Required practical 1: rate of an enzyme-controlled reaction - AQA A-Level Biology (Paper 3) Exam Questions

Practise 17 exam-style questions on Required practical 1: rate of an enzyme-controlled reaction for AQA A-Level Biology (Paper 3). Write your answer as you would in the exam and get it marked in seconds against the mark scheme, point by point, with the Study Helper on hand if you get stuck.

17 questions
1–8 marks each
Instant marking
No sign-up needed

17 questions, marked as you go

Sample questions with model answers

Here are 4 of the questions in this topic, each with a model answer, an explanation of why it earns the marks and the mark scheme an examiner would use.

Question 1

2 marks

A teacher planned a class investigation into the effect of pH on the rate of starch hydrolysis by amylase. Some students suggested using amylase from their own saliva. The teacher decided to use a commercial amylase solution of known concentration instead.

Suggest two reasons for the teacher's decision.

Model answer

1. The concentration of amylase in saliva differs between people and at different times, so the enzyme concentration would not be controlled and results from different groups could not be compared.
2. Saliva contains other substances, such as hydrogencarbonate ions, that could change the pH of the reaction mixture.

Explanation

A valid investigation of pH must keep enzyme concentration constant and change only the pH. Saliva is a variable mixture: its amylase concentration and its buffering ions differ from person to person, and handling other people's body fluids is a hygiene risk. A commercial enzyme of known concentration gives repeatable, comparable results.

Mark scheme

1 mark
The concentration/activity of amylase in saliva varies between people / at different times, so the enzyme concentration would not be controlled (or results could not be compared/repeated)
Max 2. One mark for each reason. List rule: if more than two reasons are given, each wrong or contradictory extra answer cancels one correct answer. Accept "you would not know how much amylase was in each sample".
1 mark
Saliva contains other substances that could affect the reaction or the pH, e.g. hydrogencarbonate ions/buffers, chloride ions, food particles, mucus or other enzymes
Accept "saliva has its own pH/buffers, so the pH of the buffer could be changed".
1 mark
Risk of infection / saliva (a body fluid) may contain pathogens that could be passed to others
Accept "hygiene risk when saliva samples are handled or spilled".
1 mark
Ethical reason: students may not want to give saliva / would need to give consent (and should not be pressured)
Ignore cost or time.

Question 2

3 marks

A student made a serial dilution of a solution of a protease from a fungus. The stock solution had a concentration of 4.0 g dm⁻³.

She transferred 1.0 cm³ of the stock solution into a tube containing 9.0 cm³ of pH 8 buffer and mixed it (tube 1). She transferred 1.0 cm³ from tube 1 into 9.0 cm³ of buffer and mixed it (tube 2), and made tube 3 from tube 2 in the same way.

To start a reaction, she added 2.0 cm³ of the solution from tube 3 to 8.0 cm³ of a suspension of milk protein in pH 8 buffer.

Calculate the concentration of protease in this reaction mixture. Give your answer in g dm⁻³, in standard form.

Explain why she diluted the protease with buffer rather than with distilled water.

Model answer

Each tube is a 1 in 10 dilution of the one before.
Tube 1 = 4.0 ÷ 10 = 0.40 g dm⁻³; tube 2 = 0.040 g dm⁻³; tube 3 = 0.0040 g dm⁻³
In the reaction mixture, 2.0 cm³ of tube 3 is in a total volume of 2.0 + 8.0 = 10.0 cm³:
0.0040 × 2.0 ÷ 10.0 = 0.00080 g dm⁻³ = 8.0 × 10⁻⁴ g dm⁻³

Diluting with buffer keeps the pH the same in every tube, so the only variable that changes between tubes is the concentration of protease.

Explanation

In this serial dilution 1.0 cm³ goes into a total of 10.0 cm³, so each step divides the concentration by 10. Adding the enzyme to the substrate dilutes it again: 2.0 cm³ in a total of 10.0 cm³ divides it by 5, and it is the concentration in the reaction mixture that affects the rate. A common error is to divide by the volume of substrate (8.0 cm³) rather than by the total volume. Diluting with buffer rather than water means that pH, which affects enzyme activity, is a controlled variable.

