Polysaccharides are sugar chains that became fuel, armour and paper
Polysaccharides are long chains of simple sugars, sometimes thousands of units long. Linked one way they become starch in potatoes or glycogen in your liver, ready to be burned; built another way they become cellulose in plant walls or chitin in arthropod shells, and few creatures can digest cellulose at all.
The name joins Greek words for many and sugar. Their building blocks are monosaccharides such as glucose and fructose, and as a rough rule a chain of more than ten units counts as a polysaccharide, while three to ten make an oligosaccharide. When the units are six-carbon sugars, a chain typically holds somewhere between 40 and 3,000 of them. A classic test uses iodine: the coiled helices of starch trap it and turn dark blue, while straight cellulose cannot hold it and shows no change.
Plants store glucose as starch, a blend of roughly 15 to 20 percent straight-chained amylose and 80 to 85 percent branched amylopectin. Potatoes, rice, wheat and maize supply most of the starch people eat, and enzymes called amylases break its links. Animals keep a more tightly branched version, glycogen, nicknamed animal starch, which can be mobilised faster, a good fit for creatures that move around. Soon after a meal it can make up 8 percent of liver weight, about 100 to 120 grams in an adult, and only that liver store can be shared with other organs; muscles hold just 1 to 2 percent. The uterus even stockpiles it in pregnancy to feed the embryo.
Structural polysaccharides are built for strength. Cellulose, in plant cell walls, may be the commonest organic compound on the planet, and it underpins paper, textiles, rayon and celluloid. Chitin, in the shells of arthropods and the walls of some fungi, carries nitrogen-containing side branches that make it tougher, and it is used for surgical thread. Cows and termites manage cellulose only with help from microbes.
Some chains pass through humans largely undigested as dietary fibre. Soluble fibre binds bile acids in the small intestine, which lowers blood cholesterol, and slows sugar absorption. Inulin, a fructose chain that some plants store in their roots instead of starch, was approved by the US Food and Drug Administration in 2018 as a fibre ingredient for processed foods.
Source: Polysaccharide
