• Monosaccharides?can join together via?condensation reactions?to form?disaccharides
  • A condensation reaction is one in which two molecules join together via the formation of a new chemical bond, with a?molecule of water?being?released?in the process
  • The new chemical bond that forms between two monosaccharides is known as a?glycosidic bond
  • To calculate the chemical formula of a disaccharide, you add all the carbons, hydrogens and oxygens in both monomers then subtract 2 H and 1 O (for the water molecule lost)
• Common examples of disaccharides include:
  • Maltose?(the sugar formed in the production and breakdown of starch)
  • Sucrose?(the main sugar produced in plants)
  • Lactose?(a sugar found only in milk)
• All three of the common examples above have the formula?C12H22O11

Common Disaccharides and their Monosaccharide Monomers Table

The disaccharide maltose is formed from two α-glucose monomers (sub-units)

The disaccharide sucrose is formed from α-glucose and fructose monomers (sub-units)

• Starch, cellulose?and?glycogen?are?polysaccharides
• Polysaccharides are macromolecules that are?polymers?formed by?many?monosaccharides joined together by?glycosidic?bonds
• The bonds form from?condensation?reactions, resulting in polysaccharide 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)
    • The monosaccharide glucose lowers the osmolarity of a cell causing water to move into cells
    • If too much water enters an animal cell it will burst
    • Plant cells have developed thicker cell walls to prevent this
• Cellulose?is a structural polysaccharide because it is:
  • Strong?and?durable
  • Insoluble?and slightly?elastic
  • Chemically?inert?(hardly any organisms possess enzymes that can hydrolyse it)
  • Is an ideal material for plant cell walls
  • The main constituent of?dietary fibre?for animals that eat plants

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?as well as?1,6?glycosidic bonds?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 the molecule more efficiently
• 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

Cellulose

• Cellulose is a polymer of?β-glucose?monomers
• β-glucose differs very slightly in structure to α-glucose
• The hydroxyl group on the C1 atom?sits above the carbon ring?in β-glucose, whereas it sits below the ring in α-glucose

A comparison of the structure of alpha-glucose and beta-glucose

• Alpha-glucose and beta-glucose are?isomers
• This?seemingly minor example of isomerism?has?far-reaching consequences?on the functions of the polymers
• It means that in order to form a glycosidic bond with a molecule of β-glucose, the next molecule of β-glucose in the chain must?invert itself

Two beta-glucose molecules orientation in a position where they are able to bond to each other

• This results in a chain of?repeatedly inverted?β-glucose monomers
• The alternating pattern of the monomers allows the chain to grow in?long, straight lengths?which gives?great fibrous strength
• Hydrogen bonding?occurs between strands of β-glucose monomers, adding strength to the polymer

The alternating pattern of glycosidic bonds in cellulose

How cellulose fibres band together to provide plant strength

Summary of Polysaccharides Table