Starch & Glycogen (AQA A Level Biology): Revision Note

Starch & glycogen: structures & functions

• Starch and glycogen are polysaccharides

• Polysaccharides are macromolecules formed by many monosaccharides joined by glycosidic bonds in a condensation reaction to form long 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)

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

  • Amylopectin

Amylose

• Amyllose comprises 10 - 30% of starch

• It has an 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

Amylopectin

• Amylopectin is70 - 90% of starch

• It has 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

Glycogen

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

• Liver and muscle 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