TOPIC 3 (and 4) 
Core Practical 
Exam Questions

Core Practical 5: 
Prepare and stain a root tip squash to observe the stages of mitosis

Core Practical 6: 
Identify sclerenchyma fibres, phloem sieve tubes and xylem vessels and their location within stems through a light microscope

Questions

Q1.
 
Cell division can be affected by various chemicals. The effects of these chemicals can be studied by observing tissues.
Herbicides are chemicals used to kill weeds. Some herbicides work by disrupting the cell cycle.
An investigation was carried out to study the effect of different concentrations of the herbicide Agil on mitosis in onion root tips.
The roots of onions were exposed to different concentrations of Agil for 24 hours.
Root tip squashes were then prepared and the mean mitotic index was calculated for each Agil concentration.
The results are shown in the table.

(i)  Describe the effects of Agil concentration on mitosis in onion root tips.
(2)
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(ii)  Devise an investigation to determine the effect of exposure time to Agil on the rate of mitosis in onion root tips.
(6)
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(Total for question = 8 marks)

Q2.
 
A student prepared a root tip squash to view stages of mitosis.
The diagram shows the drawing made by the student.

(a)  The ratio of cells in the drawing showing anaphase compared to those showing metaphase is
(1)
   A    2:1
   B    1:2
   C    1:1
   D    3:1

(b)  The mitotic index is the percentage of cells undergoing mitosis.
The student calculated the mitotic index at different distances from the root tip. 
She used this information to plot the graph shown.

Calculate the distance from the root tip of the cells in the student's drawing.
(2)

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

(Total for question = 3 marks)

Q3.
 
The root tip squash procedure can be used to observe cells undergoing mitosis.
(a)  Explain the role of mitosis in the development of roots.
(2)
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(b)  (i)  The flow chart below describes the stages involved in staining a root tip squash to show mitosis.
Complete the flow chart by writing the most appropriate word or words on the dotted lines.
(3)

(ii)  Describe one safety precaution that should be taken when carrying out this procedure.
(1)
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*(c)  Following mitosis, some cells undergo differentiation to become specialised tissues, such as xylem.
Explain how cells differentiate to become specialised tissues.
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(d)  Similar staining techniques can be used to observe cells undergoing meiosis.
(i)  Name a process that may be observed in cells undergoing meiosis but not mitosis.
(1)
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(ii)  Explain how meiosis can give rise to genetic variation in the gametes produced.
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(Total for question = 13 marks)

Q4.
 
The electron micrograph shows a cross-section through part of a vascular bundle, containing phloem tissue.

A sieve tube element is labelled 'ST'. A cell called a companion cell is labelled 'CC'.
Calculate the magnification of this image.
(1)

 Answer: × ........................................................... 
 
(Total for question = 1 mark)

Q5.
 
Plant fibres and oil-based plastics have both been used to make ropes.
Tissues that contain lignin provide strength to plant fibres.
Describe the positions in the stem of those tissues that contain lignin.
(2)
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(Total for question = 2 marks)

Examiner's Report

Q1.
 
(i)
This question scored strongly on the first marking point but the second was rarely awarded. Significance of data is a well explored idea that seems to be overlooked.

Results Plus: Examiner Comments
Although this candidate was almost there they stopped just short of a second mark. Although they considered the SE values given in the table, and referred to significance, they did not correctly identify an overlap.

Results Plus: Examiner Tip
Although a larger standard deviation does indicate a potential lack of accuracy in a mean, always look for overlaps with adjacent values.

Results Plus: Examiner Comments
This starts as a text book answer, comparing variables, as one increases, the other decreases. The candidate then gives the correct (negative) correlation. After that they have then simply quoted data. 1 mark was awarded.

Results Plus: Examiner Tip
To gain marks using data you will need to do more than just read them from a table. A graph maybe, but not a table.

(ii)
Although this was a standard question regarding a well-established core practical there were few candidates who obtained all 6 marks.
There were some well-described methods that neglected to change the correct variable, exposure time, instead describing a change in concentration. Another rarely seen mark was the counting of cells undergoing mitosis, many attempting to 'observe the mitotic index' or simply 'looking at the cells'.

Results Plus: Examiner Comments
This response shows a common problem. The candidate has used the example from the previous question (effect of concentration) rather than the exposure time as requested.

Results Plus: Examiner Tip
Always make sure you have the correct independent variable before you start.

Results Plus: Examiner Comments
This candidate showed an understanding of the core practical method and has adapted it to the scenario. They obtained full marks, giving excellent detail.

Results Plus: Examiner Tip
Always try to keep a logical sequence when explaining practical technique.

 

Q2.
No Examiner's Report available for this question

Q3.
 
