Topic 4 
Core Practical Exam Questions

7. Investigating plant mineral deficiencies 
8. The strength of plant fibres 
9. Investigate the antimicrobial properties of plants, including the aseptic techniques for the safe handling of bacteria

Q1.
 
Copper ions in soil can affect cell division in roots in plants.
Copper ions affect bonds in proteins, changing their shape. If the proteins are enzymes this may change the shape of the active site.
The effect of copper ions on the growth of mustard seedling roots was investigated. Mustard seeds were germinated on filter paper soaked in copper sulfate solution. These solutions provided copper ion concentrations ranging from 0 parts per million (ppm) to 200 ppm.
After 5 days, the lengths of the roots were measured.
The results are shown in the graph below.

(i)   Analyse the data to explain the effect of copper ions on the growth of roots.
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(ii)   Explain how this investigation could be improved.
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(Total for question = 6 marks)

Q2.
 
Mineral ions in the soil affect the growth of the peanut plant, Arachis hypogaea.

The effect of mineral ions on the production of fruit by these plants was investigated.
Young peanut plants were grown in soil containing all the mineral ions required.
After one week, 10 of these plants were moved into soil without calcium ions. Another 10 plants were moved into soil without magnesium ions.
Ten plants were left in the original soil.
After leaving the plants to grow, the mean number of flowers per plant and the percentage of these flowers that formed fruit were recorded.
The results are shown in the table.

Devise an investigation to determine the effect of nitrate ion concentration on the growth of young peanut plants.
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(Total for question = 5 marks)

Q3.
 
Plant fibres and oil-based plastics have both been used to make ropes.
Climbing ropes can be made from manila (a plant fibre) or nylon (an oil-based plastic). Various factors affect the tensile strength of these ropes, including diameter and storage conditions.
The table shows the effect of rope diameter on tensile strength.

It is claimed that the ideal storage conditions for rope are 18 °C and 60% relative humidity.
Devise a valid investigation to determine the effect of storage temperature on the tensile strength of manila rope and nylon rope.
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(Total for question = 4 marks)

Q4.
(a)  A student used the apparatus shown in the diagram below to investigate the tensile strength of plant fibres.

She added a 50 g mass to the middle of the fibre and measured distance X. She repeated this by adding additional 50 g masses.

The results are shown in the table below.

(i)  Describe the effect on distance X of increasing the mass.
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(ii)  Another student thought that the data did not show the mass needed to break the fibre. He suggested that it could be anywhere between 200 g and 250 g.

Suggest how a more accurate result could be determined.
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(b)  Suggest how you would use this apparatus to enable a valid comparison of the tensile strength of fibres from two different plants.
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(Total for Question = 9 marks)

Q5.
 
Some plants, such as garlic, show antimicrobial properties.
(a)
Explain what is meant by the term antimicrobial properties.
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(b)
The flow chart below shows some stages in a practical experiment to investigate the antimicrobial properties of garlic.

(i)
Suggest why the bacteria are mixed with the agar in stage 1.
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(ii)
Explain why the Petri dish must be sterile at stage 2.
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(iii)
In stage 3, the lid is taped on using two pieces of sticky tape as shown below.

Explain why the two halves of the Petri dish are not completely sealed with sticky tape.
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(iv)
Suggest a suitable temperature for the safe incubation of the agar plates in a school laboratory. Give an explanation for your answer.
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(v)
The diagrams below show the Petri dish at the beginning and end of stage 4.

Describe and explain the results shown at the end of stage 4.
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(Total for question = 11 marks)

Q6.
 
Many plants produce chemicals that have antimicrobial properties.
The suitability of using wood from three types of tree to make kitchen chopping boards was investigated.
In this investigation, 50 cm3 of a bacterial culture was added to 100 g of wood chippings from each of three types of tree.
The growth of bacteria was measured at the start (Day 0), and then after 1, 2, 4 and 8 days.
The results are shown in the graph.

(i)  Describe the antimicrobial properties of these three types of wood.
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(ii)  It was concluded from this investigation that spruce caused bacterial growth to decrease at a constant rate after the first day.
Criticise this conclusion.
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(iii)  Explain how this investigation could be improved to provide additional data to determine whether pine or larch have greater antimicrobial properties.
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(Total for question = 8 marks)

Q1.
No Examiner's Report available for this question

Q2.
 
This question was not particularly novel and allowed candidates to achieve high marks by recounting simple experimental technique in the correct context. Strong responses gave consideration to the needs of growing young plants. Responses recounting a practical involving germinating seeds were not able to score highly. Some responses gave no details instead inviting the reader to devise ways of controlling variables themselves and therefore could not score marks.

Results Plus: Examiner Comments
This response shows an acceptable range of five concentrations as well as a control of 0%. Frequently candidates gave 0% as one of the concentratons and were not awarded the mark.

 

Q3.
 
