Chemistry · Ch 14 — Biomolecules
Polysaccharides
Polysaccharides
Polysaccharides consist of large numbers of monosaccharide units bonded together by glycosidic bonds and are the most common form of carbohydrate found in nature. Because they lack the sweet taste of the smaller sugars, they are called non-sugars, and depending on the pattern of bonding they can form either linear or branched-chain molecules. They are classified according to how many different kinds of monosaccharide they are built from: homopolysaccharides are composed of only one type of monosaccharide (examples: starch, cellulose and glycogen), while heteropolysaccharides are composed of more than one type (examples: hyaluronic acid and heparin).
Starch is the energy-storage polysaccharide of plants, abundant in potatoes, corn, wheat and rice. It is a polymer of glucose in which the glucose units are linked by α(1,4) glycosidic bonds, and it separates into two distinct fractions: water-soluble amylose, making up about 20% of starch, and water-insoluble amylopectin, making up about 80%. Amylose consists of unbranched chains of up to about 4000 α-D-glucose units joined solely by α(1,4) bonds. Amylopectin consists of much larger chains of up to about 10000 α-D-glucose units, also linked mainly by α(1,4) bonds, but with additional branch points where a new side-chain of 24-30 glucose units is attached through an α(1,6) glycosidic bond. The two fractions can be told apart with iodine solution: amylose gives a deep blue colour while amylopectin gives a purple colour.
Cellulose is the major structural constituent of plant cell walls -- cotton is almost pure cellulose -- and it hydrolyses to give D-glucose. Unlike starch, cellulose is a straight, unbranched chain in which the glucose units are linked by β(1,4) glycosidic bonds rather than α(1,4) bonds. This single stereochemical difference has huge practical consequences: humans cannot digest cellulose as food, because our digestive system lacks the enzymes (glycosidases or cellulases) able to hydrolyse a β(1,4) linkage, even though we can readily digest starch's α(1,4) linkage. Cellulose is instead used extensively to manufacture paper, cellulose fibres, rayon and explosives such as gun cotton (the nitrated ester of cellulose). …
What this figure shows. Side-by-side chain diagrams of starch's two fractions: amylose drawn as a long, unbranched chain of α-D-glucose units joined by α(1,4) bonds, and amylopectin drawn as a much larger, branched network with the same α(1,4)-linked backbone chains but with additional α(1,6)-linked side branches sprouting at regular inter …
What this figure shows. A repeating-unit chain diagram of cellulose showing three consecutive β-D-glucose pyranose rings joined by β(1,4) glycosidic oxygen bridges in a straight, unbranched line (labelled '(β-D-glucose)ₙ'), with each ring flipped 180° relative to its neighbour -- the alternating-ring geometry that the β(1,4) linkage forces and that α(1,4 …