cpac 1

• Investigate a factor affecting the initial rate of reaction


  1. Determine the effect of changing a single variable on the rate of reaction

  2. Keep all other variables constant

  3. Measure the initial rate of reaction
• Initial rate of reaction

  • Rate of an enzyme-controlled reaction is high because enzymes act as biological catalysts, so concentration of reactants changes rapidly
  • The only point during the reaction when concentration of reactants and products is known
• Factors affecting the rate of reaction of an enzyme-controlled reaction

  • Temperature
  • pH
  • Concentration of the substrate
  • Concentration of the enzyme
• Method


  1. Dilute stock solution of trypsin with distilled water to produce solutions with concentrations of 0.2%, 0.4%, 0.6% and 0.8%

  2. Make a control by adding 2cm3 of trypsin solution and 2cm3 of distilled water, use this to set the colorimeter absorbance to zero

  3. Add 2cm3 of milk suspension and 2cm3 of the stock trypsin solution to a cuvette, mix, place in the colorimeter and measure absorbance at 15 second intervals for 5 minutes

  4. Rinse the cuvette with distilled water, repeat step 3 at all trypsin concentrations
• Milk contains a white protein called casein which, when broken down, causes the milk to turn colourless
• Trypsin is a protease enzyme which hydrolyses the casein
• As concentration of trypsin increases

  The number of enzyme-substrate complexes forming also increases because enzymes and substrates are more likely to collide, so the rate of reaction increases up to the optimum enzyme concentration
• The rate plateaus


  At the point where all substrates occupy an active site, increasing the enzyme concentration won't increase rate as substrate concentration is limiting the rate