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  • Biological catalyst
  • Is a globular protein
  • Speed up chemical reactions by reducing activation energy
  • Intracellular: 
  1. Enzymes that work inside the cell
  • Extracellular:
  1. Enzymes are secreted outside the cell for example in fungi.
Diagram, schematic

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Lock and key theory:

  • Substrate shape is complementary to the active site of the enzyme and so the enzyme is specific for the substrate. The substate will fit exactly in the active site.

Induced fit theory:

  • Substrate shape is complementary to the active site of the enzyme and is specific, but the active site can change its shape slightly to ensure a perfect fit. This makes catalysis more efficient.

Active Site:

  • Area on the enzyme where the substrate binds.

Lysozyme – an enzyme found in tears, saliva, and other secretions. Defence against bacteria as it breaks the polysaccharide chains in the cell wall.

Enzymes speed up reactions by reducing activation energy.

  • Activation energy: Energy need to make a reaction take place.

Measuring rate of product formation in breakdown of Hydrogen Peroxide:

  • Total volume of oxygen collected in a certain amount of time.
  • Reaction begins quickly, a large volume of product (oxygen) is formed at the start of the reaction.
  • As time passes, the volume produced reduces, this is because most of the substrate has been broken down by the enzyme.

Using a colorimeter:

  • Colorimeter is an instrument that measures the colour of a solution by measuring the absorption of different wavelengths of light.

Factors affecting enzyme action:

 Temperature: Chart

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  • Every enzyme works best at its optimum temperature.
  • Higher the temperature, the kinetic energy of the molecules increases and so more enzymes and substrate collide with each other frequently. So, bonds are broken easily.
  • Above a certain temperature, bonds holding the enzyme molecule break. The active site denatures above this temperature. The substrate no longer fits at all, and the reaction stops.


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  • Some enzymes work good in acidic conditions, some in alkaline conditions.
  • Lower the pH, higher the hydrogen ion concentration. Hydrogen ions interact with charged R groups on the amino acid of enzyme.
  • pH different from optimum pH can cause denaturation of the active site of an enzyme.

Concentration of enzyme:

  • Greater the enzyme concentration, higher the rate of reaction. This is because there’s more collisions between enzymes and the substrate due to the availability of more active sites.
  • A graph of initial rate of reaction vs Enzyme concentration can be plotted.

Concentration of substrate:

  • Initial rate of reaction is high, but it soon decreases.
  • As more substrate is added with constant enzyme concentration, at a point, every active site will be full. The enzyme cannot work faster. The enzyme works at the maximum possible rate Vmax.

Vmax: Theoretical maximum rate of an enzyme-controlled reaction, obtained when all the active sites of the enzyme are occupied.

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Michaelis-Menten constant (Km): The substrate concentration at which an enzyme works at half of its maximum rate (1/2Vmax), used as a measure of the efficiency of an enzyme; the lower the value of Km, the more efficient the enzyme.


                                                        Enzyme inhibition:

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Competitive inhibition: When a substance reduces the rate of activity of an enzyme by competing with the substrate molecules for the enzymes active site. Increasing substrate concentration decreases the degree of inhibition; increasing inhibitor concentration increases the degree of inhibition.

Non- competitive: When a substance reduces the rate of activity of an enzyme but increasing the concentration of the substrate does not reduce the degree of inhibition; many non-competitive inhibitors bind to areas of the enzyme molecule other than the active site.

Immobilising enzymes:

Advantages –

  1. Enzyme can be reused.
  2. Enzymes are more tolerant of temperature and pH changes.
  3. Final product will be enzyme free.


  • Mix enzyme with sodium alginate solution.
  • Using a syringe, add this mixture drop by drop in calcium chloride solution.
  • Sodium alginate and calcium chloride react to form a bead.
  • The enzyme is immobilised inside the bead.
  • Pack beads int a column and add the liquid containing enzymes substrate.
  • As substrate runs over the beads, enzymes convert the substrate into the product.
  • Product is the collected and enzyme beads are reused.

Note: All images and some text have been adapted from Cambridge AS and A level Biology Coursebook.