CIE A Level Biology复习笔记2.2.5 Starch & Glycogen

• 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

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