Fast and slow twitch muscles + epigenetics Questions

Q1. The photograph below shows Usain Bolt, an elite sprinter. He won a gold medal in the 100 metre final at the 2012 Olympic Games in a time of 9.63 seconds.

(a)  The skeletal muscles of elite sprinters are likely to have many fast twitch muscle fibres. Suggest why these muscles are less red in colour than muscles with many slow twitch muscle fibres.
(2)
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(b)  The pH of the blood of a sprinter falls during a race and returns to its original level after the race.
(i)  State the homeostatic control mechanism that returns the pH of blood to its original level.
(1)
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*(ii) Explain how the pH of the blood of a sprinter is returned to its original level after a race.
(5)
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(iii)  During the race, heat is generated and is lost from the body through the skin.
Describe how muscle, present in blood vessels in the skin, helps to increase heat loss from the body.
(4)
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Q2. Dark chocolate contains a chemical called epicatechin.
An investigation was carried out to study the effect of epicatechin on mice.
Three groups of one-year-old male mice, group A, group B and group C, were used in an investigation lasting 15 days.
The table below shows how each group of mice was treated.

All other variables were kept constant and after 15 days skeletal muscle from the mice in each group was studied.
(a)  The ability of the skeletal muscle to contract was compared. The time taken for the muscle to start to fatigue (fail to contract) was recorded.
The results are shown in the table below.

A statistical test was carried out on the results.
The test showed that the addition of epicatechin had a significant effect on the time taken for mouse skeletal muscle to fatigue. The test also showed that extra exercise had no significant effect.
Use the results table to supply evidence that the addition of epitcatechin had a significant effect but extra exercise had no effect.
(4)
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(b)  The mean number of capillaries per muscle fibre was found for the skeletal muscle from groups A and B.
The bar chart below shows the results.

(i)  Use the information in the bar chart to describe the effect of epicatechin on the mean number of capillaries per muscle fibre.
(2)
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*(ii)  Using your knowledge of respiration and the results table for groups A and B, suggest an explanation for the effect of the change in the numbers of capillaries on the time taken for the muscle to fatigue.
(5)
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Q3. Smoking is an environmental factor that can cause lung cancer.
Smoking can cause epigenetic changes to the AHRR gene.
People who smoke cigarettes have less DNA methylation of this gene than non-smokers.
The AHRR gene is involved in regulation of cell growth and differentiation.
(i)  Explain how differential gene expression allows cells to become specialised.
(4)
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(ii)  Deduce the reasons why reduced DNA methylation of the AHRR gene can increase the risk of lung cancer.
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Q4. Fibrosis of the liver is a chronic condition that can lead to cirrhosis of the liver and liver failure.
In fibrosis of the liver, liver cells secrete excess collagen.
There is an association between fibrosis of the liver and alcohol consumption in humans.
Scientists have investigated the effects of alcohol intake on collagen synthesis in rats.
The results are shown in the table.

(i)  Deduce the effect of alcohol intake on fibrosis of the liver in rats.
(3)
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(ii)  The scientists have suggested that epigenetic events are involved in regulating collagen synthesis in rats given alcohol.
Explain how epigenetic events might be involved in regulating collagen synthesis in rats given alcohol.
(2)
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Q5. The phenotype of an organism may be affected by both genotype and the environment. For example, the risk of developing skin cancer is affected by the activity of the p53 gene and exposure to ultraviolet (UV) light.
(a) The p53 gene plays an important role in the cell cycle in humans.
Explain the role of the cell cycle.
(2)
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(b) The p53 gene is called a 'tumour suppressor gene'. Cancers can form when the p53 gene does not function properly. UV light can cause mutations in this gene. The mutant gene results in the production of p53 mutant cells which may become cancerous.
(i) Name one environmental factor, other than UV light, that can cause a cell to become cancerous.
(1)
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(ii) Suggest how the cell cycle will be affected in cells that have become cancerous.
(2)
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(c) In an investigation, isolated skin cells were exposed to UV light for different lengths of time. The number of p53 mutant cells produced was recorded.
The graph below shows the effect of UV light on the number of p53 mutant cells produced.

