Testing for Biological Molecules (CIE A Level Biology)

A solution is mixed with dilute hydrochloric acid, heated and then cooled before being neutralised with sodium hydrogencarbonate until the solution is slightly alkaline. The mixture is then heated gently with Benedict’s reagent. 

The final observation of the solution is a blue colour. This is the same observation of the solution when it was gently heated with Benedict’s solution without treatment with acid and subsequent neutralisation. 

What can be determined about the composition of the solution? 

A starch solution was mixed with amylase.

Which of the following reagents could be used to confirm that a reaction had occurred and what observation would be made?

	reagent	observation of mixture
A	iodine in potassium iodide solution	blue-black
B	Benedict’s solution	blue
C	iodine in potassium iodide solution	orange
D	Benedict’s solution	brick-red

A group of students carry out several tests on a sample of a solution. 

The table below shows the results.

test	observations
Benedict’s	yellow
Biuret	lilac
Ethanol	clear
iodine in potassium iodide solution	orange

Which of the following molecules would be present in the sample tested based on the student’s observations?

A group of students tested three solutions for sugars. The colours observed after testing are shown in the table below.  

	test 1	test 2
solution	heated with Benedict’s solution	boiled with hydrochloric acid, neutralised, then heated with Benedict’s solution
1 2 3	brown blue yellow	blue red orange

Which solution may contain reducing sugars?

A group of students tested four solutions by heating them with Benedict’s reagent. The table below shows the students’ observations after testing. 

Which solution contains a non-reducing sugar?

	heated with Benedict’s reagent	boiled with hydrochloric acid, neutralised and then heated with Benedict’s reagent
A	orange	blue
B	orange	orange
C	blue	orange
D	blue	blue

A group of students carried out a series of tests on a liquid solution obtained from a plant. The table below shows the results of their tests:

reagent	observation
Benedict's solution biurets ethanol and water	orange lilac clear

Which of the following biological molecules were found in the plant liquid solution?

A group of students tested four solutions for sugars. The colours observed after testing are shown in the table below. 

	test 1	test 2
solution	heated with Benedict’s solution	hydrolysed, then heated with Benedict’s solution
1 2 3 4	brown blue yellow brown	blue red green orange

 

Which solution may contain non-reducing sugars?

Using a large number of aphids, a solution from the sieve tube elements of a tomato plant was extracted. 

The following tests and observations were made:

reagent	observation
Benedict’s solution Biuret ethanol and water	brown precipitate lilac colour clear

Which row correctly shows the molecules found in the solution?

	fatty acids	protein	reducing sugar
A	✓	X	X
B	X	✓	X
C	✓	X	✓
D	X	✓	✓

Key: ✓ = present, X = absent

A teacher demonstrated several food tests on a sample solution with an unknown composition of biological molecules. 

test	observation
non-reducing sugar lipid starch protein	blue cloudy orange lilac

Which row of the table below correctly identifies the molecules present in the sample?

	fatty acids	starch	protein	non-reducing sugar
A	✓	✓	X	X
B	X	✓	✓	✓
C	✓	X	✓	X
D	✓	✓	✓	X

Key: ✓ = present, X = absent

The diagram below shows a biological molecule. 

Which of the following tests would give a positive result for this molecule?

Four tests were conducted to determine the types of biological molecules present in a sample solution. Observations from these tests are shown in the table below. 

test	observation
emulsion iodine Benedict’s biuret	clear orange brick-red purple

Which molecules are present in this solution? 

Four sugar solutions were tested with Benedict’s solution. The observed colour of each solution is given in the table below. 

solution	colour
1 2 3 4	yellow green blue brick-red

Which of the following options would be the best interpretation of the observed results?

	solution 1	solution 2	solution 3	solution 4
A	0.1% reducing sugar	0.05% reducing sugar	0.5% non-reducing sugar	1.0% reducing sugar
B	0.5% reducing sugar	1.0% reducing sugar	1.0% non-reducing sugar	1.5% reducing sugar
C	0.05% reducing sugar	0.1% reducing sugar	1.0% reducing sugar	0.5% reducing sugar
D	1.5% reducing sugar	1.0% non-reducing sugar	1.5% reducing sugar	1.0% non-reducing sugar

Four tests were conducted to determine the types of biological molecules present in a sample solution. Observations from these tests are shown in the table below. 

test	observation
emulsion iodine Benedict’s biuret	cloudy blue-black green blue

Which molecules are present in this solution?

