Biological Molecules (Cambridge (CIE) IGCSE Biology)

Exam Questions

3 hours31 questions

6 marks

Grapes are soft fruits that contain sugars. Some students investigated the concentration of reducing sugars in grapes.

Step 1	The students determined the volume of the grapes by placing 50 cm³ of distilled water into a measuring cylinder.
Step 2	Four grapes were placed into the measuring cylinder and the total volume of the distilled water and grapes was measured.

Fig. 1 shows the total volume of the distilled water and grapes in the measuring cylinder.

Fig. 1

Table 1

Total volume of distilled water and grapes in the measuring cylinder / cm³	Total volume of the grapes / cm³

(i)

Calculate the total volume of the four grapes using the information in Step 1 and Fig. 1

Write your answers in Table 1

[1]

(ii)

Calculate the average volume of one grape using your answer to a) i).

Give your answer in cm³.

[1]

Step 3	Three test-tubes were labelled, S1, S2 and S3.
Step 4	5.0 cm³ of a sugar solution (S) was added to test-tube S1.
Step 5	1.0 cm³ of solution S and 4.0 cm³ of distilled water were added to test-tube S2. The contents of S2 were mixed.
Step 6	0.2 cm³ of solution S and 4.8 cm³ of distilled water were added to test-tube S3. The contents of S3 were mixed.
Step 7	Grape juice was extracted from ten grapes.
Step 8	5.0 cm³ of the extracted grape juice was placed into a test-tube labelled G.
Step 9	5.0 cm³ of Benedict’s solution was added to each of test-tubes S1, S2, S3 and G.
Step 10	Test-tubes S1, S2, S3 and G were placed into an 80 °C water-bath.
Step 11	The time at which a colour change first appeared in each test-tube was recorded.
Step 12	Steps 3 to 11 were repeated to obtain a second set of results.

Fig. 2 shows the students’ results in minutes and seconds.

Fig. 2

(iii)

Prepare a table to record the results, shown in Fig. 2

Your table should include:

The solutions tested.
The time, in seconds, of the first appearance of a colour change in each solution.

How did you do?

3 marks

(i)

The concentration of reducing sugar in solution S1 is 200 g per dm³.

The concentration of reducing sugar in solution S3 is 8 g per dm³.

Calculate the concentration of reducing sugar in solution S2, using the information in step 5. Give your answer in g per dm³.

[2]

(ii)

State a conclusion for the reducing sugar investigation.

[1]

How did you do?

5 marks

(i)

State one variable that was kept constant in the reducing sugar investigation.

[1]

(ii)

The method used to estimate the concentration of reducing sugar in grapes contains potential sources of error.

State one source of error and suggest an improvement to minimise the error.

[2]

(iii)

Identify one safety precaution that should be used when carrying out this investigation and give a reason for this precaution.

[2]

How did you do?

2 marks

Grapes develop in large groups attached to their parent plant. As they develop, grapes increase in size and ripen.

Fig. 3 shows one group of grapes.

Fig. 3

A student suggested that the concentration of reducing sugars in grapes changed as the grapes developed and ripened.

Describe how the method used in steps 3 to 12 could be modified to determine if there is a change in the concentration of reducing sugars in grapes during development.

How did you do?

9 marks

Some students placed eight grapes, that had been picked at different ages, into water. They measured the change in the volume of the grapes after 24 hours.

Table 2 shows the results of this investigation.

Table 2

Age of grapes when picked / days	Starting volume of grapes / cm³	Final volume after 24 hours / cm³	Percentage change in volume
12	5.0	5.5	10
24	7.6	8.5	12
36	12.0	13.7	14
48	17.0	19.7	16
60	22.0	26.0	18
72	25.0	30.0	20
84	30.0	36.6
96	36.0	45.0	25
108	42.0	54.6	30
120	55.0	74.3	35

(i)

Calculate the percentage change in the volume of grapes aged 84 days.

Write your answer in Table 2.

Show your working.

[2]

(ii)

Plot a line graph on the grid of the age of the grapes against the percentage change in volume.

[4]

(iii)

Describe the trends shown by the results in Table 2 and your graph.

[2]

(iv)

State the variable that was changed (independent variable) in this investigation.

[1]

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5 marks

A class investigated the protein content of powdered milk. Each class group had five standard solutions of milk powder and two unknowns. They used a calibration curve from which to estimate the concentration of protein in the two unknown samples of milk powder.

Each group carried out the following method:

Set out seven test tubes and label them 1%, 2%, 3%, 4%, 5%, X and Y
To each tube, add 10 cm³ of the milk powder solutions at each concentration and two unknown concentrations.
Add 5 cm³ of enzyme solution to the first tube (1%). Shake to mix thoroughly, and immediately start the stopwatch.
Record the time it takes for the solution to become clear by marking a cross on a piece of white paper, view the cross through the mixture, and record the time it takes for the cross to become visible.
Repeat with the other concentrations of milk powder, both known and unknown.

(i)

Explain why it is necessary to start the stopwatch immediately after the enzyme has been added to the milk powder solutions.

[2]

(ii)

Suggest how many repeat readings the students should carry out.

[1]

(iii)

Describe what the students should do if they identify anomalous results.

[2]

How did you do?

5 marks

The results collected quantitative results. The test for protein is usually a qualitative test.

(i)

Describe the test for protein using a qualitative reagent and include what would be expected from a positive result.

[2]

(ii)

Explain why a qualitative test for protein is less precise than a quantitative test for protein.

[1]

(iii)

A piece of equipment called a colorimeter can determine the end point for a reaction, such as the one described in part (a). The colorimeter analyses the colour of a sample compared to an existing standard, usually distilled water.

Explain why using a colorimeter would increase the accuracy of the experiment carried out by the class.

[2]

How did you do?

7 marks

Table 1 below shows the class' results.

Table 1

	time taken for solution to clear / sec
student group	1%	2%	3%	4%	5%	X	Y
1	44	56	67	81	103	51	82
2	48	59	65	85	106	48	98
3	54	63	71	81	128	56	85
4	57	66	73	86	98	45	85
5	44	49	69	78	95	52	79
6	45	53	75	74	115	48	86
7	63	51	65	81	105	56	91
8	39	58	66	84	93	51	87
9	42	53	71	88	110	54	86
10	45	59	73	91	117	55	83
Mean	48	57	70	83	107

(i)

Calculate the mean values for the samples X and Y.

[2]

(ii)

Plot a graph of mean time taken for the solution to clear against concentration.

[3]

(iii)

Use the graph plotting in part (ii) to determine the concentration of samples X and Y.

[2]

How did you do?

4 marks

One group of students used a beaker to measure out 10cm³ of the milk powder solution. Another group used a measuring cylinder.

(i)

Explain which piece of apparatus is more accurate.

[2]

(ii)

Identify two sources of error that could have occurred as part of the method outlined in part (a).

[2]

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