CIE A Level Biology (9700) 2019-2021

Revision Notes

2.2.3 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

  • 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

Starch and Glycogen_ Amylose, downloadable IGCSE & GCSE Biology revision notes
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

Starch and Glycogen_ Amylopectin, downloadable IGCSE & GCSE Biology revision notes
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

Starch and Glycogen_ Glycogen, downloadable IGCSE & GCSE Biology revision notes
Glycogen, the highly branched molecule used as a storage polysaccharide in animals and fungi

Summary of storage polysaccharides table

Summary of storage polysaccharides table, downloadable AS & A Level Biology revision notes

Exam Tip

Be clear about the differences between starch (amylose and amylopectin) and glycogen.

Author: Catherine

Cate has over 20 years’ experience teaching Biology to IGCSE, IB and A-level students in seven different countries across Asia, Europe, North America and the Middle East. This has given her a fine appreciation of different cultures, places and teaching methods. Cate has a keen interest in producing Biology revision resources that will help students engage with the subject.
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