1. Unit 3 Science Investigation Skills
Topic D: Enzymes in action
Lessons 8–10

2. Lessons 8–10 Learning objectives
To identify the factors that affect the rate of enzyme-catalysed reactions.
To explain the effects of these factors.
To investigate the effect of substrate concentration on an enzyme-catalysed reaction.

3. The effects of pH and temperature on proteins
The structure of an enzyme is formed and maintained by various types of interactions between different amino acid R groups.
Changes in pH and temperature can affect these interactions. Irreversible changes cause the enzyme to be denatured.
When an enzyme is denatured the shape of the active site is lost and the substrate can no longer bind. This means the enzyme cannot catalyse the reaction.

4. The effects of temperature on an enzyme
For a chemical reaction to be catalysed by an enzyme, the enzyme and substrate molecules need to collide to form an enzyme-substrate complex. When heated, the molecules will gain kinetic energy, and collide more frequently.
What effect would this have on the rate of reaction?
The rate of reaction would increase as the temperature increased.
Heating an enzyme above a certain temperature can cause hydrogen bonds to break, and also covalent and disulfide bonds at higher temperatures.
What effect could such a rise in temperature have on the structure of an enzyme?
The secondary and tertiary structure could change greatly.
How could high temperatures affect the active site and function of an enzyme?
The active site could denature, making the enzyme unable to catalyse the reaction of the substrate(s).

5. Effect of temperature on enzyme activity
You should be able to explain why an enzyme has an optimum temperature at which its activity is highest, and why its activity is reduced at temperature values above and below the optimum.

6. The effects of pH on an enzyme
Charged R groups will lose their charge at particular pH ranges.
What effect could a change in pH have on bonds within a polypeptide?
Ionic bonds could break due to the R groups losing their opposite charges. Hydrogen bonds are also affected.
What effect could this have on the protein structure?
The secondary and tertiary structure could change greatly.
How could a change in pH affect the active site and function of an enzyme?
The active site could denature, making the enzyme unable to catalyse the reaction of the substrate(s).

7. Effect of pH on enzyme activity
You should be able to explain why an enzyme has an optimum pH at which its activity is highest, and why its activity is reduced at pH values above and below the optimum.

8. Substrate Concentration
In which situation would you expect the rate of reaction to be faster, A or B?
What is the effect of increasing the substrate concentration?
= enzyme =substrate
A B

9. Substrate Concentration
Would you expect the rate of reaction to be faster in D than in C?
= enzyme =substrate
C D

10. Substrate Concentration
In C and D, the rate of reaction will be at the same maximum rate because the enzyme molecules are all occupied. The enzyme is saturated.
Increasing the substrate concentration cannot increase the rate of reaction in C or D.
C D

11. Substrate Concentration
You should be able to explain why the rate of reaction increases initially and why it reaches a maximum.

12. Practical Investigation
You will investigate the effect of substrate concentration on the rate of this reaction catalysed by the potato enzyme catalase:
2H2O2  2H2O + O2
Write a hypothesis to be tested.
Plan a method using the following: hydrogen peroxide solutions of various concentrations; potatoes; flask with bung and delivery tube; apparatus for collecting gas over water; balance; stopwatch.
Include a risk assessment.

13. Lesson 9: Practical investigation
Learning objective:
To carry out an investigation into the effect of substrate concentration on the reaction catalysed by potato catalase.

14. Lesson 10: Presenting results and data analysis
Learning objectives:
To present a suitable table of results.
To plot a suitable line graph, taking any anomalies into account.
To write a conclusion with reference to your hypothesis.
To evaluate your data and suggest improvements to your method.

15. Identifying Anomalous Data
Calculate the means to complete the table.
Any clearly anomalous results should be omitted from your calculation.
Rate of reaction / cm3 min-1
Substrate concentration/%
Run 1
Run 2
Run 3
mean 10 5 6 4 20 13 11 15 30 32 34 12 40 33 35

16. Identifying anomalous data
Check your answers.
Did you spot the anomalous result?
Rate of reaction / cm3 min-1
Substrate concentration/%
Run 1
Run 2
Run 3
mean 10 5 6 4 20 13 11 15 30 32 34 12 33 40 35