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Starch & Glycogen (CIE AS Biology)

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Cara Head

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Cara Head

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Starch & Glycogen: Structures & Functions

  • Starch and glycogen are polysaccharides
  • Polysaccharides are polymers formed from many monosaccharides joined by glycosidic bonds to form chains; these chains may be:
    • Branched or unbranched
    • Folded
    • Straight or coiled
  • Starch and glycogen are storage polysaccharides because they are:
    • Compact
      • Many molecules fit into a small space, so large volumes can be stored inside cells
    • Insoluble
      • They do not dissolve in the cell cytoplasm, so have no osmotic effect on cells; soluble molecules like glucose would lower the water potential of cell cytoplasm, drawing water into cells by osmosis

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 structure diagram

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

Amylose is one of the two polysaccharides that forms 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 structure diagram

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

Amylopectin is the other of the two polysaccharides that forms starch

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 structure diagram

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

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

Summary of storage polysaccharides table

Feature
Starch Glycogen
Amylose Amylopectin
Monomer α-glucose α-glucose α-glucose
Branched No Yes  Yes
Helix shape Yes No No
Glycosidic Bond Present 1,4 1,4 and 1,6 1,4 and 1,6
Source Plants Plants Animals

Examiner Tip

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

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Cara Head

Author: Cara Head

Expertise: Biology

Cara graduated from the University of Exeter in 2005 with a degree in Biological Sciences. She has fifteen years of experience teaching the Sciences at KS3 to KS5, and Psychology at A-Level. Cara has taught in a range of secondary schools across the South West of England before joining the team at SME. Cara is passionate about Biology and creating resources that bring the subject alive and deepen students' understanding