Starch & Glycogen (AQA A Level Biology)

Starch & Glycogen: Structures & Functions

• Starch and glycogen are polysaccharides

• Polysaccharides are macromolecules that are polymers 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 e.g. starch and glycogen)

  • Straight (making the molecules suitable to construct cellular structures e.g. cellulose) or coiled

• Starch and glycogen are storage polysaccharides because they are:

  • Compact (so large quantities can be stored)

  • Insoluble (so will have no osmotic effect, unlike glucose which would lower the water potential of a cell causing water to move into cells, plant cells would then have to have thicker cell walls, and animal cells would burst under the increased pressure)

Starch

• Starch is the storage polysaccharide of plants. It is stored as granules in plastids (e.g. chloroplasts)

• Due to the many monomers in a starch molecule, it takes longer to digest than glucose

• 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

Amylose – one of the two polysaccharides that is used to form starch (the storage polysaccharide in plants)

• 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

  • The branches result in many terminal glucose molecules that can be easily hydrolysed for use during cellular respiration or added to for storage

Amylopectin – one of the two polysaccharides that is used to form starch (the storage polysaccharide in plants)

Glycogen

• Glycogen is the storage polysaccharide of animals and fungi, it is highly branched and not coiled

• Liver and muscles cells have a high concentration of glycogen, present as visible granules, as the cellular respiration rate is high in these cells (due to animals being mobile)

• Glycogen is more branched than amylopectin making it more compact which helps animals store more

• The branching enables more free ends where glucose molecules can either be added or removed allowing for condensation and hydrolysis reactions to occur more rapidly – thus the storage or release of glucose can suit the demands of the cell

Glycogen, the highly branched molecule used as a storage polysaccharide in animals and fungi

Summary of Storage Polysaccharides Table