(i) Use the graph to find the number of p53 mutant cells produced after an exposure time of 12 a.u.
(1)
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(ii) Using the information in the graph, describe the effect of exposure to UV light on the number of p53 mutant cells produced.
(3)
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(iii) Name the type of cell division responsible for the increase in number of the p53 mutant cells.
30(1)
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(d) When cells divide out of control to produce a tumour, the cells may not become specialised.
Describe the process by which cells usually become specialised following cell division.
(3)
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(Total for question = 13 marks)

Examiner's Report

Q1.
 
(a)
This item required candidates to suggest why muscles with a predominance of fast twitch muscle fibres appeared less red that those muscles with a predominance of slow twitch muscle fibres. It was generally handled well by candidates. All marking points were seen regularly.
This is a good answer that offers three reasons. Firstly it gave the most common response. Then, in the second sentence it referred appropriately to less capillaries present and then to less blood.

Results Plus: Examiner Comments
Marking points 1, 3 and then 2, gained.

 
Results Plus: Examiner Tip
Always take note of the mark allocation in a question.

(b) (i)
A wide variety of answers was proffered for this item, with about half stating correctly the homeostatic control mechanism.
 
(b) (ii)
This 5 mark item was also the first with marks for Quality of Written Communication (QWC). Candidates were expected to explain how a lowered human blood pH was returned to its original pH. It delivered the full mark range.
The mark scheme allowed candidates to approach this from the perspective of the fate of lactate or dissolved carbon dioxide.
Whilst this is a short response it delivered three credit-worthy points.

Results Plus: Examiner Comments
The first sentence correctly links the increase in carbon dioxide, as an alternative to acid, with a decrease in pH for marking point 1.
The second sentence correctly refers to the chemoreceptor detecting a change in the pH. This is marking point 8.
The third sentence expresses marking point 9 in two different, and acceptable, ways. The addition of the word 'more' was important with reference to impulses from the medulla for this marking point. Likewise, the candidate then refers to an increase in breathing rate rather than just affecting breathing.

 
Results Plus: Examiner Tip
If tempted to write that something changes, perhaps consider the direction of that change eg heart rate increases rather than heart rate alters.

(b) (iii)
This question focussed on how the smooth muscle surrounding blood vessels in the skin could help increase heat loss.
Whilst there was a number of very accomplished answers, it was not uncommon for candidates to refer to muscles in capillaries and blood vessels moving closer to the skin.
This is a short but precise answer that gained four marks.

Results Plus: Examiner Comments
The short first sentence gained marking points 1 and 2 whilst the second sentence was sufficient to achieve marking point 5. The third sentence suitably referred to more heat loss through radiation for marking point 6.

 
Results Plus: Examiner Tip
Precision in answers is important. A number of candidates referred to blood being directed onto the skin surface, which would not qualify for marking point 5.

Q2.
No Examiner's Report available for this question

Q3.
 
(i) Students were asked to explain how cells become specialised via differential gene expression. Many students discussed the effect of epigenetic factors on gene expression rather than answer the actual question. There were still many excellent answers that achieved full marks, for explaining how a stimulus caused a gene to be activated, resulting in the transcription of that gene and the subsequent translation of the mRNA formed to produce a protein that determined the structure of function of the cell.
This is a good example of response that clearly gained all four marks.

(ii) This question required students to deduce (work out from information provided) why reduced DNA methylation of the AHRR gene can increase the risk of lung cancer. Whereas part (i) was straightforward recall of knowledge questions, this required applying that knowledge to an unfamiliar context. In order to gain marks, students had to state that reduced methylation would allow for the activation of the AHRR gene, which could then be transcribed. The deduction would then require the conclusion that this would result in uncontrollable cell division causing lung cancer. Increased cell growth was not accepted – there had to be reference to cells dividing out of control, a mark that could be given even if the rest of the answer was incorrect.
There were not many answers achieving full marks, but this was one of the more challenging questions in the paper. However, this response did gain full marks.

Q4.
No Examiner's Report available for this question

Q5.
 
(a) This question should have been accessible to most candidates but many weaker responses referred to cell growth and cell repair rather than tissue growth and repair. Strong candidates often gained both marks. Some candidates mixed this up with meiosis and some simply restated the information in the text about p53.
Good responses referred to the role of the cell cycle in providing new cells to replace old/ damaged cells and repairing tissues.