Samples of four different fruit juices (A, B, C and D), were gently heated with Benedict’s solution. 

A second sample of the same four juices were heated with dilute hydrochloric acid and then neutralised before being heated with Benedict’s solution. 

The table below shows the masses of precipitates formed in each test. 

Which juice contains the highest mass of non-reducing sugar?     

	mass of precipitate before acid hydrolysis /mg	mass of precipitate after acid hydrolysis /mg
A B C D	60 20 55 40	70 45 75 40

Question 25 - CIE IAS 2.1 Testing for Biological Molecules - HARD

A teacher demonstrates the following biological tests on four food samples to her class:

• starch
• reducing sugar
• protein
• lipids

One of the food samples, a banana milkshake, was found to contain reducing sugar, protein and lipid. 

Which of the food samples shown in the table below could be a banana milkshake?

Food	observation
	adding Biuret reagent	heating with Benedict’s solution	adding iodine in potassium iodide solution	mixing with ethanol and adding to water
A	purple	blue	orange	clear
B	pale blue	orange precipitate	blue-black	clear
C	purple	orange precipitate	orange	milky emulsion
D	pale blue	blue	blue-black	milky emulsion

A teacher demonstrates several biological tests on two food samples to her class.

• Sample 1 was a mixture of phospholipids and starch. 
• Sample 2 tested positive for lipids and protein, but negative for reducing sugars.  

food	observation
	adding Biuret reagent	heating with Benedict’s solution	adding iodine in potassium iodide solution	mixing with ethanol and adding to water	hydrolysed and heated with Benedict’s
1	purple	orange	blue-black	clear	blue
2	purple	blue	orange	milky	brick-red
3	pale blue	orange	orange	milky	blue
4	pale blue	blue	blue-black	milky	brick-red

Which of the following options would be correct?

	sample 1	sample 2
A B C D	1 4 2 4	3 2 3 1

Birch trees can be tapped to extract a solution from their sieve tube elements. 

Different samples of the solution extracted from several Birch trees were tested.

Which of the following would be the correct result?

	observation		iodine in KI
	before hydrolysis	after hydrolysis
A	blue	red	blue-black
B	green	blue	brown
C	blue	red	brown
D	red	yellow	blue-black

Using a large number of aphids, the solution from the sieve tube elements of a strawberry plant was extracted, as shown in the image below. 

Different samples of the extracted solution were tested in the lab. 

Which of the following would be the correct result?

	observation		iodine in KI
	before hydrolysis	after hydrolysis
A	blue	red	blue-black
B	red	blue	blue-black
C	red	blue	brown
D	blue	red	brown

A sample solution underwent a series of biological tests to determine the substances present in the solution. 

When the biuret test was carried out a pale blue colour was observed; the same observation was made when the sample was heated with Benedict’s solution. 

When the sample was shaken in an organic solvent and then added to water, a milky emulsion was observed. 

Based on the information given, which of the following molecules are not present in this solution? 

Two solutions, one containing glucose and amylase, the other containing starch and sucrose, were tested with a variety of reagents to confirm their identity. 

The table below shows the conclusions recorded following the outcome of the biological tests. 

Which row correctly identifies the two solutions?

	add biuret		add iodine in KI		boil with Benedict’s solution		boil with Benedict’s solution after acid hydrolysis
	1	2	1	2	1	2	1	2
A	+	ー	ー	+	ー	+	ー	+
B	+	ー	+	ー	+	ー	ー	+
C	ー	+	+	ー	ー	+	+	ー
D	+	ー	ー	+	+	+	ー	+

Key: + = biological molecule present, ー = biological molecule absent