Biology 9700/35 — May/June 2021

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

A farmer who grows grapes needs to pick them when they have the highest reducing sugar content.

You will be using the Benedict’s test to determine the concentration of reducing sugars in different samples of grape juice, U1, U2 and U3.

You are provided with the materials shown in Table 1.1.

Table 1.1

labelled	contents	hazard	volume / $\text{cm}^3$
U1	unknown concentration of reducing sugars	none	20
U2	unknown concentration of reducing sugars	none	20
U3	unknown concentration of reducing sugars	none	20
W	distilled water	none	100
B	Benedict’s solution	harmful irritant	50
R	2% reducing sugar solution	none	50

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

It is recommended that you wear suitable eye protection.

(a)

The Benedict’s test requires you to heat test-tubes containing some of the above solutions in a boiling water-bath.

Read step 1 to step 5 on page 3 and then assess the risk of using a boiling water-bath in these steps as low, medium or high.

State the level of risk. ______

(i)

State one reason for your answer.

(ii)

Carry out step 1 to step 9.

Set up a water-bath and heat to boiling ready for step 5, step 15 and step 22.
Label 3 test-tubes U1, U2 and U3.
Put $2\,\text{cm}^3$ of U1 in the test-tube labelled U1.
Put $2\,\text{cm}^3$ of B into the test-tube containing U1. Shake gently to mix.
Put this test-tube into the boiling water-bath. Start timing.
Measure the time taken for the first appearance of a colour change in the test-tube. If there is no colour change after 120 seconds, stop timing.
Record the result in (a)(ii). If there is no colour change after 120 seconds, record as ‘more than 120’.
Remove the test-tube from the water-bath and put it in the test-tube rack.
Repeat step 3 to step 8 for U2 and U3.

Record your results for U1, U2 and U3 in an appropriate table.

(iii)

Using your results in (a)(ii), state which sample, U1, U2 or U3, has the highest concentration of reducing sugars.

(iv)

You will need to carry out a serial dilution of the 2% reducing sugar solution, R, to reduce the concentration by half between each successive dilution.

Fig. 1.1 shows the first 2 beakers you will use to make your serial dilution.

Fig. 1.1

Complete Fig. 1.1 by drawing as many extra beakers as you need for your serial dilution of reducing sugar solution.

For each beaker add labelled arrows to show:

The volume of reducing sugar solution transferred
The volume of water added.

Under each beaker, state the concentration of reducing sugar solution.

(v)

Carry out step 10 to step 19.

Prepare the concentrations of reducing sugar solution as shown in Fig. 1.1.
Label test-tubes with the concentrations you prepared in step 10.
Put $2\,\text{cm}^3$ of 2% reducing sugar solution into the appropriately labelled test-tube.
Put $2\,\text{cm}^3$ of B into the same test-tube. Shake gently to mix.
Repeat step 12 to step 13 with the other concentrations you prepared in step 10.
Put all the test-tubes into the boiling water-bath. Start timing.
Leave the test-tubes in the boiling water-bath for 1 minute.
Remove the test-tubes with a test-tube holder and put them into a test-tube rack.
Observe the colour in each test-tube. You may see the same colour in more than one test-tube.
Record the colours in (a)(v) using the words:

red orange brown green blue

Record your results in an appropriate table.

(vi)

State the colour observed in step 25 ______

Using your results in (a)(v), estimate the concentration of reducing sugars in this sample.

(vii)

A student carries out the same procedure and finds that one of the unknown samples gives a negative result for reducing sugars. The student thinks that the sample may contain non-reducing sugars.

Describe how you would modify the procedure to test for non-reducing sugars.

(b)

As grapes ripen, the concentrations of the different reducing sugars change.
A scientist measured the concentration of glucose and fructose at different stages of ripening.
The results are shown in Table 1.2.

Table 1.2

stage of ripening	percentage concentration of reducing sugar
	glucose	fructose
unripe	9	1
ripening	7.5	9.5
ripe	3	22

Draw a bar chart of the data in Table 1.2 on the grid in Fig. 1.2.

Use a sharp pencil for drawing bar charts.

Fig. 1.2

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Q2Use of the Light Microscope · Analysis, Conclusions and Evaluation20M

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