Results Plus: Examiner Comments
This response gains one mark for reference to asexual reproduction and the production of genetically identical cells (either of which would have gained the same mark). If the phrase 'growth and repair' had been qualified as 'growth of the organism and repair of tissues', full marks would have been awarded.

Results Plus: Examiner Comments
One mark was awarded for asexual reproduction.One for was also given for either repair of the damaged tissues or replacement of damaged cells - even though the candidate appeared to have been trying all combinations!

Results Plus: Examiner Comments
This is a clearer response - gaining one mark for 'allows cells to multiply' (could also have been given for growth of organism), and one for replacing damaged tissues. There is also a mark that could have been given for reference to asexual reproduction. A very good answer.

(b) (i) The responses to this question indicated two trends - the first being an obvious failure to read the question, as made clear by the number giving UV light or sunlight as an answer, despite a clear instruction to provide an example other than UV. Others read the reference to 'environmental factor' and provided answers relating to abiotic factors, including temperature, weather and, even, altitude.
Also evident is the lack of understanding of what factors do give rise to cancer - anything from carbon monoxide to infra red and just plain radiation, without any qualification of the form of radiation provided. At this level, candidates are expected to know that radiation alone cannot cause cancer, it has to be a specific form of radiation.

Results Plus: Examiner Comments
This is too vague to gain a mark - radiation is a form of heat transfer.

Results Plus: Examiner Comments
Good qualification of the type of radiation that does give rise to cancer - 'gamma rays'.

Results Plus: Examiner Tip
If this candidate had not checked their answer and altered it to make it more specific, they would not have gained a mark for 'harmful rays'. Always check your answers - if they look vague, they are not going to gain full marks.

Results Plus: Examiner Comments
The phrase 'lifestyle - whether it is healthy or not' is obviously far too vague to gain credit. The next part of the response stating 'protection from UV light, e.g. wearing sun-cream or not' shows that this candidate has failed to read the instructions to name an environmental factor other than UV light.

Results Plus: Examiner Tip
Read all parts of the question with care - do not skim read the instructions or you will miss vital details.

Results Plus: Examiner Comments
Although 'smoking' was accepted as an answer, it is preferable to produce an answer like this one, making it clear that cigarette smoking causes cancer because of the carcinogenic chemicals in the tar.

Results Plus: Examiner Tip
Be clear and add vital details if you know them.

(b) (ii) This was found to be demanding by most candidates, with only 10% of candidates achieving full marks. Where candidates gained credit, it was typically for the idea of uncontrolled division. Candidates often hinted at the idea of the cycle occurring at a more rapid rate, but often phrased it in the context that growth or mitosis would be faster. Many thought that interphase would disappear completely, and in some cases implied that DNA replication would never occur. Few gained both marks. Most candidates only gained one mark for reference to cell division happening out of control, with a few referring to the Hayflick limit.
Many could have achieved better marks if they had read the question with more care, there were many responses referring to the effects on cell division, instead of the cell cycle.

Results Plus: Examiner Comments
This response gains one mark for reference to lack of 'stops' on the cell cycle.

Results Plus: Examiner Comments
This candidate has understood the concepts involved and has gained full marks. There is reference to uncontrolled cell division and lack of mechanisms to suppress cell division. One mark can also be given for 'very short interphase period' as well as reference to cells being at the varying stages of mitosis for more time.

(c) (i) This was a straightforward question asking candidates to read a point off a graph. There was no manipulation of data required. However, many failed to get the mark as a result of not reading the scales correctly or drawing lines freehand.
The use of a ruler and a sharp pencil to draw parallel lines would vastly improve the frequency of correct responses to these types of question.

Results Plus: Examiner Comments
This candidate has drawn a cross where the lines intersect and has then written 20 instead of 30. Perhaps the act of drawing the line across to the correct point on the y axis would have prevented this error.

Results Plus: Examiner Tip
Draw the lines on the graph to make sure you're reading the correct point.

Results Plus: Examiner Comments
For some reason this candidate has overcomplicated the task - they have marked off the scale on the x axis and then come up with the figures of 50 and 22. Subtraction has given them 28.

