Name: _______________________________________________ 	

  	
  	
Topic 2 Genes & Health	
 
Plasma membrane, Protein, DNA replication, Mutation
 	
Date:	

  	
  	
Time: 30 minutes 	
  	
Total marks available: 27 	
  	
Total marks achieved: ______ 	

  	

  	

  	

  	

  	

  	

  	
Data analysis focus 	

	

Questions

Q1.
 
Molecules are transported across the cell membrane in a number of different ways.
(a) Describe the structure of a cell membrane.
(3)
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(b) Cells were placed in a solution containing two different solutes, solute P and solute R.
The diagram below represents the concentration of the two solutes outside one of the cells, when this cell was placed in the solution.

The cells were left in the solution for 50 minutes.
The diagrams below represent the concentrations of the two solutes, inside and outside the cell after 10, 20, 30 and 40 minutes in the solution.

(i) Using the information in the diagrams, describe the changes that have taken place in the concentrations of solute P and solute R, in the 40 minute period.
Suggest an explanation for these changes.
(5)
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(ii) Complete the diagram below, to show the concentration of solute P and solute R inside and outside the cell, after 50 minutes.
(1)

(Total for question = 9 marks)

Q2.
 
Mutations in genetic material such as DNA often result in the formation of new alleles.
The graph gives information about chloride transport in the human respiratory system with the normal allele for the CFTR protein and with the three mutated CFTR alleles.

The table gives information about the CFTR protein produced by cystic fibrosis (CF) sufferers with mutated alleles.

Assess the effect that these mutations have on the human respiratory system.
(6)
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(Total for question = 6 marks)

Q3.
 
DNA is a polymer made from monomers called nucleotides.
Different theories for DNA replication have been suggested. Figure 1 illustrates two of these theories.

Figure 1
Meselson and Stahl carried out experiments to test these theories for DNA replication.
Figure 2 shows the results from one of their experiments.

Figure 2
Analyse the data to explain why Meselson and Stahl accepted one of the models for DNA replication and rejected the other.
(4)
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(Total for question = 4 marks)

Q4.
 
Cells in the walls of the alveoli secrete a fluid.
This fluid, containing proteins and lipoproteins, acts as a surfactant.
The layer of surfactant, shown in the diagram, prevents the alveoli collapsing.

Alveolar cells undergoing protein synthesis were investigated.
Cells containing amino acids labelled with a fluorescent marker were monitored for 30 minutes.
The percentage of fluorescence inside and outside the cells was measured.
The results are shown in the table.

Explain how these labelled amino acids would be incorporated into a surfactant protein.
(4)
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(Total for question = 4 marks)

Q5.
 
Some fish live in very cold parts of the sea where ice can form.
Many of these fish produce anti-freeze proteins, which help to stop ice forming inside the fish.
Some fish produce another anti-freeze protein, called AFP II.
The tissues of these fish were tested for the presence of AFP II and the mRNA coding for AFP II.
The results are shown in the table.

Explain the distribution of the AFP II protein and AFP II mRNA.
(4)
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(Total for question = 4 marks)

Examiner's Report

Q1.
 
(a)  This question scored very highly with over 70% of candidates gaining all three marks available. Virtually all of the candidates knew that the main part of the membrane was made up from phospholipids and due to their chemical properties there had to be a bilayer -however, a significant number of candidates didn't explain about the molecules' orientations, expecting credit marks for stating that the phospholipid was hydrophilic at the head end and hydrophobic on the tails. Most candidates recognised the presence of protein with many providing further clarification e.g. with reference to carrier/channel/intrinsic protein or glycoprotein. A significant number of candidates also recognised the presence of cholesterol in the membrane structure.
Quite a few candidates aided the clarity of their answers through the use of clear annotated diagrams.
Unfortunately, a few candidates described function rather than structure and therefore did not score any/many marks.
This response scores all three marks available.

Results Plus: Examiner Comments
This response gains credit for:
- phospholipids;
- the orientation of the phospholipids;
- channel proteins.
These points are also supported with a helpful labelled diagram that shows the orientation of the phospholipids and the position and nature of the channel protein and would have been able to gain the marks itself.

