Required Practical: Effect of pH on Enzyme Activity — AQA GCSE Biology
This required practical investigates how pH affects the rate at which the enzyme amylase breaks down starch.
The principle
Amylase breaks down starch into sugars. We can follow the reaction using iodine solution, which is blue-black when starch is present and stays orange/brown once the starch has been broken down.
Method
- Place drops of iodine in the wells of a spotting tile.
- Mix amylase, starch and a buffer solution of a known pH in a test tube, kept at a constant temperature (e.g. 35 °C in a water bath).
- Every 30 seconds, take a sample and add it to a fresh iodine drop.
- Record the time taken for the iodine to stop turning blue-black (i.e. all starch is digested).
- Repeat at different pH values using different buffers.
Analysing results
- The shorter the time, the faster the reaction (rate = 1 ÷ time).
- The pH giving the fastest rate (shortest time) is the enzyme's optimum pH.
Control variables
Keep the temperature, the volumes and concentrations of starch and amylase, and the timing intervals constant — only the pH should change.
Exam tips
- Iodine is the indicator: blue-black = starch present, orange/brown = starch gone.
- Rate = 1 ÷ time; the fastest rate is at the optimum pH.
- The independent variable is pH (changed with buffers); control everything else.
- Explain a slow rate at extreme pH by the enzyme being denatured.