The Role of Enzymes (Edexcel A (SNAB) A Level Biology)

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Cara Head

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Enzymes as Catalysts

  • 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 undergoing permanent change
    • They speed up reactions by reducing the activation energy of reactions
  • Enzymes are globular proteins with complex tertiary structures
    • Some are formed from a single polypeptide, whilst others are made up of two or more polypeptides and therefore have a quaternary structure
  • 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
  • All enzymes are proteins that are produced via the process of protein synthesis inside cells
  • Some enzymes remain inside cells, whilst others are secreted to work outside of cells
  • 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)

Intracellular and Extracellular Enzymes Table

Enzymes table

Enzymes and the lowering of activation energy

  • All chemical reactions are associated with energy changes
  • For a reaction to proceed there must be enough activation energy
  • Activation energy is the amount of energy needed by the substrate to become just unstable enough for a reaction to occur and for products to be formed
    • Enzymes speed up chemical reactions because they reduce the stability of bonds in the reactants
    • The destabilisation of bonds in the substrate makes it more reactive
  • Rather than lowering the overall energy change of the reaction, enzymes work by providing an alternative energy pathway with a lower activation energy
  • Without enzymes, extremely high temperatures or pressures would be needed to reach the activation energy for many biological reactions
    • Enzymes avoid the need for these extreme conditions (that would otherwise kill cells)

enzymes--activation-energy

The activation energy of a chemical reaction is lowered by the presence of a catalyst (i.e. an enzyme)

Examiner 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.

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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