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First teaching 2023

First exams 2025

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Macromolecules (SL IB Biology)

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Cara Head

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Cara Head

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Formation of Macromolecules

  • 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

Macromolecule Monomer
Carbohydrates (polysaccharides) Monosaccharides
Lipids Fatty acids, glycerol, phosphate groups
Proteins (polypeptides) Amino acids
Nucleic acids Nucleotides

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

forming a glycosidic bond

  • 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

peptide-bond-dipeptide-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

phosphodiester-bond-formation-diagram-

Digestion of Polymers

  • 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

Hydrolysis of a disaccharide

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Cara Head

Author: Cara Head

Expertise: Biology

Cara graduated from the University of Exeter in 2005 with a degree in Biological Sciences. She has fifteen years of experience teaching the Sciences at KS3 to KS5, and Psychology at A-Level. Cara has taught in a range of secondary schools across the South West of England before joining the team at SME. Cara is passionate about Biology and creating resources that bring the subject alive and deepen students' understanding