Polysaccharides: Structure
- Starch, glycogen and cellulose are polysaccharides
- Polysaccharides are macromolecules (polymers) that are formed by many monosaccharides joined by glycosidic bonds in a condensation reaction to form chains
- These chains may be:
- Branched or unbranched
- Folded (making the molecule compact which is ideal for storage eg. starch and glycogen)
- Straight (making the molecules suitable to construct cellular structures e.g. cellulose) or coiled
Starch
- Starch is constructed from two different polysaccharides:
- Amylose (10 - 30% of starch)
- Unbranched helix-shaped chain with 1,4 glycosidic bonds between α-glucose molecules
- The helix shape enables it to be more compact and thus it is more resistant to digestion
- Amylopectin (70 - 90% of starch)
- 1,4 glycosidic bonds between α-glucose molecules but also 1,6 glycosidic bonds form between glucose molecules creating a branched molecule
- Amylose (10 - 30% of starch)
Amylose – one of the two polysaccharides that is used to form starch (the storage polysaccharide in plants)
Amylopectin – one of the two polysaccharides that is used to form starch (the storage polysaccharide in plants)
Glycogen
- Glycogen is a polysaccharide found in animals
- It is made up of α-glucose molecules
- There are 1,4 glycosidic bonds between α-glucose molecules and also 1,6 glycosidic bonds between glucose molecules creating a branched molecule
- Glycogen has a similar structure to amylopectin but it has more branches
Glycogen, the highly branched molecule used as a storage polysaccharide in animals and fungi
Summary of Storage Polysaccharides Table
Cellulose
- Cellulose is a polysaccharide found in plants
- It consists of long chains of β-glucose joined together by 1,4 glycosidic bonds
- β-glucose is an isomer of α-glucose, so in order to form the 1,4 glycosidic bonds consecutive β-glucose molecules must be rotated 180° to each other
To form the 1,4 glycosidic bond between two β-glucose molecules, the glucose molecules must be rotated to 180° to each other
- Due to the inversion of the β-glucose molecules, many hydrogen bonds form between the long chains giving cellulose its strength
Cellulose has high tensile strength due to the many hydrogen bonds that form between the long chains of β-glucose molecules
Examiner Tip
Be clear about the differences between starch, glycogen, and cellulose.