Name: _______________________________________________ 	

  	
  	
Topic 2 Genes & Health	
 
Cell membrane permeability, Enzyme reaction
 	
Date:	

  	
  	
Time: 35 minutes 	
  	
Total marks available: 32 	
  	
Total marks achieved: ______ 	

  	

  	

  	

  	

  	

  	

  	
Practical skill focus 	

	

Questions

Q1.
 
The cell vacuoles of beetroot (Beta vulgaris) contain the red pigment betalain.
A student investigated the effect of temperature on the permeability of beetroot cell membranes.
In this investigation, five pieces of beetroot were used. One piece of beetroot was left in 10 cm3 of distilled water for
20 minutes at 20 °C.
After 20 minutes, the piece of beetroot was removed and the intensity of the colour of the water was measured using a colorimeter.
This was repeated with the other pieces of beetroot using water temperatures of 40 °C, 60 °C, 80 °C and 90 °C.
(a)  The student's results are shown in the graph below.

(i)  Using the information in the graph, describe the effect of temperature on the permeability of the cell membranes.
(2)
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
(ii)  Using the information in the graph and your knowledge of membrane structure, explain the effect of temperature on the cell membranes.
(3)
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 

*(b)  Using all the information given in the question, describe how this investigation could be carried out to provide valid and reliable results.
(5)
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
(Total for question = 10 marks

Q2.
 
Catalase is an enzyme present in many tissues of most living organisms, but can be found in high concentrations in liver cells. Its role is to break hydrogen peroxide down into oxygen and water. Hydrogen peroxide is produced by cells and is very harmful if it is not broken down.
A student carried out an investigation into the action of catalase. Some liver was chopped into small pieces, and added to hydrogen peroxide. The volume of oxygen gas produced was recorded and a graph was drawn.

(i)   Calculate the initial rate of reaction.
(3)

Answer ...........................................................

(ii)   Analyse the graph to explain the change in the total volume of oxygen produced over the course of reaction.
(4)
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 

(iii)   Draw on the graph the line you would expect if the student repeated the investigation with the same concentration of hydrogen peroxide but with double the mass of liver.
(2)
  
(Total for question = 9 marks)

Q3.
 
The cell vacuoles of beetroot (Beta vulgaris) contain the red pigment betalain.
A student investigated the effect of ethanol on the permeability of beetroot cell membranes. 
In this investigation, 10 identical pieces were cut from one beetroot. One piece of beetroot was left in
10 cm3 of 30% ethanol for 20 minutes at 20 °C.
After 20 minutes, the piece of beetroot was removed and the intensity of the colour of the ethanol solution was measured using a colorimeter.
This was repeated with other pieces of beetroot that were left in ethanol concentrations of 0%, 50%, 70% and 100%, at 20 °C.
The student repeated this investigation with the other five pieces of beetroot at the same temperature of 20 °C. The graph below shows the results of these investigations.

(a)  Using the information in the graph, describe the effect of ethanol concentration on the intensity of colour.
(3)
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 

(b)  Using the information in the graph and your knowledge of membrane structure, explain the effect of ethanol on the permeability of beetroot cell membranes.
(4)
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 

(c)  Suggest why the results for these two investigations are different.
(2)
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
(Total for question = 9 marks)

Q4.
 
Spiders inject a mixture of digestive enzymes into the body of their prey and feed on the products of this digestion.
The photograph shows a spider with its prey.

The effect of substrate concentration on the rate of an enzyme reaction was investigated.
Two substrate solutions, A and B, were used. Solution A had a higher concentration than solution B. The optimum pH for the enzyme was pH 7.
The product lowered the pH of the solution.
The rate of reaction was determined at 1-minute intervals for 10 minutes.
The graph shows the results of this investigation.

It was concluded that the product reduces the activity of the enzyme.
Comment on the validity of this conclusion.
(4)
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 
(Total for question = 4 marks)

Q1.
No Examiner's Report available for this question

Q2.
No Examiner's Report available for this question

Q3.
 
Question (a)
Most candidates correctly stated the general trend shown in the graph. A significant number of candidates just stated data from the table, without manipulating it or making it expressly clear that this linked to a non-linear increase. A number of candidates made reference to speed / time in relation to the data. Many candidates did attempt manipulation of figures, rather than just quoting them. However, some misread the scale on the graph and used inaccurate values to give incorrect manipulated answers.
Many candidates failed to recognise that the intensities of colour for test 2 were consistently higher than in test 1, despite subsequent questions asking about this. Some candidates lost marks for confusing test 1 and test 2 when they were describing the non-linear trend or calculating the differences.

This response gained all three marks available.

Results Plus: Examiner Comments
One mark was awarded for the positive correlation; one for the description of the non-linear trend; and one mark for the manipulation of the figures. Although this is a good response that gained full marks, candidates should ignore that there are two sets of data on the graph and a description of the effect on ethanol should compare both sets of data provided.

