Biology 9700/31 — May/June 2020

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

questions

marks

120

minutes

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

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

Invertase is an enzyme found in honey.

Invertase breaks down sucrose into reducing sugars, as shown in Fig. 1.1.

Fig. 1.1

You will carry out an investigation to determine the concentration of invertase in a sample of honey.

The presence of the reducing sugars can be detected by using Benedict’s solution. The time taken for the Benedict’s solution to first show a colour change can be used to estimate the concentration of invertase in the honey extract, H.

You will need to prepare a serial dilution of 1.0% invertase solution, E.

You are provided with the materials shown in Table 1.1.

Table 1.1

labelled	contents	hazard	volume / $\text{cm}^3$
H	honey extract	none	10
E	1.0% invertase solution	harmful irritant	10
B	Benedict’s solution	harmful irritant	30
S	5% sucrose solution	none	25
W	distilled water	none	150

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

It is recommended that you wear suitable eye protection.

(a)

(i)

Think about the hazards of using the invertase solution, E, as shown in Table 1.1.

State whether the risk of using the invertase solution, E, is low, medium or high. Give a reason for your answer.

risk ______

reason ______

(ii)

You need to carry out a serial dilution of the invertase solution, E, to reduce the concentration of invertase by a factor of 10 between each successive dilution.

Fig. 1.2 shows how to prepare the 1.0% and 0.1% concentrations of invertase solution.

Complete Fig. 1.2 by drawing as many extra beakers as you need for your serial dilution of invertase solution.

For each beaker:

state, under the beaker, the volume and concentration of invertase solution available for use in the investigation
use one arrow with a label, above the beaker, to show the volume and concentration of invertase solution added to prepare the concentration
use another arrow with a label, above the beaker, to show the volume of W added to prepare the concentration.

Fig. 1.2

(iii)

Carry out step 1 to step 14 to determine the concentration of invertase in the honey extract, H.

Prepare the concentrations of invertase solution as you decided in (a)(ii) and shown in Fig. 1.2. Use a glass rod to mix the invertase solutions and water.
Set up a water-bath and heat the water to approximately $30^{\circ}\text{C}$ .
Label test-tubes with the concentrations of invertase solution prepared in step 1.
Put $2\,\text{cm}^3$ of each concentration of invertase solution into an appropriately labelled test-tube.
Label another test-tube H.
Put $2\,\text{cm}^3$ of honey extract, H, into the test-tube labelled H.
Put $2\,\text{cm}^3$ of S into each labelled test-tube, including the test-tube labelled H.
Put all of the test-tubes into the water-bath at approximately $30^{\circ}\text{C}$ . Leave the test-tubes in the water-bath for 10 minutes. You do not need to keep the water-bath at $30^{\circ}\text{C}$ .

While you are waiting, use your time to continue with question 1.

After the 10 minutes remove the test-tubes from the water-bath and put them in a test-tube rack.
Put $4\,\text{cm}^3$ of Benedict’s solution, B, into each of the test-tubes.
Heat the water-bath to boiling and then stop heating.
Put the test-tube labelled H into the water-bath. Start timing.
Measure the time taken to the first colour change. Record the result in (a)(iii). If there is no colour change after 180 seconds, record as ‘more than 180’.
Repeat step 12 and step 13 using each of the concentrations of invertase solution you prepared in step 1, instead of H. Record your results in (a)(iv).

State the result for H.

result for H = ______ $\text{seconds}$

(iv)

Record your results in an appropriate table for the concentrations of invertase solution you prepared in step 1.

(v)

Use your results from (a)(iii) and (a)(iv) to estimate the concentration of invertase in H.

concentration of invertase in H = ______ $\%$

(vi)

Suggest how you would make improvements to this investigation in order to obtain a more accurate estimate of the invertase concentration in H.

(b)

Honey contains a mixture of sugars. The percentage concentration of these sugars changes over time.

The percentage concentration of sucrose in honey was measured for a period of 25 weeks. The results are shown in Table 1.2.

Table 1.2

storage time / weeks	percentage concentration of sucrose
0	2.3
3	1.8
6	1.3
12	0.9
25	0.5

(i)

Plot a graph of the data in Table 1.2 on the grid in Fig. 1.3.

Use a sharp pencil for drawing graphs.

Fig. 1.3

(ii)

Describe the change in percentage concentration of sucrose shown by your graph in Fig. 1.3.

(iii)

State which sugars, other than sucrose, will be present in the honey after 25 weeks. Explain your answer.

sugars present ______

explanation ______

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Q2Use of the Light Microscope · Analysis, Conclusions and Evaluation18M

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