Polysaccharides


3. Polysaccharides

As you may imagine, polysaccharides are formed of more than 2 monosaccharides joining together in a condensation reaction.

It may be several thousands monosaccharide units long.

Polysaccharides are not sugars though.

The most important polysaccharides are starch, glycogen and cellulose, all of which are polymers of glucose. Glucose by itself is a reactive molecule and it might interfere with normal cell chemistry, or affect osmotic properties of cells so it is therefore converted to either one of the polysaccharides above, which are compact, inert, and insoluble.

Starch and Glycogen

  • A mixture of two substances — Amylose, and Amylopectin. Amylose is made by condensations between Alpha-Glucose molecules. Amylopectin is also made of many 1,4 linked Alpha-Glucose molecules, but it’s shorter than amylose and branch out to the sides. The branches are formed by 1,6 linkages.
  • Build up into grains, which are commonly found in chloroplasts and in storage organs such as tubers or seeds.
  • Are easily seen under light microscope, especially if stained with iodine-potassium iodide solution.
  • Starch is never found in animal cells, but only glycogen.
  • Glycogen molec. clump up together to form granules, which are visible in liver cells and muscle cells, where they form an energy reserve.

starch

glycogenCellulose

  • The most abundant organic molecule on the planet, due to its presence in plant cell walls and its slow rate of breakdown in nature.
  • It has a structural role, unlike starch and glycogen.
  • Cellulose is a polymer (chain of alike molecules) of Beta-Glucose, not Alpha-Glucose like starch and glycogen.
  • Successive units are linked at 180 degrees to each other.
  • Form quite weak hydrogen bonds (H+ bonded to O-), but numerous.
  • 60-70 molecules become tightly cross-linked to form bundles called microfibrils. In turn, these are held together in bundles called fibers (good for the digestive system in animals).

A typical cell wall (only plant cells), has several layers of fibers, running in different directions to increase strength.

Cellulose fibers have very high tensile strength, almost equal to that of steel (Makes it possible for the cells not to burst when they become turgid/ makes it possible for the cells to withstand the large pressures that develop within it as a result of osmosis).

Despite their strength cellulose fibers are freely permeable.

cellulose_microfibrilsGO FURTHER TO :

1.1.4 Practicals

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