• 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

     

   

Amylose – one of the two polysaccharides that is used to form starch (the storage polysaccharide in plants)

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

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

Summary of Storage Polysaccharides Table

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

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

• 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

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