2.2.1 Carbohydrates

• 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

• 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