Results Plus: Examiner Tip
Annotated diagrams are often an effective way to clarify points and gain the marks available, particularly when describing structures.

Results Plus: Examiner Comments
This response gains just one mark for recognising that phospholipids are an important part of a membrane. The statements about the heads and tails is not enough for further credit as it has not been used to describe the orientation of the molecules in the membrane. Describing the function of the membrane is not relevant to the question asked.

Results Plus: Examiner Tip
Remember that cell membranes are not only made from phospholipids and if you are describing the structure of a membrane describe the orientation of the phospholipids in the bilayer.

Results Plus: Examiner Comments
This is an example of a response that gained no marks because the candidate has not described the structure of the membrane as asked.

Results Plus: Examiner Tip
When asked to describe the structure of something focus on the components and arrangement of the molecules in the structure and do not be distracted by the function of the membrane.

(b) (i)  This question achieved the full range of marks. The majority of candidates recognised that solute P moved into the cell by diffusion down a concentration gradient for three marks, with many also recognising that it reaches equilibrium. However, a significant number of candidates confused osmosis and diffusion, either referring to solute P as water or stating that the particles diffused by osmosis. Others described the movement being "along a concentration gradient" that is not clear enough for credit. A number of candidates described the pattern of molecules every 10 minutes rather than stating the points clearly.
Rarely did candidates appreciate that the membrane was impermeable to solute R but often referred to the "cell not needing R" or "R was too big to fit through".
This response gains all five marks available.

Results Plus: Examiner Comments
The direction and explanation for the movement of solute P is described for four marks and they correctly state that solute R does not enter the cell.

Results Plus: Examiner Tip
The first paragraph of this response is not very useful. Do not waste time by listing the numbers of molecules (data points) for every measurement. Most credit comes from identifying the correct trends, which in this case are successfully covered in the second and subsequent paragraphs.

This response scores four of the five marks available.

Results Plus: Examiner Comments
This response gives a good description and explanation of the changes that take place with solute P and therefore gains the maximum marks for that part of the question. However, they have not stated that R does not go into the cell or that the membrane is impermeable to R so fail to gain the full 5 marks.

Results Plus: Examiner Tip
The concentration remains the same is not clear enough for an alternative to recognising that solute R does not enter the cell when comparing P and R. When answering questions aim to make positive clear statements.

This response gains two of the five marks available.

Results Plus: Examiner Comments
The reference to osmosis in this response is clearly incorrect, but it did not stop them gaining credit for recognising that the solute moved down a concentration gradient until equilibrium was reached.

Results Plus: Examiner Tip
Don't forget that osmosis is the movement of water not solutes.

(b) (ii)  The vast majority of candidates gained the mark for this question. There were a small number of careless answers missing a circle and also some with 2 larger black circles inside the cell and 2 outside. Any crossing out was generally clearly done, allowing access to the mark.

Results Plus: Examiner Comments
This is an example of the correct response that gained the mark.

Results Plus: Examiner Comments
This response gains no marks because solute P has disappeared from inside the cell.

Results Plus: Examiner Tip
The question asks candidates to show the concentration both inside and outside the cell so do not just partially answer the question asked.

Results Plus: Examiner Comments
This response did not gain the mark because a molecule of R has slipped into the cell.

Results Plus: Examiner Comments
This response did not gain the mark because there are only five molecules of solute P drawn outside the cell.

Q2.
 
