Biological moleculesEdexcel A-Level Biology B: Topic test

20 questions, 54 marks

Edexcel A-Level Biology B

Biological molecules topic test

Total 54 marks

Name

Class

Date

1
A sugar beet plant stores energy as sucrose in its swollen root, whereas a potato plant stores energy as starch in its tubers. Sucrose is a disaccharide and starch is a polysaccharide.
(a)
Which pair of monosaccharides is joined to form a molecule of sucrose?
[1 mark]
AGlucose and galactose
BGlucose and fructose
CTwo molecules of glucose
DFructose and galactose
(b)
A starch molecule is an unbranched chain of 200 glucose units joined by condensation reactions. How many molecules of water are released when this chain is formed?
[1 mark]
A100
B198
C199
D200
(c)
State two differences between the structure of amylose and the structure of amylopectin.
[2 marks]
Total for question 1: 4 marks
2
Sunflower seeds store energy mainly as triglycerides in which most of the fatty acids contain carbon-carbon double bonds. A seed is small and cannot photosynthesise until after it has germinated.
(a)
Which bond joins a fatty acid to glycerol in a triglyceride?
[1 mark]
AEster bond
BGlycosidic bond
CPeptide bond
DPhosphodiester bond
(b)
Which statement correctly describes the fatty acids in sunflower oil?
[1 mark]
AThey contain no hydrogen atoms bonded to carbon
BThey have only single bonds between carbon atoms
CThey are joined to glycerol by hydrogen bonds
DThey are unsaturated because they contain at least one C=C bond
(c)
Explain why storing energy as triglyceride rather than as starch is an advantage to a small seed.
[2 marks]
Total for question 2: 4 marks
3
A mutation in a collagen gene causes fragile skin and weak blood vessel walls. In one patient the smallest amino acid, glycine, which normally occurs at every third position in the polypeptide, is replaced by a larger amino acid at one position in each chain. The three chains can no longer pack tightly together at that point.
(a)
Explain how the replacement of one amino acid in a polypeptide can change the properties of the protein.
[3 marks]
(b)
Compare the structure of collagen with the structure of haemoglobin.
[4 marks]
Total for question 3: 7 marks
4
The enzyme hexosaminidase A breaks down a lipid in nerve cells. It is coded for by a gene on chromosome 15. A common mutation inserts four extra bases into the coding region of this gene. People who are homozygous for this mutation develop Tay-Sachs disease because the lipid accumulates in their nerve cells.
(a)
Explain how the insertion of four bases changes the polypeptide made, and why the insertion of three bases would be likely to have a smaller effect.
[6 marks]
(b)
Explain why a change in the sequence of amino acids in hexosaminidase A can result in the enzyme being unable to catalyse its reaction, and why this causes lipid to build up in the nerve cells of people with Tay-Sachs disease.
[6 marks]
Total for question 4: 12 marks
5
An earthworm moves by a hydrostatic skeleton. Its body is divided into fluid-filled compartments, and contraction of muscles around a compartment changes its shape. About 85% of the mass of an earthworm is water.
(a)
Which property of water makes it suitable for use in a hydrostatic skeleton?
[1 mark]
AHigh specific heat capacity
BSurface tension
CIncompressibility
DMaximum density at 4 °C
(b)
Which statement explains why water molecules attract one another?
[1 mark]
AThe oxygen is slightly negative and the hydrogens slightly positive, so hydrogen bonds form between molecules
BWater molecules are non-polar, so they are held by ionic bonds
CWater molecules form covalent bonds with each other
DHydrogen bonds form between the two hydrogen atoms within one molecule
(c)
Suggest how the high water content of an earthworm helps to stabilise its body temperature.
[2 marks]
Total for question 5: 4 marks
6
A gardener buys three soil supplements: calcium nitrate, magnesium sulfate and superphosphate, which is a source of phosphate ions. She uses them to improve the growth of vegetable plants in poor soil.
(a)
What is the role of calcium ions in a plant?
[1 mark]
ATo make chlorophyll
BTo make ATP
CTo make DNA
DTo form calcium pectate in the middle lamella
(b)
Plants given magnesium sulfate develop dark green leaves, while plants lacking magnesium develop yellow leaves. Which statement explains this?
[1 mark]
AMagnesium ions are used to make amino acids
BMagnesium ions are needed to make chlorophyll
CMagnesium ions are used to make cell walls rigid
DMagnesium ions are part of the structure of ATP
(c)
Explain why nitrate ions and phosphate ions are both needed for the growth of a plant.
[2 marks]
Total for question 6: 4 marks
7
Methanol is converted by the enzyme alcohol dehydrogenase into methanal, which is toxic. A patient who has swallowed methanol is given an infusion of ethanol. Ethanol is also a substrate of alcohol dehydrogenase and has a molecular shape similar to methanol.
(a)
Explain how giving the patient ethanol reduces the rate at which toxic methanal is produced.
[3 marks]
(b)
In a laboratory investigation with methanol alone, 0.36 mmol of methanal formed in the first 90 seconds. Calculate the initial rate of reaction in mmol min⁻¹ and explain why the initial rate, rather than the rate over a longer period, is measured when comparing the effects of inhibitors.
[4 marks]
Total for question 7: 7 marks
8
Salivary amylase is a globular enzyme that hydrolyses starch. The gene for amylase can be present in several copies, and people from populations with a high-starch diet tend to have more copies. Cellulose and starch are both polymers of glucose, but humans cannot digest cellulose.
(a)
Explain the roles of mRNA and tRNA in the synthesis of the amylase polypeptide at a ribosome.
[6 marks]
(b)
Explain why amylase hydrolyses starch but cannot hydrolyse cellulose, even though both are polymers of glucose.
[6 marks]
Total for question 8: 12 marks

End of questions

Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).