CIE A Level Biology (9700) 2019-2021

Revision Notes

3.1.1 Enzymes

Enzymes as Proteins

  • Enzymes are biological catalysts
    • ‘Biological’ because they function in living systems
    • ‘Catalysts’ because they speed up the rate of chemical reactions without being used up or changed
  • Enzymes are also globular proteins
  • Critical to the enzyme’s function is the active site where the substrate binds
  • Metabolic pathways are controlled by enzymes in a biochemical cascade of reactions
    • Virtually every metabolic reaction within living organisms is catalysed by an enzyme – enzymes are therefore essential for life to exist
  • Enzymes can be intracellular or extracellular referring to whether they are active inside or outside the cell respectively
  • Intracellular enzymes are produced and function inside the cell
  • Extracellular enzymes are secreted by cells and catalyse reactions outside cells (eg. digestive enzymes in the gut)

Enzymes table

Enzymes table, downloadable AS & A Level Biology revision notes

Exam Tip

Don’t forget that enzymes are proteins and so anything that could denature a protein, rendering it non-operational (extremes of heat, temperature, pH etc.) would also denature an enzyme.


Alistair graduated from Oxford University in 2014 with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems and Societies.

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