AQA A Level Biology

Revision Notes

1.1.1 Biological Molecules: Key Terms

Biological Molecules: Key Terms

  • There is much variety of life within and between organisms but the biochemical basis of life is similar for all living things
  • The key molecules that are required to build structures that enable organisms to function are:
    • Carbohydrates
    • Proteins
    • Lipids
    • Nucleic Acids
    • Water
  • 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

5 Biological molecules (1), downloadable IGCSE & GCSE Biology revision notes

5 Biological molecules (2), downloadable IGCSE & GCSE Biology revision notes

The key biological molecules for living organisms

  • Carbohydrates, proteins, lipids and nucleic acids contain the elements carbon (C) and hydrogen (H) making them organic compounds
  • Carbon atoms are key to the organic compounds because:
    • Each carbon atom can form four covalent bonds – this makes the compounds very stable (as covalent bonds are so strong they require a large input of energy to break them)
    • Carbon atoms can form covalent bonds with oxygen, nitrogen and sulfur
    • Carbon atoms can bond to form straight chains, branched chains or rings
  • Carbon compounds can form small single subunits (monomers) that bond with many repeating subunits to form large molecules (polymers) by a process called polymerisation
  • Macromolecules are very large molecules
    • That contain 1000 or more atoms therefore having a high molecular mass
    • Polymers can be macromolecules, however not all macromolecules are polymers as the subunits of polymers have to be the same repeating units


  • Carbohydrates are one of the main carbon-based compounds in living organisms
  • All molecules in this group contain C, H and O
  • As H and O atoms are always present in the ratio of 2:1 (eg. water H2O, which is where ‘hydrate’ comes from) they can be represented by the formula Cx (H2O)y
  • The three types of carbohydrates are monosaccharides, disaccharides and polysaccharides

Types of Carbohydrates Table

Types of carbohydrate table, downloadable AS & A Level Biology revision notes

Exam Tip

When discussing monomers and polymers, give the definition but also name specific examples eg. a nucleic acid is a polymer, made of nucleotide monomers.

Remember, lipid molecules are not made from monomers or polymers as each fatty acid joins to a glycerol molecule, rather than to each other. Separate lipid molecules, such as triglycerides, are not held together by covalent bonds and therefore lipids cannot be classed as polymers.

Author: Amelia

While studying Biochemistry at Oxford University, Amelia started her own tutoring service, helping to connect Science tutors with students in her local area. Amelia has experience teaching the sciences and Maths at all levels to UK and international students and, as well as being our Biology Lead, designs revision resources for Chemistry.

Join Save My Exams

Download all our Revision Notes as PDFs

Try a Free Sample of our revision notes as a printable PDF.

Join Now
Already a member?
Go to Top