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
- types
- Storage (starch, glycogen, galactogen) and structural (cellulose, chitin, hemicellulose)
- function
- Energy storage and structural support in plants, animals, fungi, and bacteria
Lore & Background
Polysaccharides are formed from monosaccharides with the general formula (CH2O)n where n is three or more. As a rule of thumb, polysaccharides contain more than ten monosaccharide units, whereas oligosaccharides contain three to ten. They are often heterogeneous, containing slight modifications of the repeating unit, and may be amorphous (e.g., starch) or insoluble in water (e.g., cellulose). 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 glycogen, more densely branched and sometimes called 'animal starch.' Glycogen's properties allow it to be metabolized more quickly, suiting active animals. Galactogen, a polysaccharide of galactose, functions as energy storage in pulmonate snails and some Caenogastropoda, found only in the female snail reproductive system and perivitelline fluid.
Reader's Guide
Polysaccharides are an important class of biological polymers, with functions in living organisms usually either structure- or storage-related. Starch is a major energy source in the human diet, found in potato, rice, wheat, and maize, and is digestible by amylases. Cellulose, the most abundant organic molecule on Earth, is used in plant cell walls and in industries such as paper and textiles, and as feedstock for rayon, cellulose acetate, celluloid, and nitrocellulose. Chitin, with nitrogen-containing side branches increasing its strength, is found in arthropod exoskeletons and fungal cell walls, with uses including surgical threads. Nutritional polysaccharides are common energy sources; many organisms break down starches into glucose, but few metabolize cellulose. Some polysaccharides serve as dietary fiber, enhancing digestion: soluble fiber binds bile acids, lowers cholesterol, attenuates sugar absorption, and produces short-chain fatty acids; insoluble fiber is associated with reduced diabetes risk. Regulatory authorities in many developed countries recommend increases in fiber intake.
Did You Know?
- Starch holds iodine molecules in its helical portion, producing a dark blue color; cellulose does not, so no color change occurs with iodine.
- 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 it is used in surgical threads.
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