Topic 3 Practical Skill Exam Questions

Q1.
 
Cell division can be affected by various chemicals. The effects of these chemicals can be studied by observing tissues.
The photograph shows onion root cells undergoing mitosis.

(i)  Draw a line labelled M to one cell at metaphase and a line labelled A to one cell at anaphase.
(2)
(ii)  The rate at which cells are dividing can be determined by calculating the mitotic index.
The mitotic index is the percentage of cells in a sample undergoing mitosis.
The table shows the number of cells at different stages of the cell cycle in one sample.

Calculate the mitotic index for the sample of cells shown in the table.
(2)

Answer ........................................................... %
 
(Total for question = 4 marks)

Q2.
 
* There are many products that may increase the ability of sperm to swim. One of these products contains zinc sulphate.
Devise an investigation to obtain valid data to show whether taking zinc sulphate tablets affects the ability of sperm to swim.
(6)
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(Total for question = 6 marks)

Q3.
 
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)

Q4.
 
DNA synthesis in bacterial cell cultures has been investigated.
In an experiment, a mixture of radioactive bases was added to a culture of dividing bacteria.
The results are shown in the graph.

(i)  Calculate the fastest rate of uptake of bases by these bacteria.
(2)

Answer ........................................................... 
(ii)  Deduce how many times the bacteria in the culture have divided during this experiment.
(2)
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(iii)  Explain why the experiment would be improved if all the bases were provided but only the thymine was radioactive.
(2)
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(Total for question = 6 marks)

Examiner's Report

Q1.
 
(i)
The vast majority of candidates gained both marks for this question.

Results Plus: Examiner Comments
This response gets both marks for correctly identifying the cells in each stage.

Results Plus: Examiner Tip
Labelling onto a diagram can be unclear. Make sure you follow the guidance in the question.

Results Plus: Examiner Comments
The candidate has made a clear indication of the correct cells and is awarded 2 marks.

(ii)
Most candidates correctly identified the stages of mitosis and therefore were able to gain full marks. The difficulties were often in rounding up too far or incorrectly, although some candidates neglected to multiply by 100.

Results Plus: Examiner Comments
This candidate rounded up their answer to 13 and lost the final mark. Thankfully they gained one mark as they showed how they arrived at the answer.

Results Plus: Examiner Tip
Always show your working, questions like this gain marks for each stage in the answer.

Results Plus: Examiner Comments
A good response, this candidate has also shown that they have given their answer to 3 significant figures. Both marks were awarded.

 

Q2.
No Examiner's Report available for this question

Q3.
 
(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.

 

Q4.
 
(i)
Candidates generally coped well with this calculation of a rate. The main reason for not gaining both marks was for the omission of units with the final answer.
Because candidates needed to judge where to draw a gradient, answers between 1.6 and 2 au min-1 were accepted.

Results Plus: Examiner Comments
This response gained one mark. The numerical value calculated was correct but the candidate did not include any units.

Results Plus: Examiner Tip
Always provide suitable units with the answers to a calculation.

(ii)
Candidates thinking about this question and part (iii) realised that thymine is incorporated into DNA and not RNA so gained bullet point 1. Some went on to explain why this would improve the experiment. Many candidates did not appreciate the significance of thymine. Some candidates produced answers focussing on determining the ratios of thymine to adenine while others made statements about improved specificity or accuracy.

Results Plus: Examiner Comments
In this response, the candidate correctly suggests the bacteria divided twice and gains one mark (BP1). However, instead of then explaining why this was the case they describe when the bacteria were dividing. To gain the second mark they need to explain why they could say the bacteria were dividing twice, e.g. because there were two periods of increase in base uptake.

Results Plus: Examiner Comments
This response gained both available marks.

(iii)
Candidates thinking about the question realised that thymine is incorporated into DNA and not RNA so gained bullet point 1. Some went on to explain why this would improve the experiment. Many candidates did not appreciate the significance of thymine. Some candidates produced answers focussing on determining the ratios of thymine to adenine while others made statements about improved specificity or accuracy.

Results Plus: Examiner Comments
This response gained both marks. The second sentence was accepted for bullet point 3.

 

Mark Scheme

Q1.

 

Q2.

Q3.

 

Q4.