Syllabus Edition

First teaching 2023

First exams 2025

|

Testing for Biological Molecules (CIE AS Biology)

Exam Questions

2 hours40 questions
1a
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4 marks

A student wants to determine whether a sample of food contains lipids. 

Describe the test the student should carry out and include the positive result. 

1b
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1 mark

Describe how the student could determine the concentration of the lipid in the sample. 

1c
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1 mark

State one health and safety precaution the student should take when using the reagent to test for a lipid.

1d
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1 mark

Fig. 1 shows the structure of a lipid, also known as a triglyceride. 

structure-of-a-triglyceride

Fig. 1

Lipids cannot cross the cell-surface membranes of cells lining the gut during the absorption of nutrients. 

Suggest why it cannot cross cell-surface membranes.

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2a
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1 mark

Iodine in potassium iodide is used to test the presence of starch. 

Describe what you would expect to observe in a positive result. 

2b
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2 marks

Starch is a carbohydrate often stored in plant cells.

Fig. 1 shows a starch grain in a section of plant tissue at a magnification of ×500.

starch-grain

Fig. 1

The size of the diameter of the starch grain on the image is 18 mm. 

Calculate the actual diameter of the starch grain between points A and B. Give your answer in micrometers. 

2c
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2 marks

State the type of microscope that was used to obtain the image shown in Fig. 1 and explain the evidence for this. 

2d
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1 mark

A different reagent is used to test for the presence of proteins. 

Give the name of this reagent. 

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3a
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4 marks

Describe the test for non-reducing sugars.

3b
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1 mark

Give the name of a non-reducing sugar. 

3c
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4 marks

The presence of other molecules can also be tested for with a range of biochemical reagents. 

Complete Table 1 to show which reagents are used for which molecule.

Table 1

reagent biological molecule
 potassium iodide  
 Benedict's  
 biuret  
 ethanol  

3d
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1 mark

Some of the biological molecules identified in Table 1 can be described as polymers.

Define the term polymer.

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4a
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3 marks

Complete the following sentences. 

The presence of protein may be tested for using __________ reagent. In the presence of proteins the reagent will turn from __________ to __________.

The reagent reacts with _________ bonds present in the protein. 

4b
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2 marks

Fig. 1 shows two amino acids. 

alanine

Fig. 1

Draw the chemical bond that forms when these two amino acids are joined during a condensation reaction. You need only draw the parts of the molecules shown in the box.

4c
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2 marks

The chemical test for a protein is described as a qualitative test. 

Explain why this is the case. 

4d
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1 mark

A student was using the reagent, named in part (a), to test for the presence of an amylase enzyme. The student accidentally heated the reaction solution, containing amylase, as part of their experiment and then tested the reaction solution using the reagent for proteins. 

Suggest why the error in their method may cause an inaccurate result. 

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5a
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3 marks

Benedict's reagent can be used to test for the presence of reducing and non-reducing sugars. 

Outline the method for test for a reducing sugar. 

5b
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2 marks

The Benedict's test for reducing sugar is often referred to as a semi-quantitative test. 

(i)

Suggest what is meant by a semi-quantitative test.                                                                                           

[1]

(ii)

Describe the colour change in Benedict's solution you would expect to see as the concentration of the test substance increases.                   

[1]

5c
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1 mark

Fig. 1 shows the colour change scale of five known concentrations of reducing sugar solutions. 

concentrations-of-sugard

Fig. 1

Estimate the concentration of the unknown sample. 

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1a4 marks

Students are presented with an unknown solution which they need to test for the presence of starch, reducing sugar and protein.

(i)

Outline the test for reducing sugar.

[3]

(ii)

State how students could estimate the concentration of reducing sugar present in the solution.

[1]

1b3 marks

Samples of the original solution are removed and iodine solution is added to one of the samples (A), while biuret reagent is added to the other sample (B). The results of these tests are found in Table 1.

Table 1

Sample End colour of the solution
A Orange-brown
B Purple

Discuss the results obtained for each sample.

1c3 marks

Explain how the test for reducing sugar, outlined in part (a) (i), could be modified to test whether the solution contained sucrose.

1d1 mark

After following the procedure outlined in part (c) the mass of precipitate that formed was heavier than when the test for reducing sugar was performed.

State the significance of these results.

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2a3 marks

The enzyme amylase breaks down starch to form the reducing sugar maltose. Iodine solution can be used to measure the progress of this enzyme-controlled reaction during a practical investigation.

Explain how iodine solution can be used to measure the rate of this enzyme-controlled reaction.

2b2 marks

After all the starch was broken down into maltose by amylase, the enzyme maltase was added to the solution. A sample (sample 1) was taken from the solution immediately after the maltase was mixed with the solution. The remaining solution (sample 2) was incubated at 37 °C for an hour.

