Biology 5090/32 — October/November 2012

Cambridge O-Level · Practical Test · worked solutions for every part, with the mark scheme

questions

marks

minutes

Topics Experimental Contexts · Observations and Measurements · Planning Experiments and Investigations · Microscopy and Biological Drawing · Analysis, Conclusions and Evaluation · Use of Techniques, Apparatus and Materials

Q120MExperimental ContextsObservations and MeasurementsAnalysis, Conclusions and EvaluationUse of Techniques, Apparatus and MaterialsPlanning Experiments and InvestigationsFree sample

Starch is broken down into reducing sugars by the enzyme amylase.

Iodine solution is used to test for the presence of starch. When no starch is present the yellow-brown iodine solution does not change colour. If starch is present the iodine solution turns blue-black.

You are required to investigate the effect of sodium chloride on the breakdown of starch by amylase.

You are provided with

two test-tubes each containing $5\text{ cm}^3$ of $1\%$ starch solution, labelled A and B,
two test-tubes each containing $2\text{ cm}^3$ of $1\%$ amylase, labelled A2 and B2,
test-tube containing $1\text{ cm}^3$ of water, labelled water,
test-tube containing $1\text{ cm}^3$ of $0.5\%$ sodium chloride solution labelled sodium chloride,
two dropping pipettes,
Iodine solution,
two white tiles.

Proceed as follows:

Label one white tile, A and the other white tile, B.
Add 10 separate drops of iodine solution to each tile, as shown in Fig. 1.1.

Fig. 1.1

To test-tube A add the $1\text{ cm}^3$ water and the contents of A2.

Record the time ______

Carefully shake the mixture.
After one minute remove a drop from this mixture and add it to the first drop of iodine solution on the white tile A.
Record the colour in Table 1.1.
Continue taking one drop from the mixture every minute, recording the colour until the yellow-brown iodine solution does not change colour.
To test-tube B add the $1\text{ cm}^3$ of $0.5\%$ sodium chloride solution and the contents of B2.

Record the time ______

Carefully shake the mixture.
Repeat the procedure of taking one drop from this mixture, adding it to the iodine solution drops on the white tile B. Record the colours in Table 1.1 until the yellow-brown colour of the iodine solution does not change colour.

(a)

Time 0 minutes has been done for you.

Table 1.1

time / mins	test-tube A	test-tube B
0	blue-black	blue-black
1
2
3
4
5
6
7
8
9
10

(b)

(i)

Using your data in Table 1.1, state how long it took for the starch to be completely broken down in test-tubes A and B.

test-tube A = ______
test-tube B = ______

(ii)

Describe the effect of sodium chloride on the breakdown of starch by amylase.

(c)

Explain why the following procedures were carried out

(i)

‘use a white tile’

(ii)

‘shake the mixture’

(iii)

‘add the $1\text{ cm}^3$ of water to test-tube A’

(d)

Some students carried out an investigation on the effect of pH on the activity of amylase. The results are shown in Table 1.2.

Table 1.2

pH	time taken to break down starch / minutes
3	20
4	13
5	3
6	7
7	12
8	22

(i)

Using the data in Table 1.2 construct a graph to show the effect of pH on the time taken for the breakdown of starch.

Fig. 1.2

(ii)

State the optimum pH for the activity of this enzyme.

______

(iii)

Describe and explain the effect of pH on the activity of this enzyme.

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