(a)
Although the majority of candidates grasped the question, they were not always able to provide AS level detail in their answers, often simply stating 'cell division for growth and repair'. Surprisingly few discussed the idea of increasing cell number. Only a small proportion of candidates gained the mark for replacing dead cells or repairing damaged tissue – a common error being to refer to the repairing of damaged 'cells' instead of damaged 'tissues'. Some referred to the 'formation of identical cells' without describing these cells as genetically identical. There were also many references to asexual reproduction, probably as a consequence of describing the role of mitosis generally instead of in the context of root growth, as directed in the question. Some candidates focussed on the role of roots, showing that they had not understood the question.
Candidates need to be aware of the fact that answers learnt from prior mark schemes usually lose marks if the context of the current question is not taken into account.

Results Plus: Examiner Comments
This is typical of the type of response that is too vague and imprecise to gain marks.
If the student had referred to new root 'cells' they would have gained one mark.

Results Plus: Examiner Comments
This example gained one mark for explaining that mitosis allows cell division and the production of more cells.

Results Plus: Examiner Comments
This response gained 2 marks for reference to cell division and the production of genetically identical cells.

(b)(i)
Considering that the 'root tip squash' is a core practical, there were many very disappointing responses to this question. Many candidates were unable to name a microscope slide and a coverslip, and/or confused a microscope slide with a watch glass or Petri dish. Correct spelling of a suitable stain, or even a reasonable suggestion of one, was surprisingly rare. Overall this question was poorly answered, it not only challenged spelling, it also demonstrated that common equipment could not be identified. It also appears to indicate that there are candidates who had not carried out this core practical, a requirement of the course.

Results Plus: Examiner Comments
This answer gained two marks – the first for identifying hydrochloric acid and correctly stating its function in the root tip squash practical, and the second mark for 'osin' accepted as a phonetically correct attempt to spell orcein.
We were lenient on the spelling of stains in this question as it was not Quality of Written Communication(QWC) assessed and the focus was on learnt practical skills.
However, 'viewing lens' is not the equivalent of a 'cover slip'.

Results Plus: Examiner Comments
There were many answers like this, where the candidate knew the theory – the fact that hydrochloric acid was used to soften the tissue and that an acceptable stain was acetic orcein, but had no idea of the names of basic laboratory equipment. This really does suggest that either the practical was not carried out, or that the student has not learnt the names of standard items such as a microscope slide.

(b)(ii)
Candidates cannot expect to score marks with generic answers such as "wear safety glasses" without some justification. Most answered correctly with reference to protecting the eyes from the acid or stain, although some inadvertently spelt 'goggles' as 'googles' that we allowed as a slip of the pen. It must be noted that the wearing of a lab coat to 'protect clothing' is not a safety precaution, it is a laundry consideration – however, if linked to keeping acid from spilling onto the skin, credit could be given.
A safety precaution is one that protects the individual carrying out the experiment from harm.

Results Plus: Examiner Comments
There were a few who were concerned for the safety of the root tip rather than themselves.

Results Plus: Examiner Tip
Remember – a safety precaution is one that protects you!

Results Plus: Examiner Comments
The wearing of lab coats cropped up frequently – although they do indeed keep your clothes clean and protected, this does not count as a safety precaution!

Results Plus: Examiner Comments
This form of protective clothing does gain a mark for linking the wearing of gloves to preventing burns caused by the acid.

(c)
There were many good answers to this question, with answers to questions concerning differential gene expression clearly well 'rehearsed'. However, a few misconceptions still remain, mRNA is not transcribed (DNA is transcribed to produce mRNA) or DNA translated (the mRNA is translated at the ribosome), genes are activated or switched on (a standard phrase, linked to gene switching) they are not turned on. Although most candidates knew that cells are stimulated to become specialised, they were unspecific about the nature of said stimulus – only the better answers referred to a chemical stimulus. Poor expression cost marks here – and this was a question assessed for QWC.

Results Plus: Examiner Comments
Yet again, a bullet pointed response clearly sets out the answer to the question.
Note that the student does refer to mRNA being transcribed, a common mistake that can be ignored if the context is correct – in this case the fact that transcription is described in relation only to the genes that are switched on.

Results Plus: Examiner Tip
Bullet points are a good way to arrange your answers to questions that ask you to describe a sequence of events.

Results Plus: Examiner Comments
This is a good answer which refers to a chemical stimulus activating some genes, transcription at the active genes and then describes translation correctly. This answer also takes into account the context of the question in the description of specialised cells forming a specialised tissue.

(d)(i)
The key to this question was the word 'observed' – there were many answers concerning events that happened during meiosis but not mitosis, however, not all can be observed using a microscope. There were also a number of incorrect responses, concerning events that occur in both mitosis and meiosis, such as interphase or cytokinesis. 'Crossing over' was by far the most common correct answer seen.

