood also has been used as a building material from simple wooden huts to the timber framed buildings of today.  Strong, flexible, relatively light and easily shaped it is an ideal material.  It is still the most commonly used material for furniture.
Copyright of William G Woodward
Plant cells link together combining the cellulose in their cell walls to make fibres.  Across the world the fibres of many plants have been used for weaving cloth for clothes, sheets, carpets, sails and ropes.   Hemp provided strong fibres for weaving canvas used in sails and tents.  Some varieties of hemp also produce the drug cannabis.  Flax or linseed, makes a smooth, cool, fabric called linen used for bed sheets and clothing.  Flax also produces an edible oil. In the nineteenth century, one plant fibre came to dominate the market - cotton.  The cotton mills of Lancashire and elsewhere were as much a sign of Britain’s industrial development as the coal mines, ironworks and engineering projects such as ships and railways.   Cotton replaced other plant fibres for most uses.  Today linen is made from cotton although hemp and flax are making a comeback.

Two thousand years ago the Chinese discovered that cellulose could be turned into a flat sheet instead of a fibre.  They had invented paper.  Hemp was the first material used but just about any plant cellulose can be turned into paper – even grass after it has been chewed and egested by  sheep (sheep poo paper).   Old rags were an important source of cellulose for paper in the nineteenth century.  However the growth in the number of books, magazines and newspapers meant that paper manufacturers looked for another source.   Wood was the obvious choice but separating the cellulose from the lignin was a problem.  A solution was found by German, Carl Dahl in 1879.  His process mixed wood chips with sodium hydroxide and sodium sulfide to make wood pulp that could be turned into paper.  Unfortunately the process produces a lot of waste and the paper is acidic and less durable than that made from other sources of cellulose.

Peter Ellis

Peter Ellis taught science (mainly chemistry) in secondary schools to GCSE and A level for 35 years and was a head of department for twenty years.  He is now a freelance writer of educational materials in science and dabbles in writing fiction.  

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