Name: _______________________________________________ 	

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

  	
  	
Time: 35 minutes 	
  	
Total marks available: 31 	
  	
Total marks achieved: ______ 	

  	

  	

  	

  	

  	

  	

  	
Longer answer 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.
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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.
 
Muscle cells contain globular and fibrous proteins.
Compare and contrast the molecular structures of globular and fibrous proteins.
(4)
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(Total for question = 4 marks)

Q3.
 
Transcription and translation are processes involved in the synthesis of proteins.
(a)  Put a cross  in the box to complete each of the following statements.
(i)  Individual mononucleotides in a molecule of mRNA are joined together by
(1)
            A    glycosidic bonds
            B    hydrogen bonds
            C    peptide bonds
            D    phosphodiester bonds

(ii) The maximum number of amino acids coded for by a molecule of mRNA that is 600 mononucleotides long is
(1)
            A    100
            B    200
            C    300
            D    600

(iii) When a gene that contains 22% adenine is transcribed, the mRNA produced will have
(1)
            A    22% adenine
            B    0% cytosine
            C    0% thymine
            D    28% uracil

(b)  Describe the structure of a mononucleotide found in RNA.
(2)
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*(c)  Describe the process of protein synthesis that occurs in the cytoplasm.
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(Total for question = 10 marks)

Q4.
 
Mutations to DNA can affect the structure of proteins produced in the cell.
Removing one base from a DNA sequence will affect the primary structure of a protein.
Changing one base for another may not affect the primary structure of a protein.
Explain why these two types of mutation have different effects on protein structure.
(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.
 
It was surprising that very few candidates could compare and contrast the molecular structures of globular and fibrous proteins. Many candidates who did understand what was meant by molecular structures lost marks by not being comparative in their answers. They would, for example, state that globular had hydrophilic groups on the outside without going on to describe the same aspect for fibrous proteins.
Many incorrectly described the different functions of these proteins, referring to the structural role of fibrous proteins for example.
Another common mistake was to list examples of each type of protein, for example haemoglobin and keratin without describing their structure.
This question provided many examples of how marks were lost due to vague answers that failed to address the question.

Results Plus: Examiner Comments
This clearly written and succinct answer gains 2 marks for comparing the overall shape and bonds found in the molecules. They gain a further mark for correctly stating that fibrous have secondary structures whereas globular have tertiary.

Results Plus: Examiner Tip
Comparing and contrasting means you need to make comparative statements. Make sure you say similarities and differences.

Results Plus: Examiner Comments
Here the candidate understands what is meant by the phrase 'molecular structures', but incorrectly names bonds holding the primary, secondary and tertiary structures and therefore gains no marks.

Results Plus: Examiner Comments
This is an example of a response where the candidate states some comparative features of globular and fibrous proteins but does not link these characteristics to the molecular structures. They also name some proteins which is also not credit worthy. This response gains 0 marks.

Results Plus: Examiner Tip
Check what specific part of your knowledge you should use to answer a question.

Results Plus: Examiner Comments
This answer gains 2 marks for comparing the overall structures and bonding of the two types of proteins.
Although the candidate comments on the properties of solubility and insolubility, they do not relate this to the molecular structure and where the hydrophilic/ hydrophobic groups could be found.

 

Q3.
 
Question (b)
Most candidates did well on this question. Where errors were made they confused the structure of a DNA nucleotide with an RNA nucleotide e.g. by naming deoxyribose and/or including thymine as a possible base. A few candidates implied that one nucleotide had several bases attached to it, others described a base pair rather than the structure of a single base.
This response gains both marks available.

Results Plus: Examiner Comments
Although the diagram was not needed as both marks have been gained by the written response, the annotated diagram would also have gained the marks.

Results Plus: Examiner Tip
If you include a diagram in your response make sure you annotate it clearly or it will not gain any credit.

This response gains both marks available, but also shows some errors with the bonding.

Results Plus: Examiner Comments
One mark gained for the phosphate and ribose and the other mark for the base. Glycosidic bonds are not involved, but this has not been penalised here.

This response gained one mark.

Results Plus: Examiner Comments
This response gained the mark for the base, but is an example of a common error where the structure of an RNA nucleotide and a DNA nucleotide has been muddled and they have included deoxyribose instead of ribose.

Question (c)
This question achieved the full range of marks from 0 to 5, with equal proportions of candidates on each mark. Many candidates demonstrated a detailed and sound understanding of the process of translation writing a logical account with plenty of specific details e.g. the complementary base pairing between the codon and anticodon. Several candidates even managed to match all points on the mark scheme in the same sequence. The majority of candidates gained credit for recognising the role of ribosomes and that peptide bonds form between the amino acids, but several lost marks for careless wording e.g. a tRNA carries amino acids (rather than just one).
Unfortunately, many candidates ignored the context stated in the question 'in the cytoplasm' and therefore wrote in detail about transcription and only wrote a few comments about translation at the end such as the mRNA goes to the ribosome. Some other candidates included a significant amount of unnecessary detail about the structure and folding of proteins rather than how they were synthesised.
Some candidates appear confused about the relationship between tRNA, mRNA and amino acids and have tRNA being formed by replication of the mRNA and several candidates describe amino acids being formed/produced by the tRNA. A few candidates even described reverse transcription and had the mRNA making new DNA molecules in the ribosomes without mentioning proteins at all. There was some confusion with a few candidates over the types of bonds between bases, and those between amino acids. Some candidates stated that amino acids were bases.
This response gained all five marks available.

Results Plus: Examiner Comments
Marks were awarded for:
 mRNA attaching to the ribosome;
 anticodon and codon pairing between the tRNA and mRNA;
 correct reference to the hydrogen bonding;
 tRNA carrying an amino acid;
 correct reference to the peptide bond;
 tRNA being released;
 correct reference to a stop codon.

Results Plus: Examiner Tip
For an extended description like this it helps clarity (and credit) if your response is written in a clear logical sequence like this response.

This response gained no marks.

Results Plus: Examiner Comments
This is a typical response of the candidates who did not read the question and just described transcription and gave up just as they reached the description of translation.

Results Plus: Examiner Tip
Always read the question carefully to avoid silly mistakes. If the question asks you to describe what happens in the cytoplasm, don't just describe what happens in the nucleus.

This response gained two of the five available marks.

Results Plus: Examiner Comments
This response gained marks for the reference to the ribosome and the pairing of the mRNA and tRNA. This candidate is typical of many who think that amino acids are formed rather than arranged by the process of translation.

Q4.
 
This question was answered poorly with 80% of candidates gaining half marks or less. Marks were more commonly awarded for the substitution of a base explanation than for the deletion of a base explanation.
Many did refer to the idea that substituting a base may result in the same amino acid being coded for, but then failed to explain why this may have been the case.
With respect to the deletion of a base concept, the main mark awarded was for a reference to a frame shift.
Marking point 2 was very rarely awarded.
This question provided many examples of how marks were lost due to vague answers that failed to address the question.

Results Plus: Examiner Comments
This response gains full marks as they correctly stated and explained points for each type of mutation.

Results Plus: Examiner Comments
This is an example of a response where the candidate gains just 1 mark for correctly stating that deletion of a base could cause a frameshift.
They have just repeated information from the question stem for the substitution concept and therefore gain no additional marks.

Results Plus: Examiner Tip
Make sure you are explaining information you have been given and not just repeating it.

 

Mark Scheme

Q1.

Q2.

 

Q3.

Q4.