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3.1.3Additional lessons

Enzyme Activity Skills — IB Biology SL Revision Notes & Topical Questions

5 min read 6 parts Lesson 52 of 142

📚 IB Biology SL Hub / Topical Revision
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Part 1 of 6Common Investigations

Catalase
  • Catalase Catalase in liver or yeast breaks down hydrogen peroxide, and the oxygen produced can be measured as volume or foam height.
Amylase
  • Amylase Amylase digests starch, and the time taken for starch to disappear is measured with iodine solution.
Independent variable
  • Independent variable The factor changed on purpose, such as temperature in a water bath or pH using buffer solutions.
Dependent variable
  • Dependent variable The measured result, such as volume of oxygen, time to the end point or mass of product formed.
Control variables
  • Control variables Keep the volume and concentration of enzyme and substrate, and the other conditions, the same in every test.

Part 2 of 6Method

1
Prepare buffers and solutions, and bring enzyme and substrate separately to the test temperature before mixing.
2
Mix them and start the stopwatch, then measure the product or sample at fixed intervals.
3
For amylase, test drops of the mixture with iodine until the blue-black colour no longer appears.
4
Repeat each condition several times and calculate the mean to improve reliability.
5
Calculate the rate as 1 ÷ time, or as the change in product per unit time.

Part 3 of 6Evaluating Results

Evaluating Results
Reliability
  • Reliability Repeat results that are close together suggest the data are reliable.
Anomalies
  • Anomalies Identify results far from the others, check how they might have occurred and exclude them with a reason.
Limitations
  • Limitations Temperature may change during the test, and judging the colour end point is subjective.
Safety
  • Safety Wear eye protection, since hydrogen peroxide and iodine solutions can irritate the skin and eyes.

Part 4 of 6Choosing a Method

Choosing a Method
Range
  • Range Choose at least five values of the independent variable spread over a suitable range.
Timing
  • Timing Start the clock as soon as the enzyme and substrate are mixed, and stop at a clear end point.
Presentation
  • Presentation Plot the independent variable on the x axis and the rate on the y axis, with units on both axes.

Part 5 of 6Worked Example

Question

  • Starch disappears in 40 s at 30 °C and in 25 s at 40 °C with amylase.
  • Calculate the rate at each temperature and the percentage increase.

  1. Rate at 30 °C = 1 ÷ 40 = 0.025 s⁻¹.
  2. Rate at 40 °C = 1 ÷ 25 = 0.040 s⁻¹.
  3. Increase = (0.040 − 0.025) ÷ 0.025 × 100 = 60 %.

Answer 0.025 s⁻¹ and 0.040 s⁻¹, an increase of 60 % (example data).

Part 6 of 6Exam Checklist & Tips

3.1.3 – Key Points for the Exam

  • Rate The amount of change per unit time.
  • Buffer A solution that holds pH steady at a set value.
  • Dependent variable The variable that is measured in an experiment.
  • Control A test that keeps all conditions the same except the one being investigated.

Common Mistakes

  • Putting the enzyme and substrate together before they have reached the test temperature.
  • Using time taken as the rate; the rate is the reciprocal, 1 ÷ time.
  • Not repeating results, so one anomaly changes the conclusion.
  • Varying more than one factor at once, which makes the effect unclear.

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