OCR A Level Biology:复习笔记2.4.1 The Role of Enzymes

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


  • 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


Site of action of enzymes

  • 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 therefore 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

  • As well as amylase (which hydrolyses starch into maltose), another example of an extracellular digestive enzyme that is secreted by the pancreas and enters the small intestine is trypsin, which breaks proteins down into peptides and amino acids
  • In fact, some organisms can only feed using a form of extracellular digestion in which the digestive enzymes are actually secreted outside of their bodies
    • For example, the hyphae of fungi secrete the necessary enzymes directly onto the food they are consuming (e.g. wood) so that the food is digested into smaller, simple molecules that the fungi can then absorb through the walls of the hyphae


Exam Tip

Don't forget that enzymes are proteins, meaning that anything that could denature a protein and make it non-operational (such as extremes of heat, temperature, pH etc.) would also denature an enzyme. This is one reason why homeostasis (the maintaining of internal body conditions) is so important: to ensure that enzymes within the body can function properly, as they are essential to the survival of organisms.