Investigating Enzymes
Build confidence in Investigating Enzymes for GCSE Biology with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. The required practical measures how pH affects the enzyme amylase breaking down starch, timed until iodine no longer turns blue-black.
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Topic overview
This required practical investigates how pH affects the activity of amylase, the enzyme that breaks down starch into sugars.
Amylase and starch are mixed in a buffer solution at a set pH. At regular intervals a drop of the mixture is transferred to iodine on a spotting tile. While starch remains, the iodine turns blue-black; once all the starch has been digested, the iodine stays orange-brown.
The time taken for the iodine to stop turning blue-black is recorded. A shorter time means a faster reaction, so the pH giving the shortest time is the optimum for that enzyme.
Revision notes
The method
Mix amylase, starch and a pH buffer. Every 30 seconds transfer a drop to a spotting tile containing iodine.
Record the time at which the iodine no longer turns blue-black — this is the point at which all the starch has been digested. Repeat at a range of pH values.
Control variables
Keep the temperature constant using a water bath, and keep the volumes and concentrations of amylase and starch the same.
Only pH should change. Without these controls the comparison between pH values is not valid.
Interpreting the results
A shorter time means a faster rate of reaction. The pH giving the shortest time is the optimum.
Rate can be calculated as 1 ÷ time. Plotting rate against pH gives a curve peaking at the optimum, falling away either side as the enzyme is denatured.
Key points
- Iodine turns blue-black while starch is present.
- The end point is when iodine stays orange-brown.
- A shorter time means a faster reaction.
- Rate = 1 ÷ time.
- Control temperature, volumes and concentrations.
- The optimum pH gives the shortest time.
Worked examples
Example 1
Explain how you know when all the starch has been digested. [2 marks]
Model answer
A drop of the mixture is added to iodine on a spotting tile (1 mark). When the iodine no longer turns blue-black and stays orange-brown, all the starch has been broken down (1 mark).
Example 2
A reaction takes 50 seconds. Calculate the rate of reaction. [2 marks]
Model answer
Rate = 1 ÷ time = 1 ÷ 50 (1 mark) = 0.02 per second (1 mark).
Example 3
Name two variables that must be controlled in this investigation and explain why. [3 marks]
Model answer
Temperature must be kept constant (1 mark) and the concentration of amylase must be kept the same (1 mark), because both affect the rate of reaction — if they changed, any difference in the results could not be attributed to pH alone (1 mark).
Common mistakes
Recording the time when iodine first turns blue-black.
The end point is when it STOPS turning blue-black, meaning the starch has gone.
Forgetting to control temperature.
Temperature affects enzyme activity, so it must be held constant for a valid comparison.
Calculating rate as time rather than 1 ÷ time.
A longer time means a slower rate, so rate is the reciprocal.
Not repeating the experiment.
Repeats improve reliability and allow anomalies to be identified.
Exam tips
- Describe the end point precisely — iodine remaining orange-brown.
- Use rate = 1 ÷ time and include the unit per second.
- Name control variables and say why each matters.
- Link the optimum pH back to enzyme shape if asked to explain.
Key terms
- Buffer
- A solution that keeps pH constant.
- Spotting tile
- A tile with wells used for testing small drops.
- End point
- The moment a reaction is judged complete.
- Optimum
- The condition at which an enzyme works fastest.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.