Starch & Glycogen (Cambridge (CIE) A Level Biology): Revision Note

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

• 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

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