This question provided the candidates with two pieces of scientific information about three cystic fibrosis mutations. Candidates were expected to analyse and interpret these pieces of data in order to help them answer the question.
Candidates needed to assess the effect that these mutations have on the human respiratory system.
A wide variety of responses were seen.
A small minority of responses did not answer the question asked and instead referred to the effect of cystic fibrosis on the reproductive or digestive systems.
Approximately 30% of responses gained level 1. Here the responses contained some basic knowledge with some attempt to link knowledge and understanding to the given context. For example explaining why cystic fibrosis would result in thicker mucus or increased lung infections. Where there was some reference to the data, this was often limited interpretation. For example a simple statement that all three mutations reduced chloride transport.
Level 2 responses showed evidence of linkages being made. For example linking the reduced quantity of CFTR protein to reduced chloride transport, or relating these mutation to Fick's Law to help explain the effect on the respiratory system.
Very few candidates were awarded a level 3 as most responses did not have evidence of sustained scientific reasoning. Knowledge relating to mutations affecting protein structure and therefore their function were rarely seen which was disappointing.

Results Plus: Examiner Comments
This response was awarded the lower mark in level 2. 

They have described some effects of the CFTR protein problems on the respiratory system, showing some linkages and lines of scientific reasoning with some structure. 

However, they could not be awarded the higher mark in the level as they have not included evidence of analysis, interpretation and/or evaluation of both pieces of scientific information.

Results Plus: Examiner Tip
If you have been provided with data, then you need to demonstrate that you understand it and can use it to add detail to your answer.

Results Plus: Examiner Comments
This is an example of a level 3 response as there is evidence of sustained scientific reasoning. 

The candidate has recognised that the mutated alleles would result in a different tertiary structure and has started to link this knowledge to their analysis and evaluation of both pieces of scientific information.

 

Q3.
 
This question was a very good differentiator and the full range of marks was awarded.
Candidates were required to analyse the data, provided in both Figures 1 and 2, to explain why one model was accepted / rejected by Meselson and Stahl. A significant number of candidates did not refer to the data however, which limited the number of marks that could be awarded.
Where reference was made to the information provided, many candidates described instead of explained and therefore the final two marking points were rarely awarded.
This response gained four marks.

Results Plus: Examiner Comments
This response used the data to explain in a clear and succinct way why the semi-conservative model was accepted and the conservative model was rejected.

Results Plus: Examiner Tip
Make sure your answer uses all the data you have been provided with.

 

Q4.
 
This question asked candidates to explain how these labelled amino acids would be incorporated into a surfactant protein and was a very good differentiator.
Some candidates referred to the amino acids joining to a pre-existing surfactant protein which was not credit worthy.
The most commonly awarded marking point was number 4. Marking point 5 was the next most commonly awarded as many recognised that the primary structure would not be the final structure of a surfactant protein. Around half of candidates did not include any more details of translation.
When candidates did include some detail about events that occur at the ribosome, imprecise language often prevented them gaining marks. For example, they would not refer to codons or anticodons − mRNA finding a ribosome was also not sufficient.

Results Plus: Examiner Comments
This is an example of a response which gained 2 marks for the most commonly awarded marking points.

Results Plus: Examiner Comments
This response gained the first four marking points. They have described the process of translation clearly. 

However, they did not go on to explain how the primary structure of the polypeptide would be changed to form a surfactant protein's tertiary structure.

Results Plus: Examiner Tip
Think about why certain terminology is used in the question.

 

Q5.
 
This question assessed candidates' understanding of gene switching and cell specialisation: if AFP II mRNA is present only in the liver, the AFP II gene must be transcribed only in the liver, so this gene must be activated in the liver and deactivated in other tissues. The question generated a good spread of marks: many candidates realised that the protein must be produced in the liver than transported to the rest of the body, so gained 1–2 marks; fewer candidates completed the explanation with reference to gene switching, but a significant number did so successfully to gain 3–4 marks.

Results Plus: Examiner Comments
This candidate shows good understanding of the biological processes involved here (with one mistake at the end).
The answer scored the full 4 marks, as follows:
- sentence 1 gains mark scheme bullet point 5
- sentence 2 gains no marks because it simply repeats the information in the table, although it helps the logical flow of the answer
- sentence 3 gains mark scheme bullet point 1
- sentence 4 gains mark scheme bullet points 2 and 3
- sentence 5 gains no marks because the biology is incorrect (transport around the body is not in vesicles).

 

Mark Scheme

Q1.

Q2.

 

Q3.

 

Q4.

 

Q5.