Carbohydrates: Polysaccharides

This lesson covers the structure and function of the three key polysaccharides — starch, glycogen and cellulose — as required by the Edexcel A-Level Biology B specification (9BI0), Topic 1: Biological Molecules. You need to understand how the structures of these polymers relate to their biological functions and be able to explain the differences between storage and structural polysaccharides.

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What Are Polysaccharides?

Polysaccharides are polymers made of many monosaccharide monomers joined together by glycosidic bonds in condensation reactions. They are macromolecules — large biological molecules with relative molecular masses in the thousands or millions.

Key properties of polysaccharides:

• They are insoluble in water (or form colloids) — this makes them ideal for storage because they do not affect the water potential of the cell.
• They are not sweet-tasting.
• They are not reducing sugars (because very few of their anomeric carbons are free).
• They can be hydrolysed back to monosaccharides by enzymes or by boiling with dilute acid.

Key Definition: A polymer is a large molecule made up of many repeating subunits (monomers) joined by covalent bonds. Polysaccharides are polymers of monosaccharides.