Describe a chemical test you could carry out to show that maltose is a reducing sugar.

2c3 marks

The chemical test in part (b) was carried out on both samples 1 and 2. All experimental variables were kept the same when testing both of the samples. Both tubes were left for 15 minutes to allow the precipitate to settle.

State and explain how the result of sample 2 would differ from sample 1.

2d1 mark

Explain why iodine solution would not be a suitable reagent to use when testing for biological molecules present in extracts from animal cells.

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3a4 marks

Triglycerides are the main form of dietary lipids ingested by humans. They are digested by the enzyme lipase in the stomach.

Describe a biochemical test that could be performed to determine whether a sample of food contains triglycerides.

3b3 marks

Using your knowledge of the structure of triglycerides, explain the visual result of a positive test for the presence of lipids.

3c2 marks

Seeds are a good source of lipids. They also contain starch which serve as a food source for the growing plant embryo during germination.

Outline the biological test that can be used to confirm the presence of starch.

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4a2 marks

Scientists wanted to test the nutritional content of a new meal replacement shake under development. Table 1 shows some of the results that were gathered by the scientists.

Table 1

Biological molecule tested Test reagent(s) used End results observed
Glucose (i) Brick-red precipitate forms
Sucrose (ii) Blue colour
Protein (iii) Blue colour
Lipids (iv) Cloudy suspension forms

Complete the table by identifying the test reagents used in each case.

4b3 marks

Describe the procedure the scientists would have followed to test for the presence of protein and state the meaning of the results that they obtained.

4c2 marks

Discuss the results that were obtained by the scientists when they were testing for the presence of carbohydrates.

4d2 marks

While testing for the presence of sucrose, the scientists accidentally left out the sodium hydroxide after adding the hydrochloric acid to the solution.

Explain how this could have affected their results.

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1a
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1 mark

The Benedict's test for reducing sugars gives a semi-quantitative result when used on samples of reducing sugar solution.

Explain the term semi-quantitative in this context. 

1b
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3 marks

A solution with an unknown concentration of glucose is tested for reducing sugar.

Outline a method for determining the concentration of glucose in this solution. 

1c
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2 marks

Table 1 below gives the experimental protocol for making a serial dilution of a glucose solution.

The concentration of the stock solution of glucose that is to be used is 20 mmol dm-3.

Table 1

Concentration of glucose required /
mmol dm-3
Volume of stock solution required /
cm3
Volume of distilled water required /
cm3
Total volume of solution required /
cm3
0 0 100 100
2     100
4     100
10     100
15     100
20 100 0 100

Calculate the missing values and complete Table 1 with your calculated values.

1d
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2 marks

In a similar procedure to that outlined in part (c), a student wished to make up a solution of glucose of 0.1 mmol dm -3 from a laboratory stock solution with a concentration of 100 mmol dm-3.

The student evaluated two methods of producing this solution, methods A and B as shown below in Table 2.

Table 2

Method A Method B
0.1 cm3 stock solution and 99.9 cm3 of distilled water 10 cm3 stock solution and 900 cm3 distilled water, to make solution X, followed by...
1 cm3 of solution X and 9 cm3 of distilled water 

Select which method, A or B, would be the better method for the student to follow.

Give a reason for your choice. 

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2a
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2 marks

A group of students wanted to test a food sample to measure the concentration of reducing sugars.

They began their investigation by testing a series of samples of known glucose concentration using a colorimeter. Their results are shown in Table 1 below.

Table 1

Glucose concentration / mmol dm-3 % absorbance
1.0 69
2.0 58
3.0 46
4.0 39
5.0 25
6.0 17

Explain why the students began their investigation in this way.

2b
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2 marks

Calculate the percentage change in absorbance as the glucose concentration increases from 1.0 mmol dm-3 to 6.0 mmol dm-3. Give your answer to three significant figures. 

2c
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4 marks

Use the data in Table 1 to plot a graph of glucose concentration against absorbance.

graph-paper

2d
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3 marks

Describe three measures that the students should take to ensure that their results are valid.

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3a
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5 marks

The effect of storage on the sugar content of winter squash fruits was studied. The results are shown in Fig. 1 below. Note that the error bars show standard deviation.

Graph showing changes in the levels of sucrose, glucose and fructose in fruit after 25, 50 and 75 days of storage

Fig. 1

Suggest how the squash may have been tested to assess sugar content.

3b
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4 marks

Describe the effect of storage time on squash sugar content shown in Fig. 1.

3c
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2 marks

Suggest an explanation for the results observed in part (b).

3d
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2 marks

Fig. 2 shows another set of data gathered during the investigation into the effect of storage time on squash biochemistry.

cie-ial-2-1-h-q3d

Fig. 2

The researchers concluded from this data that winter squash would become more appealing to consumers over time.

Discuss this conclusion.

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