2.2 Applications and skills

2.2.5 Practical 3: Factors affecting enzyme Activity

  • This page clarifies definitions of experimental terms and gives you some tips on how to design experiments. This is very useful for your IA.
  • Use these tips to help you design a lab to investigate how enzyme activity is affected by temperature, pH or substrate concentration, and then perform it (Practical 3).

Experimental variables

Figure 2.2.5a – Testing the effect of pH on catalase activityFigure 2.2.5a – Testing the effect of pH on catalase activity

  • In the example shown in Figure 2.2.5a, a student set up test tubes containing solutions of different pHs, and used a sensor to measure changes in oxygen gas pressure during the reaction of hydrogen peroxide by the enzyme catalase.
  • In this example, the independent variable is pH and the dependent variable is gas pressure.

Control variables

  • The goal of a scientific experiment is to test the effect of one factor (the independent variable) on the outcomes of another (the dependent variable).
  • All the other known factors that can influence the outcome of the test are control variables.
  • For the student testing the effect of pH on catalase activity, temperature and substrate concentration are two control variables.
  • The student needs to make sure that the temperature and substrate concentration in each test tube are the same.
  • Other known factors may influence the experimental results. These should be controlled as well.
  • Sometimes, a test is performed to see if unknown factors influence an experiment. This is called an experimental control or a control test. For example, the student may choose to measure the changes in gas pressure when there is no catalase in the test tube, so that she can determine if her instruments are working properly.

Reliability and validity of data

Figure 2.2.5b – Dartboard analogy: accuracy, precision, reliability and validityFigure 2.2.5b – Dartboard analogy: accuracy, precision, reliability and validity

  • Accuracy refers to how close to the true value a measurement is. If your measurements are accurate, the mean of all the measurements is very close to the ‘correct’ value.
  • Precision refers to how consistent a measurement is. This means that the range, or error of measurements, is small.
  • Validity refers to whether or not your results correspond to the real world. Sometimes you take very precise measurements but your results show something that is physically impossible, or that simply doesn’t make sense scientifically. This means your experiment is invalid.
  • Reliability refers to the level to which your experiment can be repeated to get the same results.
  • Well-designed experiments yield results that are both valid (accurate) and reliable (precise).

Activity 1: Thinking about experimental design

  • A student performed an experiment to determine the effect of temperature on cellular respiration in yeast, Saccharomyces cerevisiae.
  • This is her research question:

How does temperature affect the rate of cellular respiration in yeast, Saccharomyces cerevisiae?

  • Cellular respiration is an enzymatic process that converts sugar into carbon dioxide. Carbon dioxide makes dough rise.
  • The student measured the volume of rising dough over a period of 20 minutes using 100ml size graduated cylinders in water baths at different temperatures. Here are her unprocessed results:

 

Volume at different temperatures (cm3) (+/- 0.5cm3)

Time (min)

5°C

22°C

37°C

70°C

0

20

20

20

20

20

20

19

20

20

20

19

19

2

20

21

21

21

22

22

23

24

25

21

19

20

4

22

22

22

24

24

25

28

31

30

22

20

21

6

22

23

23

28

30

30

40

39

40

22

21

22

8

23

25

25

33

36

35

50

49

50

22

22

22

10

25

26

25

38

42

32

60

58

60

23

 

23

12

26

27

27

46

48

47

68

70

70

23

22

23

14

27

28

28

50

54

55

75

73

75

23

22

23

16

28

30

29

58

60

60

75

76

78

23

22

24

18

29

30

29

67

68

68

 

86

85

24

23

24

20

29

31

30

40

73

75

100

100

100

24

23

24

  1. Identify the independent and dependent variables in the student’s experiment.
  2. Has she collected sufficient data? Why or why not?
  3. Suggest a reason for the missing data points and for the anomalous data point highlighted in yellow.
  4. Make a list of all the control variables in this experiment.

Activity 2: Designing a controlled method

  1. Why did the student choose those specific temperatures to test in her method (5°C, 22°C, 37°C and 70°C)?
  2. Do you think she has controlled some variables? Which?
  3. Write a controlled step-by-step method that would lead to you replicating the results of the student’s experiments. Print the Lab Planning Checklist to help you.
  4. Compare your method with that of  a classmate. Which of the methods:
  • yields more reliable results
  • uses laboratory time more efficiently?

Figure 2.2.5c – Electronic balanceFigure 2.2.5c – Electronic balance
The measurement accuracy of this electronic balance is +/- 0.01g.

Key concept

Accurate, quantitative measurements in enzyme experiments require replicates to ensure reliability.

Figure 2.2.5d – CylinderFigure 2.2.5d – Graduated cylinder
The measurement accuracy of this graduated cylinder is +/- 0.5 cm3.

Course links

In Section 2.1.5 you are asked to design experiments to test the effect of one factor on the rate of enzymatic activity. You may also design experiments to:

  • test the effect of limiting factors on photosynthesis 2.1.9
  • test factors that affect stem root cuttings 3.2.5
  • HL: test factors that affect transpiration rates 9.2.2 and germination rates 9.2.3.