This question required the design of a valid investigation to determine the effect of storage temperature on tensile strength of rope.
There were many good, well thought out investigations that considered all aspects, including the independent and dependent variables. However, some candidates did not take into account the word 'storage' and described carrying out the experiment in different temperatures. Many varied both temperature and humidity, rather than picking out that the humidity should be controlled and kept at 60%. Many candidates gave an appropriate range of temperatures, either side of 18°C, but a few did not go above 18°C. Some students referred to keeping the fibres in a water bath; whilst this was not penalised in this question, students should be reminded to think of investigations from a practical viewpoint and make sure that the method they describe is realistic.
Many candidates considered controlled variables and referred to keeping the diameter of the rope the same but did not refer to the length. Meanwhile, some referred to using the same species of nylon rope!
It was evident from the answers seen which candidates had taken an active part in the core practical concerning testing of tensile strength. It is imperative that students do carry out the core practicals.

Results Plus: Examiner Comments
This response was given 3/4 marks. Marks were given for:

  •  a range of temperatures above and below 18°C; 
  •  ropes kept at the different temperatures for 24 hours; 
  •  masses added until rope broke.

Results Plus: Examiner Tip
No mark for fibres being of the same diameter without mention of same length.

Results Plus: Examiner Comments
This response was given 4/4 marks. Marks were given in the following sequence:

  •  range of temperatures from 10 – 20°C; 
  •  humidity of 60% in storage area; 
  •  masses added until fibre broke; 
  •  controlled variables described as diameter and length of fibre.

Results Plus: Examiner Tip
A diagram can help make it clear what is being described and can also help the candidate to visualise the practical procedure, making it easier to then describe in words.

 

Q4.
Question (a)(i)
Many candidates seemed to be trying to recall their own results from when they carried out this investigation rather than looking at the table, as their descriptions didn't always reflect the data provided. Another issue was simply quoting data from the table instead of organising it into trends and then those who got the trends right often failed to provide relevant data to support them. Typical responses said that as mass increased, distance X increased 'up to a point', but didn't state what that point was. It was often stated that 'as mass increased so did distance until 250g when it broke'.
Most candidates did not appreciate that the distance only increased up to 150g, however, most were able to state that the distance remained constant from 150g to 200g. Also many candidates incorrectly stated that the greatest increase was from 50g to 100g because it doubled in length.

Results Plus: Examiner Comments
A typical response that gained no marks. A general statement that does not describe the effect on distance X of increasing the mass.

Results Plus: Examiner Comments
No marks for a general statement followed by repetition of the first two sets of data from the table. No trend has been identified or described.

Results Plus: Examiner Comments
No mark for stating that the distance X increases with mass up to 250g, but one mark is given for '150g and 200g both had a distance of 5cm'.

Results Plus: Examiner Tip
Check the data with care – make sure that sweeping statements are correct. A close look at the table of data shows no further increase above 150g.

Results Plus: Examiner Comments
A good response – one mark for 'as mass ... increased from 0g to 150g the distance X was increased' and one mark for 'however from 150g to 200g the distance X did not increase any further'.

Question (a)(ii)
This was very well answered by the majority of the students. Some excellent answers were seen which suggested they started with 10g increments and then set about refining the result with 1g increments.
Some gained 2 marks by stating that the experiment should be repeated at masses including 210g, 220g, 230g, 240g and 250g.
A minority suggested measuring distance X to the nearest mm, which indicated poor interpretation of the question. Others suggested using a Newton meter and one inventive candidate described attaching a beaker of water to the fibre and gradually adding more water until the fibre broke.

Results Plus: Examiner Comments
This response gains one mark for suggesting the use of 10g masses.

Results Plus: Examiner Comments
This answer gains both marks – one for using masses of 10g, and the other mark for using these smaller masses after 200g has been added.

Question (b)
Few candidates achieved full marks on this question, with candidates making the same point several times rather than being able to give a well-balanced description; however, many earned 4 out of 5 marks. All marking points were seen, but not usually within one response. Generally, the question was answered well by the majority of candidates, with marks being gained for description of variables to be controlled, standardised method of applying masses and repetition. Although repeats were often mentioned many forgot that these allowed mean results to be calculated, although several candidates 'created' averages!
Most candidates got the idea that masses should be added until the fibre breaks, although many talked about measuring extension only. Named procedures were very vague - usually restricted to 'use same' masses or same distances. A new mass was also spotted in a number of papers, 'the 0 gram weight'.

Results Plus: Examiner Comments
This response just gains 4 marks, although the points are not always clearly made.
Marks awarded: mp3 for adding the same masses each time to the middle of the fibre; mp4 for adding masses until the fibre broke; mp7 for wearing goggles to protect the eyes and mp5 for reference to repeats being used to calculate means.
mp1 was not given, although there was a reference to fibres being of the same length, there needed to be two relevant variables about the fibres to gain the mark and stating that they should be from the same plant is not relevant in an investigation comparing 2 different plants!

