Questions

Q1.
 
The muscles of the earthworm (Lumbricus terrestris) contract when it is touched. This is known as the withdrawal response.
Studies were carried out to investigate the withdrawal response in earthworms.
The duration of the withdrawal response is the length of time that the muscles remain contracted. If the stimulus is repeated, the withdrawal response is either reduced in duration or lost.
In one study, an earthworm was touched 20 times in one minute at point V as shown in the diagram. The shortening of the earthworm's body was measured after 20 stimuli.
The effect of touching point W in the same way was recorded and also the effect of alternating touches between points V and W.
The results of this study are shown in the table.

In another study, sensory neurones of an earthworm were stimulated by an electrode six times.
Nerve impulses were recorded at positions X and Y as shown in the diagram.

Nerve impulses recorded at X and Y are shown in the diagram below. The presence of a line indicates that an impulse was detected. S shows the stimulus.

* (i)  Analyse the data from these two studies to evaluate the most likely cause of the loss of the withdrawal response.
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(ii)  Devise an experiment to show that the frequency of the stimulation used in the first study should be 20 stimuli per minute.
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(Total for question = 10 marks)

Examiner's Report

Q1.
 
(i)
This was the first of the longer, extended writing questions in this paper. These style questions are new to the reformed A-level assessment and as such we recognise that candidates will have little experience of these. It is vital that all the information provided is read carefully and the phrase 'analyse the data' reinforces the need to do this before starting to answer the question. The command word here was 'evaluate'. The definition provided in the specification is as follows:
"Review information then bring it together to form a conclusion, drawing on evidence including strengths, weaknesses, alternative actions, relevant data or information. Come to a supported judgement of a subject's qualities and relation to its context."
This was attempted well by the majority of candidates, with more achieving full marks than 0 or 1 mark out of 6. The full range of marks was seen, indicating that this question discriminated well.
Most candidates referred to the results of both investigations, which was necessary to raise the response from Level 1 to Level 2. Some candidates provided a description of the data without any explanations relating to neurotransmitters and thresholds being met or not met in order to initiate a reaction.
Common responses that did not gain many marks included descriptions of the behavioural adaptations linked to habituation, rather than focusing on succinct descriptions of the evidence provided and discussions of the likely causes.
Good answers did make links between the two studies, correctly identifying that the alternating stimuli in study 1 were stimulating different neurones. When applying their knowledge, many candidates described the effect of synapses, neurotransmitters and calcium ions on transmission of impulses.
Descriptions of both experiments were frequently seen accompanied by excellent descriptions of the mechanism of habituation.
Weaknesses were often in unclear descriptions of the studies rather than the theory.
A number of candidates referred to an inhibitory neurotransmitter at the synapse rather than the depletion of neurotransmitter.
A large number of candidates seemed to anthropomorphise the earthworms and described them 'ignoring' stimuli or 'realising they weren't important'. Others changed species and discussed snails instead, their learning of the core practical having pushed to one side the actual context of the question.

Results Plus: Examiner Comments
This response was graded as a Level 1 response, 1/6 total.
Reference was made to the second study – but without evaluating the evidence of that study. A conclusion was made, based on knowledge and understanding of habituation and this is what allowed this response to gain the mark – the candidate describes how, after repeated stimuli, the 'impulse no longer passes over the synapse'.

Results Plus: Examiner Comments
This response was given Level 2, 4 marks. 
This answer evaluates the evidence from both studies, but only goes into detail for the second study, concluding that habituation occurred at the synapse between the two neurones involved in the response. Links are made to biological knowledge to some extent, but there are no details of how the mechanism of habituation occurs.

Results Plus: Examiner Comments
This excellent answer was judged to be Level 3 and gained 5/6 marks.
There was evaluation of both studies, with the main focus on the second study. Conclusions were made about habituation and this was linked to biological knowledge, with descriptions of the calcium ion channels becoming less responsive and less neurotransmitter being released across the synapse after repeated stimulation.
The reason it was not given full marks was due to gaps in the analysis of the first study – if there had been reference to V and W stimulating different neurones, which explains why habituation occurred when it did, this response could have gained full marks.

(ii)
This 'devise an investigation' question was set in the context of the data provided in the first study described in this question and asked candidates to devise an experiment to determine that the frequency of stimulation should be 20 stimuli per minute.
The context was therefore the investigations described previously – using earthworms, not snails. However, this did not stop candidates defaulting to the snail scenario, some even venturing into the use of slugs. To allow for marks to be awarded to correct procedures using the wrong species, marks were given for correct descriptions of the independent variable and the standardisation of the procedure.
This question was very poorly answered, with a third of candidates scoring 0, and less than 10% gaining 3 or 4 marks.
Approaching the question logically, some candidates did identify the independent variable – stimuli per minute – and realised that they would have to test a range of frequencies above and below 20 per minute. Those who had carried out the investigation with snails were also able to suggest some method for standardising the procedure, such as touching the earthworm on the same place each time. Marks were also gained for the idea of repeating with other earthworms of the same size or species.
The marking point seen least frequently was the one for a description of how the dependent variable would be measured – with very few correctly checking for habituation. This suggested that candidates did not understand the point of the experiment. An understanding of the core practicals is essential when it comes to applying those techniques to unfamiliar contexts.

Results Plus: Examiner Comments
This answer gained 1/4 marks.
The only mark achieved was for describing the range of stimuli to provide – with reference to a range from 10-30 stimuli per minute at intervals of 5 stimuli/minute. No other valid details were provided for this investigation.

Results Plus: Examiner Tip
If in doubt when designing an investigation, always try to describe the independent variable and how it will be applied, the dependent variable and how it will be measured. Then try to describe valid variables that will need to be controlled.
Repeats on its own is not worthy of a mark – however, reference to repeats using earthworms of the same size would have gained another mark.

Results Plus: Examiner Comments
This answer was given 3/4 marks, nearly gaining full marks for a good design of a valid experiment. Marks were given for:

  •  controlled variable – size of worm 
  •  range of stimuli frequencies from 5 – 50 per minute, with intervals of 5 
  •  method for measuring habituation described – measuring change in body length.
Although this candidate also referred to keeping the pressure of the stimuli the same – it was not stated that the location of the stimuli should also be kept the same.

Results Plus: Examiner Comments
This is an excellent answer, gaining full marks – 4/4. Marks were given for the following reasons:

  •  same species of worm used 
  •  range of stimuli from 10 to 30 per minute in intervals of 5 
  •  stimuli at the same point of the worm and of the same strength 
  •  body length measured to ascertain the number of stimuli that gave 0mm change

 

Mark Scheme

Q1.