AQA A Level Biology复习笔记1.1.8 Starch & Glycogen

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 eg. 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, cells would then have to have thicker cell walls - plants or burst if they were animal cells)



  • 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 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