Biology 9700/33 — May/June 2018

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

Fruit growers need to know the concentration of sugars, such as sucrose, in the fruit so that it can be picked at the best time.
As the fruit matures the concentration of sucrose increases.

The concentration of sucrose can be estimated in a fruit extract by carrying out the non-reducing sugar test. Known concentrations of sucrose are tested and the result for the unknown concentration of sucrose in the fruit extract is compared to them.

You will need to:

prepare known concentrations of sucrose solution using simple (proportional) dilution
carry out the non-reducing sugar test on these concentrations
carry out the non-reducing sugar test on the unknown concentration of sucrose solution in fruit extract, U
estimate the concentration of sucrose in fruit extract, U.

You are provided with the materials shown in Table 1.1.

Table 1.1

labelled	contents	hazard	volume/ $\text{cm}^3$
S5	5.0% sucrose solution	none	60
W	distilled water	none	100
U	unknown concentration of sucrose solution	none	20
H	dilute hydrochloric acid	irritant	50
A	10g sodium hydrogencarbonate powder	none	—
Benedict’s	Benedict’s solution	harmful	20

It is recommended that you wear suitable eye protection.
If H, A or Benedict’s come into contact with your skin, wash off immediately under cold water.

Read step 1 to step 19 before proceeding.

Set up a water-bath and heat to boiling ready for step 7 and step 13.

You will need to make simple (proportional) dilutions of the sucrose solution S5 and prepare the concentrations of sucrose solution:

4.0% sucrose concentration, to be labelled S4
3.0% sucrose concentration, to be labelled S3
2.0% sucrose concentration, to be labelled S2.

You will need to prepare $10\ \text{cm}^3$ of each concentration.

(a)

(i)

Complete Table 1.2 to show how you will prepare these concentrations.

Table 1.2

label	volume of S5 / $\text{cm}^3$	volume of distilled water, W / $\text{cm}^3$	final percentage concentration of sucrose solution
S5	10.0	0	5.0
S4			4.0
S3			3.0
S2			2.0

(ii)

Prepare the concentrations of sucrose as stated in Table 1.2 in the beakers provided.

The sucrose concentration can be estimated by using the non-reducing sugar test.

To test for the presence of a non-reducing sugar, any non-reducing sugars must be hydrolysed (broken down) into the reducing sugars.

For example, boiling sucrose with dilute hydrochloric acid hydrolyses sucrose into glucose and fructose, which are both reducing sugars.

The Benedict’s test can then be used to show the presence of these reducing sugars.

Put $2\ \text{cm}^3$ of S5 into a labelled test-tube.
Put $2\ \text{cm}^3$ of H into the same test-tube.
Shake the test-tube gently to mix the contents.
Repeat step 3 to step 5 for S4, S3, S2 and U.
Put all the test-tubes into the boiling water-bath (set up in step 1). Leave them for 2 minutes.
After 2 minutes, remove the test-tubes from the water-bath and put them in the test-tube rack.
Leave the test-tubes to cool for a further 3 minutes.

You will need the boiling water-bath for step 13.

After 3 minutes put a small amount of A into each test-tube. The mixture will fizz and rise up the test-tube. Repeat until there is no more fizzing. This neutralises the acid so that the Benedict’s solution can work.

Note: there may be some of A left in the bottom of some of the test-tubes. This will not affect the results.

Put $3\ \text{cm}^3$ of Benedict’s solution into the test-tube containing S5.
Shake the test-tube gently to mix the contents.
Put this test-tube in the boiling water-bath. Start timing.
Measure the time taken to the first appearance of a colour change in the test-tube.
If there is no colour change after 120 seconds, stop timing and record as ‘more than 120’.
Record the result in (a)(ii).
Remove the test-tube from the water-bath and put it in the test-tube rack.
Repeat step 11 to step 16 with each of the other concentrations of sucrose solution.
Repeat step 11 to step 14 with the sample from U.
Record the result for U in (a)(iii).

Record your results for the known concentrations of sucrose solution in an appropriate table.

(iii)

State the time taken for the appearance of the first colour change for U.

Complete Fig. 1.1 to show an estimate of the percentage concentration of sucrose solution in the sample U, using the letter U.

Fig. 1.1

(iv)

The concentration of sucrose in U can also be estimated from a graph of your results.

Draw a graph using the results you recorded in (a)(ii) on the grid in Fig. 1.2. The axes have been labelled for you.

Use a sharp pencil for drawing graphs.

Fig. 1.2

(v)

Use your graph and the time stated in (a)(iii) to estimate the percentage concentration of sucrose in U.
Show on the graph how you determined your answer.

percentage concentration of sucrose solution in U = ______

(vi)

Identify one significant source of error in this investigation.

(vii)

A student carried out the same procedure to estimate the percentage concentration of sucrose solution in a different fruit extract. The estimate for the percentage concentration of sucrose solution was found to be below 2.0%.

Apart from drawing a graph, suggest how the student could improve this procedure to obtain a more accurate estimate of the percentage concentration of sucrose solution in this fruit extract.

(b)

Maturing fruits contain many types of sugar. The proportions of these different sugars vary according to the type of fruit.

A scientist carried out some tests to determine the mass of each type of sugar found in one type of fruit. The total mass of the fruit was $55\ \text{g}$ .

The results are shown in Table 1.3.

Table 1.3

type of sugar in the fruit	mass of sugar in $55\ \text{g}$ fruit / $\text{g}$
P	13.6
Q	12.2
R	3.4
S	2.0
T	0.4

(i)

Calculate the percentage of the fruit that is made up of sugars.

Show all the steps in your working.

percentage = ______

(ii)

Draw a bar chart of the data shown in Table 1.3 on the grid in Fig. 1.3. Each bar should be separated for each type of sugar.

Use a sharp pencil for drawing bar charts.

Fig. 1.3

(iii)

Name one source of the sucrose in the plant and the process used for loading the sucrose into the phloem tissue.

source = ______

process = ______

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Q2Use of the Light Microscope · Analysis, Conclusions and Evaluation18M

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