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1.4.4Biological Molecules

Required Practical- Measuring Enzyme Activity — AQA A Level Biology 7402 Revision Notes & Topical Questions

5 min read 9 parts Lesson 20 of 49 1 quiz

📚 AQA A Level Biology 7402 Hub / Topical Revision
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Lesson summary

1.4.4 Required Practical- Measuring Enzyme Activity — AQA A Level Biology 7402 notes: Plan and carry out an investigation of the effect of a named variable on the rate of an enzyme-controlled reaction, identifying variables, controls and hazards.The AQA required practical is an i…

  • Catalase and hydrogen peroxide
  • Amylase and starch
  • iodine clearing time
  • Trypsin and milk protein or protease and gelatin
  • Exam-style question 1
  • Exam-style question 2

Section 1 of 9Choosing a method

The AQA required practical is an investigation of the effect of a named variable (such as temperature, pH, enzyme or substrate concentration) on the rate of an enzyme-controlled reaction. The rate must be measurable. Common choices:

Choosing a method
Catalase and hydrogen peroxide
Catalase and hydrogen peroxide measure volume of oxygen released in a gas syringe or by displacement of water,
  • or the height of foam in a tube, over a fixed time.
Amylase and starch
Amylase and starch take samples at intervals and test with iodine solution; record the time for the blue-black colour to disappear (the iodine clearing time); rate = 1/time.
Trypsin and milk protein or protease and gelatin
Trypsin and milk protein or protease and gelatin time taken for opacity to clear.

Section 2 of 9Method outline (amylase and starch, temperature as the independent variable)

1
Place spots of iodine solution in the wells of a white tile.
2
Set up water baths at a range of temperatures (such as 10, 20, 30, 40, 50 °C) and check with a thermometer.
3
Put equal volumes of starch solution (buffered at the same pH) and amylase solution in separate tubes and bring each to the water bath temperature for several minutes before mixing.
4
Mix, start a stopwatch, and every 30 seconds transfer a drop of the mixture to a fresh iodine spot.
5
Record the time at which the iodine stays orange-brown (no starch remains).
6
Repeat each temperature at least three times and calculate the mean time and then rate (1/time).

Section 3 of 9Variables

TypeExample
IndependentTemperature
DependentTime for starch to disappear / rate
ControlledpH (use a buffer), volume and concentration of enzyme and substrate, same source of enzyme, same stirring

Controlling variables makes it a fair test, so that any change in the rate is caused only by the independent variable. If pH were uncontrolled, an effect of temperature could not be separated from an effect of pH.

Section 4 of 9Hazards, risks and ethics

1
Hydrogen peroxide is an irritant: wear eye protection and gloves.
2
Enzymes can cause allergic reactions or irritate the skin, so avoid dust and spills and wash hands.
3
Hot water baths: take care with hot water and electrical equipment, and keep wet hands away from sockets.
4
Iodine stains skin and clothing.

A risk assessment names the hazard, the harm, and a precaution. No live animals are used, but liver or yeast should be handled hygienically and disposed of safely.

Section 5 of 9Improving reliability and validity

Repeats
Repeats and a mean reduce the effect of random error; spot anomalies and exclude them if justified.
Before
Allow solutions to reach temperature before mixing, otherwise the reaction starts at the wrong temperature.
Buffer
Use a buffer to hold pH constant; use a water bath rather than a heating plate for stable temperatures.
Control
Include a control with boiled (denatured) enzyme or water instead of enzyme, which should show no reaction.
Use appropriate apparatus with a suitable resolution (a syringe for small volumes).

Conclusions should be linked to the biology: the rate increases with temperature because of more kinetic energy and collisions, then falls when the enzyme denatures.

Section 6 of 9Writing a plan and analysing results

In exam questions on the required practical, you might be asked to plan a method, identify a source of error or analyse given results.

Writing a plan and analysing results
When planning, state the independent variable and give its range with values; say how the dependent variable is measured; list the controlled variables with how each is kept constant; state the number of repeats;
  • and mention any control.
When identifying a source of error, say which variable it affects and how, such as 'the tube was removed from the water bath while sampling,
  • so temperature fell,
  • giving a lower rate than expected'.

Using yeast or liver as a source of catalase gives variability, since the amount of enzyme in different pieces differs.

1
Using a standard mass of tissue, cut into pieces of the same size, helps.
2
Larger surface area exposes more enzyme, so finely chopped liver reacts faster than a large lump.
3
Enzyme from a purified solution is more consistent.
4
The foam produced by catalase can make volumes hard to read,
  • and a gas syringe or an inverted measuring cylinder over water gives a more accurate measurement.

When interpreting results, quote data and link to enzyme theory. Where there is an unexpected result, such as a sudden low reading in a series, discuss possible causes and whether the repeats show that it is anomalous. Finish with a conclusion that matches the data and is limited to the range tested.

Section 7 of 9Exam-style questions

  1. Describe how you would investigate the effect of pH on the rate of catalase activity. [5]5 marks

  2. In the amylase investigation, the time for starch to disappear at 30 °C was 150 s. Calculate the rate and express as s-1. [2]2 marks

  3. Explain why the enzyme and substrate solutions were placed separately in the water bath before mixing. [2]2 marks

Section 8 of 9Key terms

TermMeaning
Independent variableThe factor that is deliberately changed.
Dependent variableThe factor that is measured as the response.
Control variableA factor kept constant to ensure a fair test.
BufferA solution that resists changes in pH.
AnomalyA result that does not fit the pattern.
ReliabilityHow consistent repeated results are.
ValidityWhether the experiment measures what it claims to.
RateChange in a quantity per unit of time.

Section 9 of 9Exam technique

1
Not controlling pH in a temperature experiment, or temperature in a pH experiment.
2
Starting the reaction before the solutions have reached the water-bath temperature.
3
Using only one repeat, so anomalies cannot be recognised.
4
Reporting 'time' as the rate. Rate is 1/time or amount per unit time.

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