Biological MoleculesCambridge IGCSE Biology: Topic test

20 questions, 54 marks

Cambridge IGCSE Biology

Biological Molecules topic test

Total 54 marks

Name

Class

Date

1
A student is comparing the chemical composition of three types of large biological molecule: carbohydrates, proteins and fats. She looks up which chemical elements each type of molecule contains.
(a)
Which three elements are always found in a carbohydrate molecule such as glucose?
[1 mark]
ACarbon, hydrogen and oxygen
BCarbon, hydrogen and nitrogen
CCarbon, nitrogen and oxygen
DCarbon, hydrogen, oxygen and nitrogen
(b)
Which element, found in proteins, is not found in carbohydrates or fats?
[1 mark]
ACarbon
BNitrogen
CHydrogen
DOxygen
(c)
State the elements found in a fat molecule, and name the two types of smaller molecule that combine to form a fat molecule.
[2 marks]
Total for question 1: 4 marks
2
A student adds a few drops of Benedict's solution to a sample of grape juice and heats the mixture in a water bath. The solution, originally blue, changes colour and a coloured precipitate forms at the bottom of the tube.
(a)
What colour change indicates a positive result in the Benedict's test for reducing sugar?
[1 mark]
ABlue to purple
BOrange-brown to blue-black
CBlue to a brick-red (orange) precipitate
DColourless to pink
(b)
What must be done to the test mixture for the Benedict's test to work?
[1 mark]
AIt must be cooled in ice
BIt must be shaken vigorously without heating
CIt must be left at room temperature for 24 hours
DIt must be heated, for example in a water bath
(c)
The grape juice gives a positive result in the Benedict's test. State what this result shows about the grape juice, and name one reducing sugar that might be present.
[2 marks]
Total for question 2: 4 marks
3
A biochemist is studying three large molecules found in living organisms: starch, found in potato tubers; glycogen, found in liver cells; and cellulose, found in plant cell walls. All three are described as polymers, made from many smaller repeating units joined together.
(a)
Name the small molecule from which starch, glycogen and cellulose are all built. State one function of starch and one function of cellulose.
[3 marks]
(b)
A DNA molecule is a different type of large biological molecule, made from a different small repeating unit. Describe the structure of a DNA molecule.
[4 marks]
Total for question 3: 7 marks
4
A nutritionist is investigating the composition of a homemade energy bar made from crushed oats, milled egg white powder and vegetable oil. She wants to design food tests to confirm the presence of starch, protein and fat in the bar, and to explain the biochemistry behind each test.
(a)
Describe a test the nutritionist could use to test for the presence of starch, and a test she could use to test for the presence of protein, in a sample of the energy bar, including a positive result for each.
[6 marks]
(b)
Explain why testing for fat requires a different method from testing for starch or protein, describe the ethanol emulsion test for fat, and explain why a control tube is important when interpreting the result of a food test.
[6 marks]
Total for question 4: 12 marks
5
A student tests the vitamin C content of orange juice using DCPIP solution, which is blue and decolourises when vitamin C is present. She adds orange juice drop by drop to a fixed volume of DCPIP solution and counts the number of drops needed to just decolourise it completely. She repeats the test with a sample of apple juice.
(a)
In the DCPIP test, what colour change indicates that vitamin C is present?
[1 mark]
AThe blue DCPIP solution turns colourless
BThe solution turns from colourless to blue
CThe solution turns from orange-brown to blue-black
DThe solution turns from blue to purple
(b)
The student needed fewer drops of orange juice than apple juice to decolourise the same volume of DCPIP solution. What does this suggest?
[1 mark]
AOrange juice contains less vitamin C than apple juice
BOrange juice contains more vitamin C than apple juice
COrange juice contains no vitamin C
DApple juice contains no vitamin C
(c)
Explain why counting the number of drops needed to decolourise a fixed volume of DCPIP solution allows the student to compare the vitamin C concentration of the two juices.
[2 marks]
Total for question 5: 4 marks
6
A student is revising the building blocks of large biological molecules. She writes: 'Starch, glycogen and cellulose are all built from many glucose molecules joined together. Proteins are built from many amino acid molecules joined together. Fats are built from fatty acids and glycerol.'
(a)
Which of the following large molecules is built from amino acids?
[1 mark]
AStarch
BCellulose
CProtein
DFat
(b)
Which two types of smaller molecule combine to form a fat molecule?
[1 mark]
AGlucose and amino acids
BAmino acids and glycerol
CGlucose and fatty acids
DFatty acids and glycerol
(c)
State one similarity and one difference between the building blocks of starch and the building blocks of protein.
[2 marks]
Total for question 6: 4 marks
7
A short section of one strand of a DNA molecule is analysed and found to contain 120 bases in total: 30 are adenine (A), 30 are thymine (T), and the rest are cytosine (C) and guanine (G) in equal numbers to each other.
(a)
Calculate the number of cytosine (C) bases and the number of guanine (G) bases in this section of DNA strand, showing your working, and state the base that guanine always pairs with.
[3 marks]
(b)
Explain why, on the complementary (paired) strand of this DNA molecule, you would expect to find exactly 30 thymine bases and 30 adenine bases, and explain the general importance of specific base pairing to the structure of DNA.
[4 marks]
Total for question 7: 7 marks
8
A food scientist carries out three food tests on a sample of full-fat milk. When iodine solution is added, the milk stays orange-brown. When Benedict's solution is added and the mixture is heated, an orange-red precipitate forms. When the sample is shaken with ethanol and then poured into water, a cloudy white emulsion forms.
(a)
Using the results described above, deduce which of the following are present in the milk sample: starch, reducing sugar, and fat. Justify each conclusion using the result of the relevant test.
[6 marks]
(b)
The food scientist did not test the milk sample for protein. Describe how she could test for protein, predict the likely result given that milk is known to contain the protein casein, and explain why using several different food tests together, rather than just one, gives more complete information about a food's composition.
[6 marks]
Total for question 8: 12 marks

End of questions