Polysaccharide
Polysaccharides are abundant carbohydrates serving storage and structural roles.
Polysaccharides are compounds consisting of a large number of monosaccharides linked glycosidically. They are the most abundant carbohydrates in food, with structures ranging from linear to highly branched polymers. Examples include storage polysaccharides such as starch, glycogen, and galactogen, and structural polysaccharides such as hemicellulose and chitin. The term 'glycan' is synonymous with polysaccharide, but often glycans are discussed in the context of glycoconjugates, i.e., hybrids of polysaccharides and proteins or lipids.
- field
- Biochemistry, Carbohydrate Chemistry
- known_for
- Most abundant carbohydrates in food; key storage and structural polymers in living organisms
- examples
- Starch, glycogen, galactogen, cellulose, chitin, hemicellulose, inulin, arabinoxylans
Lore & Background
Polysaccharides are often heterogeneous, containing slight modifications of the repeating unit. They may be amorphous (e.g., starch) or insoluble in water (e.g., cellulose). As a rule of thumb, polysaccharides contain more than ten monosaccharide units, whereas oligosaccharides contain three to ten monosaccharide units, but the precise cutoff varies according to convention. Starch, a polymer of glucose, is used as a storage polysaccharide in plants, found as both amylose and branched amylopectin. In animals, the structurally similar glucose polymer is the more densely branched glycogen, sometimes called 'animal starch,' which allows it to be metabolized more quickly. Galactogen is a polysaccharide of galactose that functions as energy storage in pulmonate snails and some Caenogastropoda, found only in the female snail reproductive system and in perivitelline fluid.
Reader's Guide
Polysaccharides are an important class of biological polymers whose function in living organisms is usually either structure- or storage-related. Starch is a major source of energy in the human diet, found in potato, rice, wheat, and maize. Glycogen serves as secondary long-term energy storage in animal and fungal cells, primarily stored in the liver and muscles. Cellulose, used in plant cell walls, is the most abundant organic molecule on Earth and has significant roles in paper, textiles, and production of rayon, cellulose acetate, celluloid, and nitrocellulose. Chitin, found in arthropod exoskeletons and fungal cell walls, has nitrogen-containing side branches that increase its strength and is used in surgical threads. Nutritional polysaccharides are common energy sources; many organisms break down starches into glucose, but few metabolize cellulose. Some polysaccharides act as dietary fiber, enhancing digestion, lowering cholesterol, and attenuating sugar absorption. The starch-iodine test, producing a dark blue color, is a unique character test for polysaccharides, while cellulose does not show this color change.
Did You Know?
- Starch holds iodine molecules in its helical portion, producing a dark blue color; cellulose does not show this color change.
- Glycogen is more densely branched than starch and can be metabolized more quickly, suiting active animals.
- Chitin has nitrogen-containing side branches that increase its strength and is found in arthropod exoskeletons and fungal cell walls.
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