Macromolecules (HL IB Biology)

• Carbon compounds can be large molecules made from many small, repeating subunits
  • Monomers are the smaller units from which larger molecules are made
  • Polymers are molecules made from a large number of monomers joined together in a chain
  • The process by which monomers join to form polymers is polymerisation
• Macromolecules are very large molecules
  • They contain 1000 or more atoms and so have a high molecular mass
  • Polymers can be macromolecules, however, not all macromolecules are polymers; polymers must consist of many repeating subunits
    • E.g. lipids are not polymers, as they do not consist of repeating monomers

Key biological macromolecules table

Formation of macromolecules

• Macromolecules are formed during condensation reactions
  • A condensation reaction occurs when molecules combine together, forming covalent bonds and resulting in polymers (polymerisation) or macromolecules
  • Water is removed as part of the reaction

Examples of condensation reactions

• Polysaccharides
  • Polysaccharides are formed when two hydroxyl (OH) groups on different monosaccharides interact to form a strong covalent bond called a glycosidic bond

Glycosidic bond formation diagram

• Polypeptides
  • Polypeptides are formed by condensation reactions
  • Two amino acid monomers interact to form a strong covalent bond called a peptide bond

Peptide bond formation diagram

• Nucleic acids
  • Separate nucleotides are joined together via condensation reactions to form a phosphodiester bond
    • These condensation reactions occur between the phosphate group of one nucleotide and the pentose sugar of the next nucleotide
    • It is called a phosphodiester bond because it consists of a phosphate group and two ester bonds

Phosphodiester bond formation diagram

• Macromolecules often need to be broken down into their monomers, e.g. this happens in digestion
• The reaction that allows this to occur is a hydrolysis reaction
  • Hydrolysis means ‘lyse’ (to break) and ‘hydro’ (with water)
• In the hydrolysis of macromolecules, covalent bonds are broken when water is added
  • The -O and -OH from the water molecule are used to form the functional groups of the products
• Examples of hydrolysis reactions include:
  • The hydrolysis of glycosidic bonds in poly- or disaccharides to produce monosaccharides
  • The hydrolysis of peptide bonds in polypeptides to produce amino acids
  • Hydrolysis of ester bonds in triglycerides to produce three fatty acids and glycerol

Hydrolysis of a disaccharide diagram