Mark scheme

1 mark
Correct method: concentration in tube 3 = 4.0 ÷ 1000 = 4.0 × 10⁻³ g dm⁻³ AND this is diluted 2.0 in 10.0 (× 2.0 ÷ 10.0) in the reaction mixture; or a correct final answer of 8.0 × 10⁻⁴ with no working
A correct answer of 8.0 × 10⁻⁴ g dm⁻³ (accept 8 × 10⁻⁴) with no working scores 2 marks (points 1 and 2). Accept for 1 mark: 4.0 × 10⁻³ (the concentration in tube 3, with the dilution on mixing ignored) OR 1.0 × 10⁻³ (multiplied by 2.0 ÷ 8.0 instead of 2.0 ÷ 10.0) OR 0.0008 or 0.00080 (correct value, not in standard form) OR 8.0 × 10⁻³ or 8.0 × 10⁻⁵ (one serial dilution too few or too many). Do not award this point for a two-fold (halving) dilution at each step.
1 mark
8.0 × 10⁻⁴ (g dm⁻³)
Accept 8 × 10⁻⁴. Do not accept 0.0008 or 0.00080 for this mark (not in standard form).
1 mark
So the pH is the same in every tube / buffer keeps the pH constant, so only the protease concentration changes
Accept "diluting with water would change the pH (by different amounts in each tube)". Insufficient: "buffer is the optimum pH" alone.

Question 3

4 marks

A student investigated the effect of pH on the activity of an amylase produced by a fungus. Her method was:

1. Cut four wells, each 5 mm in diameter, in one Petri dish of starch agar.
2. Fill each well with amylase solution mixed with a buffer of pH 4, 5, 6 or 7.
3. Leave the dish on the laboratory bench for 24 hours.
4. Pour iodine solution over the agar and measure the diameter of the clear zone around each well with a ruler.

Suggest two improvements to this method. For each improvement, explain why it would improve the investigation.

Model answer

1. Use a separate Petri dish for each pH. The buffers and the enzyme diffuse through the agar, so with all four wells in one dish the pH around each well would be affected by the neighbouring wells.
2. Incubate the dishes at a constant temperature, for example 25 °C in an incubator. Temperature affects the rate of the reaction, and the temperature on a laboratory bench changes during 24 hours.

Explanation

Agar is a solid, but solutes still diffuse through it, so wells close together on one plate do not stay at separate pH values. Other improvements are keeping temperature constant, using the same volume of enzyme in every well, repeating each pH, measuring two diameters at right angles and adding a boiled-enzyme control. Each improvement must be explained by saying what it controls or what error it reduces; "to make it more reliable" on its own does not score.

Mark scheme

1 mark
Use a separate dish for each pH / put the wells for different pH values far apart
Max 4. List rule: if more than two improvements are given, each wrong or contradictory extra improvement cancels one correct improvement (with its explanation). Each improvement scores 1 mark and its explanation scores 1 mark; an explanation can score only if its improvement has scored (dependent points). Two other improvements may be credited in the same way, in place of one of the pairs below: measuring two diameters at right angles and taking a mean, or measuring the area (1), because clear zones may not be circular (1); OR a control well of boiled amylase in buffer (1), to show the clear zone is caused by amylase activity and not by the buffer alone (1). Do not credit "repeat it" without saying what is repeated.
1 mark
Buffers/enzyme diffuse through the agar, so the pH around a well is changed by the neighbouring wells / clear zones may overlap
1 mark
Incubate at a constant (stated) temperature, e.g. in an incubator at 25 °C
Accept "use a water bath/incubator" for the improvement.
1 mark
Temperature affects the rate of reaction (and of diffusion), and the temperature on the bench varies
1 mark
Use (at least) three wells/dishes at each pH and calculate a mean diameter
1 mark
To identify anomalous results / to reduce the effect of random errors / to check the results are repeatable
Insufficient: "to make it reliable" or "more accurate" alone.
1 mark
Put the same volume of amylase-buffer mixture in each well, measured with a pipette/syringe
Accept "same concentration and volume of amylase".
1 mark
The quantity of enzyme affects the size of the clear zone / filling each well by eye gives different volumes

Question 4

6 marks

Catalase catalyses the breakdown of hydrogen peroxide into water and oxygen. When a small disc of filter paper soaked in catalase solution is placed at the bottom of a tube of hydrogen peroxide solution, oxygen forms on the disc and the disc rises to the surface.

