Polysaccharides (OCR A Level Biology)

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Polysaccharides: Structure

  • Starch, glycogen and cellulose are polysaccharides
  • Polysaccharides are macromolecules (polymers) that are 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

  • 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

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)

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 a polysaccharide found in animals
  • It is made up of α-glucose molecules
    • There are 1,4 glycosidic bonds between α-glucose molecules and also 1,6 glycosidic bonds between glucose molecules creating a branched molecule

  • Glycogen has a similar structure to amylopectin but it has more branches

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

Cellulose

  • Cellulose is a polysaccharide found in plants
  • It consists of long chains of β-glucose joined together by 1,4 glycosidic bonds
  • β-glucose is an isomer of α-glucose, so in order to form the 1,4 glycosidic bonds consecutive β-glucose molecules must be rotated 180° to each other

2.2.1 Formation of Sugars 1, downloadable IB Biology revision notesTo form the 1,4 glycosidic bond between two β-glucose molecules, the glucose molecules must be rotated to 180° to each other

  • Due to the inversion of the β-glucose molecules, many hydrogen bonds form between the long chains giving cellulose its strength

Cellulose_ hydrogen bond formation between chains, downloadable IGCSE & GCSE Biology revision notes

Cellulose has high tensile strength due to the many hydrogen bonds that form between the long chains of β-glucose molecules

Examiner Tip

Be clear about the differences between starch, glycogen, and cellulose.  

Polysaccharides: Function

  • Starch and glycogen are storage polysaccharides because they are:
    • Compact
      • So large quantities can be stored

    • Insoluble
      • So they will have no osmotic effect, unlike glucose which would lower the water potential of a cell causing water to move into cells

Starch

  • Starch is the storage polysaccharide of plants. It is stored as granules in plastids such as amyloplasts and chloroplasts
    • Plastids are membrane-bound organelles that can be found in plant cells. They have a specialised function eg. amyloplasts store starch grains

  • Due to the many monomers in a starch molecule, it takes longer to digest than glucose
  • The amylopectin in starch has branches that result in many terminal glucose molecules that can be easily hydrolysed for use during cellular respiration or added 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

Cellulose

  • Cellulose is the main structural component of cell walls due to its strength which is a result of the many hydrogen bonds found between the parallel chains of microfibrils
  • The high tensile strength of cellulose allows it to be stretched without breaking which makes it possible for cell walls to withstand turgor pressure
  • The cellulose fibres and other molecules (eg. lignin) found in the cell wall forms a matrix which increases the strength of the cell walls
  • The strengthened cell walls provide support to the plant
  • Cellulose fibres are freely permeable which allows water and solutes to leave or reach the cell surface membrane
  • As few organisms have the enzyme (cellulase) to hydrolyse cellulose it is a source of fibre

Cellulose_ Structure linking to function of cellulose, downloadable IGCSE & GCSE Biology revision notes

The strength and insolubility of cellulose fibres mean it is a suitable molecule to construct cell walls

Examiner Tip

Other carbohydrate polymers exist in organisms: peptidoglycan is found in the cell walls of bacteria and chitin is found in the exoskeleton of insects.

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Lára

Author: Lára

Expertise: Biology Lead

Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.