This response gained one of the three marks available.

Results Plus: Examiner Comments
The overall trend has been correctly identified, but the candidate has just gone onto quote various points on the graph. 
To gain credit for recognising that a trend is non-linear candidates need to clearly state what is happening at each stage, even if it is just a simple statement such as the greatest increase was between 30 and 70% ethanol. 
With descriptions of data candidates will often be awarded marks for manipulating the figures (not just stating the data points) e.g. "there was a 0.1 increase (in the intensity of colour) between 0 and 30% ethanol concentration in test 1"

This response gained one of the three marks available.

Results Plus: Examiner Comments
This candidate recognised the general trend, but in trying to describe the non-linear nature of the trend they ignored what was being measured and referred to rapid and less rapid changes.

Results Plus: Examiner Tip
Always check the axis of a graph carefully for what is being measured. If there is no measurement of time then a description of an effect should not refer to time e.g. with use of words such as rapid, faster, slower, etc.

Question (b)
Many candidates were able to simply explain that the membrane had been disrupted but many were unable to explain this further as to how this disruption takes place. Very few linked this to potentially denaturing membrane proteins but some were able to comment on ethanol dissolving lipids. Some candidates lacked some detail especially to do with comments on where the pigment is escaping from or stating that it escaped from the cell membrane. Few made any comments about the vacuole. Some candidates just commented on the increased permeability and used data from the graph to describe the trend shown by the graph, instead of explaining the effect. A significant number of candidates used osmosis or diffusion of the ethanol explanations in an attempt to explain what was happening. Some thought that the disruption was due to the pH of the ethanol.
This response gained all four available marks.

Results Plus: Examiner Comments
Marks were awarded for:
 disruption of the membrane (lack of membrane);
 ethanol dissolves phospholipid;
 ethanol deforms protein channels (an equivalent allowed for denaturing the protein);
 pigment can leak out of the cell (pieced together across the response).

This response gained two of the four marks available.

Results Plus: Examiner Comments
This response is typical of those that recognise that the membrane is disrupted by the ethanol and that this is because the lipid tails are soluble in ethanol. No attempt is made to explain what is causing the colour intensity and the phosphate heads are not broken down.

This response gained no marks.

Results Plus: Examiner Comments
This is an example of the many responses that primarily gave the trend of greater permeability without explaining why it became more permeable. Some candidates also thought that the pigment is produced rather than released from the cell.

Results Plus: Examiner Tip
Make sure you know the difference between describe and explain type of questions.

Question (c)
Most candidates gained at least one mark but many only made one point or made a good point followed by a general statement such as the variables weren't kept controlled, or restated one of the control variables already stated in the question e.g. temperature or using a different beetroot. Both of these should have been ruled out as possible answers if the information in the stem of the question had been read carefully.
Most candidates who gained both marks referred described that there would be damage to the cells and there was a need to rinse the pieces which could have caused the discrepancy in results. Although some did not make it clear which set of test results would have been affected by the error (or got the test wrong e.g. test 1 was left in ethanol for a longer time).
Some candidates made a correct reference to differences in the use of the colorimeter, e.g. calibration.
A few candidates just described the data suggesting that they had not read the question properly.
This response gains both marks available.

Results Plus: Examiner Comments
Marks were awarded for beetroot cells damaged when cutting and test 2 not being washed properly. 
Although this gained full credit, be careful when suggesting that beetroot cylinders need to be dried before use as this can cause errors.

This response gained no marks.

Results Plus: Examiner Comments
Different beetroots could cause differences in results, but the question stem clearly states that the 10 identical pieces used for test 1 and test 2 were cut from the same beetroot, so this is not a credible reason for the differences in the context of this investigation - so no credit given.

Results Plus: Examiner Tip
Read all of the information given in the question carefully to make sure you do not repeat information already given.

Q4.
 
This question gave candidates various data to read and analyse. They were expected to apply their knowledge of enzyme activity and denaturation to this context to decide if the conclusion stated was valid. The majority of candidates struggled to understand why the conclusion was not valid.
Common mistakes included:

·	 commenting that the experimental procedure was not valid
·	 not understanding that the product lowered the pH of the solution and would therefore denature the enzyme, resulting in a reduced rate of reaction
·	 referring to substrate concentration limiting the rate of reaction
·	 commenting that the conclusion was 'not very valid' which was not sufficient for the mark.
The most commonly awarded mark was for describing that the rate of reaction for A did not decrease immediately.
This response scored two marks.

Results Plus: Examiner Comments
This candidate has correctly stated that the conclusion is not valid and has also given the most commonly awarded point about A's rate of reaction not decreasing immediately.

Results Plus: Examiner Tip
Make sure you use all the information given. This response has not used the information about products lowering pH.

This response scored 0 marks.

Results Plus: Examiner Comments
This is an example where the procedure was analysed and not the validity of the conclusion.

 

Mark Scheme

Q1.

Q2.

Q3.

Q4.