You are given:
- a catalase extract, made by blending celery with water and filtering (call this 100% extract)
- distilled water
- hydrogen peroxide solution in a pH 7 buffer
- filter paper and a hole punch
- forceps, boiling tubes, syringes, a ruler, a stopwatch and a water bath.

Describe how you would use these to investigate the effect of catalase concentration on the rate of this reaction.

Model answer

Make a range of catalase concentrations by diluting the extract with distilled water: for example 2, 4, 6, 8 and 10 cm³ of extract each made up to 10 cm³ with water, giving 20%, 40%, 60%, 80% and 100%.
Use the hole punch to cut discs of the same size from the same filter paper.
Put the same volume of hydrogen peroxide solution in pH 7 buffer into each boiling tube, so the depth is the same, and stand the tubes and the extracts in a water bath at 25 °C until they reach that temperature.
Using forceps, soak a disc in one extract for 10 seconds, drain it for 5 seconds, then push it to the bottom of a tube and time how long it takes to rise to the surface.
Repeat three times at each concentration, ignore any anomalous times and calculate the mean time. Calculate the rate as 1 ÷ mean time.
As a control, use a disc soaked in boiled extract, which should not rise.

Explanation

A good method states how the independent variable is changed (a dilution series), what is measured (time for the disc to rise) and how it becomes a rate (1 ÷ time), and keeps everything else the same: disc size, soaking time, volume and concentration of hydrogen peroxide, depth of liquid and temperature. Repeats allow anomalies to be identified, and a boiled-extract control shows the oxygen comes from catalase activity. A common error is to describe changing the hydrogen peroxide concentration instead of the catalase concentration.

Mark scheme

1 mark
Dilute the extract with distilled water to make a range of (at least five) concentrations, e.g. 20%, 40%, 60%, 80% and 100%
Max 4 if the answer does not describe how different concentrations of catalase are made. Max 6. Accept a correct set of volumes, e.g. 2 cm³ extract + 8 cm³ water for 20%. Accept a serial dilution, e.g. 100%, 50%, 25%, 12.5%.
1 mark
Cut discs of the same size (with the hole punch) from the same filter paper
1 mark
Soak each disc in the extract for the same time (using forceps) / drain the discs for the same time
Accept "same volume of extract on each disc".
1 mark
Use the same volume/depth and concentration of hydrogen peroxide solution in each tube
Insufficient: "same amount of hydrogen peroxide".
1 mark
Place the disc at the bottom of the tube and measure the time for it to rise to the surface
Accept "time from dropping the disc in until it reaches the top" if the depth of liquid is kept the same.
1 mark
Keep the temperature constant by standing the tubes (and extracts) in a water bath (at a stated temperature) before starting
Accept "leave the solutions in the water bath to reach the temperature first".
1 mark
Repeat (at least three times) at each concentration and calculate a mean time
Accept "repeat and exclude anomalies before calculating a mean".
1 mark
Calculate the rate as 1 ÷ (mean) time (or 1000 ÷ time)
Accept "rate = distance risen ÷ time".
1 mark
Control: a disc soaked in boiled extract (or in distilled water) does not rise, showing the disc rises because of catalase activity
Insufficient: "use a control" with no description.

Ready to try the rest? Every answer you write is marked against the mark scheme, so you can see exactly where the marks are.

Study Helper Hints first, answers after you try

Stuck on this one? Ask for a hint and I’ll point you in the right direction, without giving the answer away. Once you’ve checked your answer, I’ll show you how the marks are awarded.