AQA GCSE Biology

Revision Notes

2.2.3 Required Practical: Enzymes

Enzyme Rate

  • Aim: To investigate the effect of pH on the rate of reaction of amylase
  • You will:
    • Use the enzyme amylase to breakdown starch at a range of pH values, using iodine solution as an indicator for the reaction occurring
    • Use a continuous sampling technique to monitor the progress of the reaction
  • Amylase is an enzyme that digests starch (a polysaccharide of glucose) into maltose (a disaccharide of glucose)
  • Starch can be tested for easily using iodine solution

 

Using iodine to test for starch, IGCSE & GCSE Biology revision notes

Iodine can be used qualitatively to indicate the presence or absence of starch from a sample

Method

  • Place single drops of iodine solution in rows on the tile
  • Label a test tube with the pH to be tested
  • Use the syringe to place 2cm3 of amylase in the test tube
  • Add 1cm3 of buffer solution to the test tube using a syringe
  • Use another test tube to add 2cm3 of starch solution to the amylase and buffer solution, start the stopwatch whilst mixing using a pipette
  • After 10 seconds, use a pipette to place one drop of the mixture on the first drop of iodine, which should turn blue-black
  • Wait another 10 seconds and place another drop of the mixture on the second drop of iodine
  • Repeat every 10 seconds until iodine solution remains orange-brown
  • Repeat experiment at different pH values – the less time the iodine solution takes to remain orange-brown, the quicker all the starch has been digested and so the better the enzyme works at that pH

Investigating effect pH on enzyme activity, IGCSE & GCSE Chemistry revision notes

Investigating the effect of pH on enzyme activity

Improvements to method

  • The above method can be adapted to control temperature by using a water bath at 35℃
  • All solutions that need to be used (starch, amylase, pH buffers) 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; with a solution containing starch being darker and glucose light (as a result of the colour-change of iodine) – this will affect the absorbance or transmission of light in a colorimeter

Exam Tip

Describing and explaining experimental results for enzyme experiments is a common type of exam question so make sure you understand what is happening and, for a 7, 8 or 9, can relate this to changes in the active site of the enzyme when it has denatured, or if it is a low temperature, relate it to the amount of kinetic energy the molecules have.

Author: Jenna

Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.
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