Results Plus: Examiner Tip
Practice reading points off graphs and working out the scales.

Results Plus: Examiner Comments
Freehand drawing of lines on graphs is risky - as can be seen here. Always use a ruler, preferably a transparent one (to keep lines parallel more easily), and a sharpened pencil for accuracy.

Results Plus: Examiner Comments
This answer indicates that the use of a ruler helps the candidate read the point off the graph with ease to gain the mark.

(c) (ii) Well prepared candidates fared well with this question, two thirds of candidates achieving at least 2 marks, generally for a full description of the graph. Most were able to identify, or at least describe, the positive correlation, but fewer candidates noticed that the relationship was not linear. Although many described the shape of the graph, few used the terms exponential or non linear. Many candidates did not manipulate data, and only about 20% gave a numerical calculation, most simply quoting numbers from the graph. Those that did make an attempt at a calculation misread the graph and many referred to readings at 15au light intensity, even though the line finished before 15au, or the calculation was incorrect, especially where candidates had attempted to calculate a percentage increase.

Results Plus: Examiner Comments
One mark for positive correlation described - 'as UV light increases more mutant p53 cells are made'. No mark for manipulation of data - from 5 to 18 is not a tripling. Simply repeating data from the graph does not gain this mark.

Results Plus: Examiner Comments
This gained full marks. First of all there is a description of a positive correlation, then a description of the non-linear nature of the relationship. Finally, for the third mark, there is a correct calculation of the difference in number of p53 mutant cells between two different exposure times.

Results Plus: Examiner Tip
Don't try to over complicate the manipulation of data - go for something easy to calculate initially, such as the difference between two separate points on the graph - but do take care with scales on the axes.

(c) (iii) This question simply asked candidates to name the type of cell division responsible for the increase in number of p53 mutant cells. Despite earlier parts of Q8 prompting candidates to describe the function of the cell cycle, there were still a few that got this wrong. However, over 90% of responses were correct.
A common mistake was misspelling mitosis so that it was midway between mitosis and meiosis.

Results Plus: Examiner Comments
Careless mistake from not reading the question - a type of cell division is not going to be nuclear division.

Results Plus: Examiner Comments
Hybrid words that are neither mitosis or meiosis will not gain credit. All that stood in the way of this candidate gaining the mark was the letter 't'!

Results Plus: Examiner Comments
It is very common to see examples like this, of candidates crossing out a correct answer and replacing it with one which is incorrect. If in doubt, think first, before crossing out what's been written. Usually, instincts are correct and the answer is right the first time.

Results Plus: Examiner Tip
Always check before crossing out an answer and replacing it!

(d) This question drew some excellent, well written answers, with over 50% of candidates achieving full marks. Where candidates understood the question, they often gained full credit, although almost 20% scored nothing. Many were imprecise in the usage of the term transcription (the mRNA was often transcribing the gene, or even translating it) but were often able to gain three other mark points. As usual there was some confusion between the terms transcription and translation. A few did not fully understand the question and simply discussed cell division and cell differentiation in the embryo. Others wrote about totipotency and pluripotency, and some described the function of specialised cells e.g. goblet cells. There were even a few descriptions of the stages of mitosis.

Results Plus: Examiner Comments
This is an excellent example of a response that covers virtually every marking point. Chemical stimulus of the cell, activation of genes, transcription of activated gene, translation of mRNA and production of a protein that can permanently affect the structure and function of the cell. A perfect answer.

Results Plus: Examiner Tip
Think of the sequence of events involved and write your answer in that order, with a beginning, a middle and an end.

Results Plus: Examiner Comments
This shows the type of response that is along the right lines, but too vague to gain full marks. This is just enough to gain credit for reference to activation of genes leading to production of proteins for 2 marks. The phrase 'the cells will then do what is asked of them' suggests a conscious decision on the part of the cells!

Results Plus: Examiner Tip
Try to be specific, relevant and detailed in your responses.

Results Plus: Examiner Comments
This is an example of a response referring to totipotency and pluripotency. Many made the mistake of referring to these processes, rather than describe how the cells become specialised. This response gained no marks.

Mark Scheme

Q1.3

Q2.

Q3.

 

Q4.

 

Q5.