Starch, \mathrm{(C_6H_{10}O_5)}_n, is made of many α-glucose units, and plants store it in seeds, roots and tubers. It contains two kinds of molecules: amylose, which dissolves in warm water, and amylopectin, which hardly dissolves.
Amylose is an unbranched chain of glucose units joined by α-1,4-glycosidic bonds. Amylopectin has branches as well, where an α-1,6-glycosidic bond joins a side chain to carbon 6 of a glucose unit in the main chain.
Most starch is about 20 to 25 per cent amylose, but the starch in glutinous rice is almost all amylopectin. Animals store glucose in the liver and muscles as glycogen, which is like amylopectin but even more branched.
The chain of amylose coils into a helix, with about six glucose units in each turn.
When iodine solution is added to starch, iodine fits inside the helix and the solution turns deep blue. This is the iodine-starch reaction. Amylopectin, whose helical parts are shorter, turns red-violet.
When the solution is heated, the helix loosens and lets the iodine out, so the colour disappears. The colour comes back when the solution is cooled.
The enzyme amylase, found in saliva and pancreatic juice, breaks starch down into shorter chains called dextrins, and then into maltose.
If samples are taken in turn from starch solution containing amylase and tested with iodine solution, the colour changes from blue to red-violet and then red-brown, and finally no colour forms. The shorter the chains become, the less colour they give with iodine.
Maltose is broken down further into glucose by another enzyme, maltase. Heating starch with dilute acid also hydrolyses it all the way to glucose.
Cellulose, \mathrm{(C_6H_{10}O_5)}_n, is the main substance of plant cell walls. It is a chain of β-glucose units joined by β-1,4-glycosidic bonds. Every other unit is turned over, so the chain is straight.
The straight chains lie side by side and are held together by many hydrogen bonds between their \mathrm{{-}OH} groups, forming strong fibres. Cotton is almost pure cellulose, and paper is also made from it.
Cellulose does not dissolve in water and gives no colour with iodine. Humans have no enzyme that breaks it down, so we cannot digest it, but cows and other grazing animals can, with the help of microorganisms in their stomachs.
Fibres made by dissolving cellulose and then turning it back into fibres are called regenerated fibres. For cuprammonium rayon, cellulose is dissolved in Schweizer's reagent, a deep blue solution of tetraamminecopper(II) hydroxide, and pushed through fine holes into dilute sulfuric acid.
For viscose rayon, cellulose is treated with sodium hydroxide and carbon disulfide to make a thick solution called viscose, which is also pushed into dilute sulfuric acid. Pushing viscose through a narrow slit instead gives a film, cellophane.
Treating cellulose with acetic anhydride changes its \mathrm{{-}OH} groups into \mathrm{{-}OCOCH_3} groups. After partial hydrolysis, the product dissolves in acetone and is spun into acetate fibre, a semi-synthetic fibre made by changing part of the structure of cellulose.