Edexcel IGCSE Biology 复习笔记 2.3.4 Practical: Enzymes & Temperature

• Amylase is an enzyme that digests starch (a polysaccharide of glucose) into maltose (a disaccharide of glucose)
• The effect of temperature on the activity of amylase can be investigated 

Apparatus

• Spotting tile
• Measuring cylinder
• Test tube
• Syringe
• Pipette
• Stopwatch
• Water
• Thermometer
• Water bath
• Iodine
• Starch solution
• Amylase solution

Method

• Add 5cm³ starch solution to a test tube and heat to a set temperature using beaker of water with a Bunsen burner
• Add a drop of Iodine to each of the wells of a spotting tile
• Use a syringe to add 2cm³ amylase to the starch solution and mix well
• Every minute, transfer a droplet of solution to a new well of iodine solution (which should turn blue-black)
• Repeat this transfer process until the iodine solution stops turning blue-black (this means the amylase has broken down all the starch)
• Record the time taken for the reaction to be completed
• Repeat the investigation for a range of temperatures (from 20°C to 60°C)

Investigating the effect of temperature on enzyme activity

Results and Analysis

• Amylase is an enzyme which breaks down starch
• The quicker the reaction is completed, the faster the enzyme is working
• This investigation shows:
  • At the optimum temperature, the iodine stopped turning blue-black the fastest
    • This is because the enzyme is working at its fastest rate and has digested all the starch

     

  • At colder temperatures (below optimum), the iodine took a longer time to stop turning blue-black
    • This is because the amylase enzyme is working slowly due to low kinetic energy and few collisions between the amylase and the starch

     

  • At hotter temperatures (above optimum) the iodine turned blue-black throughout the whole investigation
    • This is because the amylase enzyme has become denatured and so can no longer bind with the starch or break it down

     

   

Limitations

• The method used to control the temperature, described above, is not very precise, an improvement would be to use water baths kept at each temperature
• The starch and amylase solutions that need to be used should be placed in a water bath and allowed to reach the temperature (using a thermometer to check) before being used
• A colorimeter can be used to measure the progress of the reaction more accurately
  • A solution containing starch will be darker than a solution containing glucose (as a result of the colour change of iodine)
  • The absorbance or transmission of light through the coloured solution can be measured using a colorimeter

   

Applying CORMS to practical work

• When working with practical investigations, remember to consider your CORMS evaluation

CORMS evaluation

• In this investigation, your evaluation should look something like this:
  • C - We are changing the temperature in each repeat
  • O - This is not relevant to this investigation as we aren't using an organism
  • R - We will repeat the investigation several times to make sure our results are reliable
  • M1 - We will measure the time taken
  • M2 - for the iodine to stop turning black
  • S - We will control the concentration and volume of starch solution, iodine and amylase used in the investigation