Polysaccharides

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Important points

  • Polysaccharides are polymers of monosaccharides having more than 10 units.
  • Polysaccharides are biomacromolecules.
  • Polysaccharides, even though classified as saccharides, are not sweet in taste.
  • During the formation of a polysaccharide, new monosaccharide residues are added at the non-reducing end of the chain.

CLASSIFICATION OF POLYSACCHARIDES

On the basis of the type of repeating units

  • HOMOPOLYSACCHARIDES
  • HETEROPOLYSACCHARIDES
On the basis of their role

  • STRUCTURAL POLYSACCHARIDES
  • STORAGE POLYSACCHARIDES

1. Structural polysaccharides

CELLULOSE

  • Cellulose is a homopolymer of β-glucose.
  • The β-glucose units are joined by β-(1,4) glycosidic bonds.
  • Cellulose is a linear, straight-chain molecule.
  • Cellulose is the most abundant organic molecule on Earth.
  • Found in the cell wall of plants and in the spores of some slime moulds.
Structure of cellulose showing beta-glucose units joined by beta-1,4 glycosidic bonds

Humans cannot digest cellulose because we lack the enzyme cellulase, which breaks β-(1,4) bonds. Ruminants digest it with the help of microbes in their gut.

CHITIN

  • Chitin is a homopolymer of N-acetyl glucosamine (NAG), an amino sugar with an N-acetylated amino group.
  • The NAG units of chitin are joined by β-(1,4) glycosidic bonds.
  • Chitin is also a linear, straight-chain molecule, but it can form helices in its secondary form.
  • Chitin is the second most abundant organic matter on Earth.
  • Found in the cell wall of fungi and the exoskeleton of arthropods.
Structure of chitin showing N-acetyl glucosamine units

PEPTIDOGLYCAN

  • Peptidoglycan is a heteropolymer of N-acetyl glucosamine (NAG) and N-acetyl muramic acid (NAM) [both NAG and NAM are amino sugars].
  • The NAG and NAM units of peptidoglycan are joined by β-(1,4) glycosidic bonds.
  • Found in the cell wall of eubacteria.
Structure of peptidoglycan showing alternating NAG and NAM units

OTHER STRUCTURAL POLYSACCHARIDES

  • Pectins
  • Hemicellulose

2. Storage polysaccharides

STARCH (AMYLUM)

  • Starch is a homopolymer of α-glucose units.
  • The α-glucose units are joined by α-(1,4) glycosidic bonds in the chains and α-(1,6) glycosidic bonds at the branch points (in amylopectin).
  • It is the reserve food in plants and is found as insoluble granules in chloroplasts and amyloplasts.
  • Starch is made up of α-amylose and amylopectin.
Starch diagram
Structure of amylose, an unbranched chain of alpha-glucose units
Structure of amylopectin, a branched chain of alpha-glucose units

GLYCOGEN

  • Glycogen is a homopolymer of α-glucose units.
  • Glycogen is similar in structure to amylopectin but is relatively more branched.
  • Glycogen is the reserve food in most bacteria, fungi and animals.
  • Gives a red-violet colour with iodine.
  • Number of non-reducing ends in a glycogen molecule = number of branch points + 1.
  • Number of reducing ends = 1 (right side).
Structure of glycogen showing its highly branched chains

Iodine test: starch gives a blue-black colour because iodine fits inside the amylose helix, while glycogen gives a red-violet colour. Cellulose does not react, as it has no helices to hold iodine.

Amylose is unbranched, with only α-(1,4) bonds. Amylopectin branches about every 24–30 glucose units through α-(1,6) bonds, and glycogen is more branched, about every 8–12 units.

INULIN

  • Inulin is a homopolymer of fructose units.
  • The fructose units are joined by β-(2,1) glycosidic bonds.
  • Stored in the roots of Dahlia and related plants.
  • Inulin is a water-soluble polysaccharide.
  • Inulin is not metabolized in the human body and hence is used for the kidney function test.
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