Biology 9700/34 — May/June 2021

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 · Manipulation, Measurement and Observation · Analysis, Conclusions and Evaluation · Use of the Light Microscope

Q1Presentation of Data and ObservationsManipulation, Measurement and ObservationAnalysis, Conclusions and EvaluationFree sample

Yeast cells contain the enzyme catalase which catalyses the breakdown of hydrogen peroxide, releasing oxygen.

When a mixture of hydrogen peroxide and yeast cells is put into a syringe, some of the mixture comes out of the syringe nozzle as the oxygen is released.

You are going to investigate the effect of pH on the activity of catalase.

You are provided with the materials shown in Table 1.1.

Table 1.1

labelled	contents	hazard	volume / $\text{cm}^3$
H	3% hydrogen peroxide solution	irritant	25
Y	yeast cell suspension	none	40
B3	buffer pH 3	none	10
B4	buffer pH 4	none	10
B5	buffer pH 5	none	10
B6	buffer pH 6	none	10
B7	buffer pH 7	none	10
U	unknown pH buffer	none	10

If any of the solutions come into contact with your skin, wash off immediately with cold water.

It is recommended that you wear suitable eye protection.

Wear gloves to protect your hands when using H.

Carry out step 1 to step 22.

Put clear plastic tubing onto the nozzle of a $10\ \text{cm}^3$ syringe, as shown in Fig. 1.1.
Put two marks on the (plastic) tubing so that one mark is at a distance of $5\ \text{cm}$ from the nozzle and the other mark is at a distance of $15\ \text{cm}$ from the nozzle, as shown in Fig. 1.1.

Fig. 1.1

Remove the tubing from the nozzle of the syringe.
Use the glass rod to stir the yeast cell suspension, Y.
Put the nozzle of the syringe into the beaker containing Y.
Pull the plunger out to the $3\ \text{cm}^3$ mark so that Y enters the syringe, as shown in Fig. 1.2.

Fig. 1.2

Remove the syringe from the beaker containing Y and wipe the nozzle with a paper towel.
Put the nozzle of the same syringe into the beaker containing the pH 3 buffer, B3.
Pull the plunger out to the $5\ \text{cm}^3$ mark so that approximately $2\ \text{cm}^3$ of the buffer enters the syringe, as shown in Fig. 1.3.

Fig. 1.3

Remove the syringe from the beaker containing pH buffer and wipe the nozzle with a paper towel.
Put the nozzle of the same syringe into the beaker containing hydrogen peroxide solution, H.
Pull the plunger out to the $6\ \text{cm}^3$ mark so that approximately $1\ \text{cm}^3$ of H enters the syringe, as shown in Fig. 1.4.

Fig. 1.4

Remove the syringe from the beaker containing hydrogen peroxide solution, H, and carefully wipe the nozzle with a paper towel to remove excess H.
Put the tubing onto the nozzle.
Put the syringe and tubing on a flat surface, as shown in Fig. 1.5.

Fig. 1.5

Start timing when the mixture reaches the mark at $5\ \text{cm}$ .
Stop timing when the mixture reaches the mark at $15\ \text{cm}$ . Record the time in (a)(i).

If the mixture does not reach the $5\ \text{cm}$ mark or if it takes more than 4 minutes to reach the $15\ \text{cm}$ mark, record as 'more than 240'.

Hold the syringe with the nozzle up and remove the tubing from the syringe. Put the tubing onto a paper towel.
Hold the syringe with the nozzle pointing downwards over the container labelled 'For waste' and empty the syringe.
Fill the syringe with water from the container labelled 'For washing'. Put the tubing onto the syringe nozzle and empty the syringe into the container labelled 'For waste'.
Repeat step 3 to step 20 with each of the other pH buffers, B4, B5, B6 and B7.

(a)

(i)

Record your results in an appropriate table.

(ii)

State the dependent variable for this investigation.

(iii)

Suggest an appropriate control for this investigation.

(iv)

Repeat step 3 to step 20 with the unknown pH buffer, U. Record the time in (a)(iv).

State the time for U. ______

Using your results from (a)(i) estimate the pH of U. ______

(v)

Identify one source of error in step 4 to step 17.

Suggest an improvement to the method which will reduce the effect of this error.

error ______

improvement ______

(vi)

The procedure described by step 1 to step 21 investigated the effect of pH on catalase activity, by measuring the time taken for the mixture to move a set distance.

Describe how you would modify the procedure to investigate the effect of changing the concentration of hydrogen peroxide on the time taken for the mixture to move a set distance.

(b)

A student investigated the effect of temperature on the activity of catalase, by measuring the concentration of hydrogen peroxide remaining in the mixture after a set time.

The student used potassium manganate(VII) solution to determine the concentration of hydrogen peroxide remaining in the mixture.

The greater the volume of potassium manganate(VII) used, the higher the concentration of hydrogen peroxide remaining.

The results are shown in Table 1.2.

Table 1.2

temperature / $^\circ\text{C}$	volume of potassium manganate(VII) used / $\text{cm}^3$
10.0	11.0
23.0	7.0
32.0	4.5
38.5	3.2
49.0	7.4
58.5	10.8

(i)

Plot a graph of the data in Table 1.2 on the grid in Fig. 1.6.

Use a sharp pencil for drawing graphs.

Fig. 1.6

(ii)

Use your graph to find the volume of potassium manganate(VII) needed when the temperature was $45\ ^\circ\text{C}$ .

volume of potassium manganate(VII) = ______ $\text{cm}^3$

(iii)

Suggest an explanation for the difference in concentration of hydrogen peroxide remaining at $38.5\ ^\circ\text{C}$ and at $58.5\ ^\circ\text{C}$ .

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