Carbohydrates: Definition, Functions & Examples (HL IB Biology)

• The monomers of carbohydrates are monosaccharides
  • Two monosaccharides can join to form a disaccharide
  • Many monosaccharides join to form a polysaccharide
• Monosaccharides can join together via condensation reactions
  • The new chemical bond that forms between two monosaccharides is known as a glycosidic bond
• Monosaccharides have the general formula C_nH_2nO_n
  • Where 'n' is the number of carbon atoms in the molecule
  • Note that this formula only applies to monosaccharides
• Monosaccharide properties include:
  • Colourless crystalline molecules
  • Soluble in water
• There are different types of monosaccharide formed from molecules with varying numbers of carbon atoms, for example:
  • Triose molecules contain 3 carbon atoms, e.g. glyceraldehyde
  • Pentose molecules contain 5 carbon atoms, e.g. ribose
  • Hexose molecules contain 6 carbon atoms, e.g. glucose

Ribose and glucose structure diagrams

Pentose sugars, such as ribose (top), can be recognised by their five-point carbon rings and hexose sugars, such as glucose (bottom) by their six-point carbon rings

Glucose

• The most well-known carbohydrate monomer is glucose
• Glucose has the molecular formula C₆H₁₂O₆
  • Glucose is the most common monosaccharide and is of central importance to most forms of life
  • Glucose is the main substrate used in respiration, releasing energy for the production of ATP
  • Glucose is produced during photosynthesis
• Glucose exists in two structurally different forms, alpha (α) glucose and beta (β) glucose, these structures are known as the isomers of glucose
  • This structural variety results in different functions between carbohydrates
  • This seemingly minor example of isomerism has far-reaching consequences on the functions of the polymers

Glucose structure diagrams

• Different polysaccharides are formed from the two isomers of glucose
  • Starch and glycogen are made from molecules of alpha glucose
  • Cellulose is made from molecules of beta glucose

Properties of glucose

• Glucose has several properties that are essential to its function in living organisms
  • Stable structure due to the presence of covalent bonds which are strong and hard to break
  • Soluble in water due to its polar nature
  • Easily transportable due to its water solubility
  • A source of chemical energy when its covalent bonds are broken

The function of carbohydrates

• Carbohydrates function as essential energy storage molecules and as structural molecules
• Starch and glycogen are effective storage polysaccharides because they are:
  • Compact
    • Large quantities can be stored in a small space
  • Insoluble
    • This is essential because soluble molecules will dissolve in cell cytoplasm, lowering the water potential and causing water to move into cells
    • If too much water enters an animal cell it will burst
• Cellulose is a structural polysaccharide because it is:
  • Strong and durable
  • Insoluble and slightly elastic
  • Chemically inert; few organisms possess enzymes that can hydrolyse it

Polysaccharide function diagram

The different structures of starch, glycogen and cellulose allow each polysaccharide to perform different functions 

Starch

• Starch is the storage polysaccharide of plants
  • Starch is stored as granules in chloroplasts
• It is made of alpha glucose monomers 
• 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)
    • Contains 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

Amylose and amylopectin structure diagrams

Amylose (top) and amylopectin (bottom); the two polysaccharides that form starch in plant cells

Glycogen

• Glycogen is the storage polysaccharide of animals and fungi
• The monomer of glycogen is alpha glucose, joined by 1,4- and 1,6 glycosidic bonds
• Glycogen is more branched than amylopectin, providing more free ends where glucose molecules can be removed by hydrolysis
  • This means that glycogen can be broken down quickly, supplying the higher metabolic needs of animal cells
• Liver and muscles cells contain glycogen as visible granules, enabling high rates of cellular respiration

Glycogen structure diagram

Glycogen is a highly branched storage molecule present in animals and fungi

• Cellulose is a structural carbohydrate found in the cell walls of plants
• Molecules of cellulose are straight and unbranched
• Cellulose is a polymer of β-glucose monomers
  • β-glucose differs very slightly in structure to α-glucose; the hydroxyl group on carbon 1 sits above the carbon ring in β-glucose, whereas it sits below the ring in α-glucose
  • It means that in order to form a glycosidic bond with a molecule of β-glucose, every alternate molecule of β-glucose in the chain must invert itself, or flip upside down

Beta glucose in cellulose diagram

• The alternating pattern of the monomers in cellulose allows hydrogen bonding to occur between strands of β-glucose monomers, adding strength to the polymer
  • Hydrogen bonds link several molecules of cellulose to form microfibrils

Hydrogen bonding in cellulose diagram

Cellulose function diagram

Polysaccharide structure summary table