Biology 9700/33 — May/June 2022

Cambridge AS Level · Advanced Practical Skills · worked solutions for every part, with the mark scheme

questions

marks

120

minutes

Topics Presentation of Data and Observations · Analysis, Conclusions and Evaluation · Manipulation, Measurement and Observation · Use of the Light Microscope

Q1Manipulation, Measurement and ObservationPresentation of Data and ObservationsAnalysis, Conclusions and EvaluationFree sample

Plant cells contain an enzyme which catalyses the breakdown of hydrogen peroxide, releasing oxygen, as shown in Fig. 1.1.

Fig. 1.1

You will investigate the effect of pH on the activity of this enzyme.

You are provided with the materials shown in Table 1.1.

Table 1.1

labelled	contents	hazard	volume / $\text{cm}^3$
H	hydrogen peroxide solution	harmful irritant	130
E	enzyme solution	harmful irritant	20
B5	pH5 buffer	none	20
B6	pH6 buffer	none	20
B7	pH7 buffer	none	20
B8	pH8 buffer	none	20
B9.5	pH9.5 buffer	none	20

If any solution comes into contact with your skin, wash off immediately with cold water.

It is recommended that you wear suitable eye protection.

When discs of filter paper that have been soaked in E are put into hydrogen peroxide solution, H, the discs rise to the surface as H is broken down. The higher the activity of E, the faster the discs rise.

(a)

(i)

Explain why the discs rise.

(ii)

A student investigated the effect of pH on the activity of enzyme E.

The student suggested the hypothesis:

As the pH of the buffer containing E increases, the activity of E decreases.

Carry out step 1 to step 16 to test the student’s hypothesis.

step 1 Label 5 beakers with E5, E6, E7, E8 and E9.5.

step 2 Put $10\,\text{cm}^3$ of B5 buffer solution into the beaker labelled E5.

step 3 Repeat step 2 by putting the appropriate buffer solution into the beakers labelled E6, E7, E8 and E9.5.

step 4 Stir E and put $2\,\text{cm}^3$ of E into each of the labelled beakers used in step 2 and step 3. Swirl each beaker to mix the contents.

step 5 Leave the beakers for 2 minutes.

step 6 Label a small beaker H and put $60\,\text{cm}^3$ of solution H into this beaker.

step 7 After 2 minutes (step 5) stir E9.5.

step 8 Pick up one disc of filter paper using forceps.

step 9 Continue to hold the disc in the forceps and:

dip the disc in E9.5 for 5 seconds
put the disc at the bottom of the liquid in the beaker labelled H.

step 10 Immediately release the disc from the forceps and start timing.

step 11 Stop timing when the disc reaches the surface of the liquid. You may find it helps to observe the disc with a piece of black card behind the beaker.

step 12 Record in (a)(ii) the time taken for the disc to reach the surface of H. If the time taken for the disc to rise to the surface is longer than 180 seconds, record as ‘more than 180’.

step 13 Remove the disc from the beaker using the forceps and put it on a paper towel.

step 14 Dip the forceps in the water in the container labelled For washing. Dry the forceps with a paper towel.

step 15 Repeat step 7 to step 14 two more times, using E9.5.

step 16 Repeat step 7 to step 15 using E8, E7, E6 and E5.

Record your results in an appropriate table, including raw results and processed (mean) results.

(iii)

The student suggested the hypothesis:

As the pH of the buffer containing E increases, the activity of E decreases.

Tick (✓) one box to show whether or not your processed results support this hypothesis.

support [ ]
do not support [ ]

Explain how the trend in your processed results provides evidence for this decision.

(iv)

State the independent variable in this investigation.

(v)

Describe an appropriate control for this investigation.

(vi)

One possible source of error in this investigation is reusing the same hydrogen peroxide solution for all the discs.

Suggest a reason why reusing the same hydrogen peroxide solution is a possible source of error.

State how you could reduce this source of error.

(vii)

Describe how you could modify your procedure to determine the optimum pH for this enzyme.

(b)

Temperature is another factor that affects the activity of an enzyme.

You will show the effect of preheating the enzyme at two different temperatures, before measuring the activity of this enzyme. You will select two temperatures and standardise the pH by choosing an appropriate buffer.

(i)

Measure the room temperature.

room temperature = ______

(ii)

Select two temperatures between room temperature and $50^{\circ}\text{C}$ .

State the temperatures you have selected.

(iii)

step 17 Prepare a water-bath at the lowest temperature you stated in (b)(ii).

You will need to standardise the pH by choosing an appropriate mixture of buffer and enzyme, E5, E6, E7, E8 or E9.5.

State the mixture of buffer and enzyme you will use.

(iv)

step 18 Label one test-tube T1 and label another test-tube T2.

step 19 Stir the mixture of buffer and E you chose in (b)(iii) and put approximately $5\,\text{cm}^3$ into each of the test-tubes T1 and T2.

step 20 Put test-tube T1 into the water-bath and leave it for 5 minutes.

step 21 Empty the small beaker labelled H into the container labelled For waste.

step 22 Put $60\,\text{cm}^3$ of H into the small beaker labelled H.

step 23 After 5 minutes (step 20) pour the mixture from test-tube T1 into a clean beaker.

step 24 Pick up one disc of filter paper using forceps.

step 25 Continue to hold the disc in the forceps and:

dip the disc in the mixture in the beaker for 5 seconds
put the disc at the bottom of the liquid in the beaker labelled H.

step 26 Immediately release the disc from the forceps and start timing.

step 27 Stop timing when the disc reaches the surface of the liquid. You may find it helps to observe the disc with a piece of black card behind the beaker.

step 28 Record in (b)(iv) the time taken for the disc to reach the surface of H. If the time taken for the disc to rise back to the surface is longer than 180 seconds, record as ‘more than 180’.

step 29 Remove the disc using the forceps and put it on a paper towel.

step 30 Dip the forceps in the water in the beaker labelled For washing. Dry the forceps with a paper towel.

step 31 Heat the water-bath to the other temperature you stated in (b)(ii).

step 32 Put test-tube T2 into the water-bath and leave it for 5 minutes.

step 33 After 5 minutes (step 32) pour the mixture from test-tube T2 into a clean beaker.

step 34 Repeat step 24 to step 28.

Record your results for T1 and T2.

T1 ______
T2 ______

(v)

Explain the effect of increased temperature on the activity of enzyme E.

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Q2Use of the Light Microscope · Presentation of Data and Observations · Analysis, Conclusions and Evaluation21M

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