Results Plus: Examiner Comments
Although independent assortment does occur in meiosis but not in mitosis, it is not readily observable under the microscope.

Results Plus: Examiner Comments
Although it is the chromatids rather than the chromosomes which cross over, the mark was given for the process of crossing over being visible.

(d)(ii)
Naming the two processes of 'crossing over' and 'independent assortment' was common, and although it seems that candidates tend to have the right sort of idea about what they are, clear descriptions were less frequently seen, especially for independent assortment. Many responses did not explain how the random assortment would produce variation in the gametes. Vague details concerning the exchange of 'genetic material' do not gain credit, whereas exchange of pieces of chromatid, or DNA, or indeed alleles between chromatids is far preferable. There were also, as usual, many incorrect discussions of fertilisation as one of the features of meiosis. Random fertilisation does give rise to genetic variation, but it is not a consequence of meiosis.

Results Plus: Examiner Comments
Although this response initially appears to contain enough relevant details to gain at least one mark, the context is incorrect and the details too vague to gain credit. Exchange of some of 'their genetic information' is not enough – DNA would be better.
Also this response implies that it is crossing over that leads to independent assortment which is incorrect.
Descriptions of 'many combinations' needs to be qualified – e.g. different combinations of alleles are created.

Results Plus: Examiner Comments
This answer gains 2 marks – one for mentioning both crossing over and independent assortment and another for correctly describing crossing over, with reference to chromatids.

Results Plus: Examiner Comments
One mark for mentioning both crossing over and independent assortment and a second mark for correctly describing how independent assortment generates genetic variation.
The incorrect details of crossing over were ignored.

Q4.
 
When the paper was written this was intended as quite an accessible question, since the labelled scale bar was provided below the micrograph. However, very many candidates seemed completely confused as to how to tackle the question – perhaps because they had not previously practised working with a scale bar. Various different features of the image were measured (unnecessarily) and diverse erroneous calculations led to answers ranging over 20 orders of magnitude. Answers less than 1 were frequently seen; in these cases a sense-check should have suggested a mistake, since it is fairly clear that the image has been magnified rather than reduced. Candidates are advised to practice magnification calculations of a variety of different types.

Results Plus: Examiner Comments
This candidate has correctly measured the scale bar below the micrograph, but unfortunately did not convert the value of 1.4cm into 14000μm – so the answer is 1000 times too small.
This was just a 1-mark question so there was no mark available for working, therefore this answer scored 0 marks.

Results Plus: Examiner Tip
When carrying out a calculation, be sure to take account of the units of measurement.

Results Plus: Examiner Comments
In this example the candidate has converted 5μm to 0.005mm before carrying out the division. This is an equivalent, valid method and the answer is correct, gaining 1 mark. The candidate has set out extremely clear working, allowing the examiner to follow exactly what has been done.

 

Q5.
 
This question tested knowledge of which tissues in a plant contain lignin and the position of these tissues in the stem. Many candidates correctly identified the tissues, but then went onto describe the location of lignin in those tissues, therefore not gaining any marks.
Many candidates knew that lignin is found in xylem and sclerenchyma, though phloem was also mentioned. There was a wide range of descriptions of its location with some very precise answers getting full marks, but vague references such as 'in the stem' or 'outer edge of stem' did not gain marks. Some answers were often lacking in detail and did not refer to vascular bundles. Many candidates also used vague terms like 'in front of', which is completely subjective and depends on the aspect of the viewer. Other candidates just went on to give functions of the xylem and/or sclerenchyma demonstrating the importance of reading questions carefully to ensure the response given is to the question being asked.
An accurately annotated diagram was given credit in this question – and it can help students if they do this to help describe biological structures.

Results Plus: Examiner Comments
This response gained 0/2 marks. Although there has been a valiant attempt at drawing a labelled diagram, the relative positions of the tissues labelled are unclear. There is also no mention of vascular bundles.

Results Plus: Examiner Tip
If drawing a diagram, label it fully and make sure it is clear where the label lines are pointing. In this case drawing the vascular bundles slightly larger would have helped, especially if it was clear that the xylem was nearest the centre of the stem and not nearer the outer edge than the phloem.

Results Plus: Examiner Comments
This answer gained 1/2 for describing the location of the sclerenchyma as 'outside the phloem'. Although the candidate obviously knew the answer, they lost a mark by missing out the word 'vascular' in the phrase 'xylem is the innermost part of the bundle'.

Results Plus: Examiner Tip
Use vocabulary with care – learn the terminology. In plants the tissues are inside vascular bundles.

Results Plus: Examiner Comments
This answer gained 2/2 marks. It is clear that the lignified tissues are correctly identified and their location in the vascular bundles is described with precision.

 

Mark Scheme

Q1.

 

Q2.

Q3.

Q4.

 

Q5.