Results Plus: Examiner Comments
This is a well written response, clearly describing the procedure.
Marks awarded as follows: mp1 for fibres of same length, diameter and part of plant extracted from; mp3 for 10 g masses used; mp4 - recording mass taken to break fibre; mp5 repeats to calculate averages and mp2 for keeping temperature constant.

Q5.
 
(a) It was surprising how many candidates got this question wrong, appearing mystified by the phrase or incapable of explaining it succinctly. Many described antimicrobials as fighting the bacteria (not saying they killed or destroyed the bacteria). There were also a large number of responses along the lines of 'the plants are protected against microbes' without explaining how.
There were relatively few correct references to antimicrobials either inhibiting the growth of microbes or killing them.

Results Plus: Examiner Comments
An example of 'fighting against bacteria' - which doesn't really say anything and was not awarded the mark.

Results Plus: Examiner Tip
Precise language gains marks. Antimicrobials either kill microorganisms or inhibit their growth

Results Plus: Examiner Comments
A good answer explaining how plants produce toxins that kill or inhibit the growth of bacteria.

Results Plus: Examiner Comments
Clearly phrased answer covering both marking points.

(b) (i) There were two alternative types of answer acceptable here - candidates could either refer to the provision of nutrients by the agar, allowing the bacteria to grow, or to the even distribution of bacteria throughout the agar.
Incorrect responses were those that tended to be too vague, e.g. agar is a good medium for growth (without saying why) or focusing on how it would make it easier to see how the bacteria worked.
Many candidates achieved this mark.

Results Plus: Examiner Comments
This is typical of the type of vague answer that doesn't really say anything and doesn't gain marks. It is not the standard of answer required at this level.

Results Plus: Examiner Comments
This response shows a familiarity with the procedure and an understanding of what is required from the question.

(b) (ii) This question was accessible to the majority of candidates, with most gaining at least one mark and many two. The majority were able to recognise the idea of contamination, although weaker candidates often referred to contamination by chemicals or dust. In fact, most candidates did not specifically refer to contamination, although they had got the idea that bacteria were being inhibited from growing. Fewer discussed competition, but there were plenty of references to the growth of pathogenic bacteria.
Some candidates understood the term 'valid', but there were many answers of the line of 'it is more accurate, reliable, valid and a fair test'. There are still a large number of candidates confused about the differences between reliability, validity and accuracy. It may be best to think of an investigation or procedure as being valid, results being reliable and measurements being accurate. There were more references to this precaution producing reliable results, than references to validity.

Results Plus: Examiner Comments
Good answer that covers 3 marking points - killing any micro-organisms that could compete with the bacteria and keeping the test valid.

Results Plus: Examiner Comments
This answer clearly address prevention of contamination and reduced competition to gain 2 marks.

(b) (iii) There were some surprising answers to this question. Considering that this question is based on a core practical, it is expected that candidates should be familiar with the procedures and understand why Petri dishes are taped the way they are. There were many vague responses about avoiding condensation or protecting humans from bacteria getting out, saving money as less tape was used or needing to see inside. There were also references to making it easier to open the dishes after incubation - a practice that should be avoided in school laboratories.
When plates are incubated they are often turned upside down to prevent condensation from dripping onto the surface of the agar, and although candidates may not be aware of this, they should know that minimal taping is to ensure the entry of oxygen to allow aerobic respiration by the bacteria being cultured and to prevent the development of anaerobic conditions that could encourage the growth of unwanted pathogenic bacteria.

Results Plus: Examiner Comments
This is an example of the common misconception that the bacteria will become harmful if allowed to grow in anaerobic conditions. Either that or poor wording conveying a different meaning than the candidate intended. 1 mark.

Results Plus: Examiner Tip
Be very careful with phrasing - 'this could make the bacteria a lot more harmful' implies that the bacteria being grown become harmful and not that anaerobic conditions encourage the growth of harmful bacteria.
Poorly worded answers lose marks, even when the candidate knows what they mean.

Results Plus: Examiner Comments
A good example of a 2 mark answer - this candidate understood that oxygen has to enter in order for the bacteria to respire aerobically.

Results Plus: Examiner Comments
An example of a candidate whose grasp on aseptic technique is worrying. Petri dishes in which bacteria have been cultured should not be opened in school laboratories. No marks.

Results Plus: Examiner Comments
Despite references to 'aerobic' and 'anaerobic', this response seems to be written by someone completely unfamiliar with the core practical and scored no marks.

(b) (iv) Although this question appeared to be very accessible, it drew a surprising number of poor answers. There were quite a few suggestions of 37 ºC (for 'maximum growth rate' as body temperature was identified as optimum for growth), or ranges from 20 - 40 ºC. A large number of candidates thought that the bacteria being cultured would automatically become pathogenic once temperatures exceeded 30ºC. Many stated that if the temperature was too high it would cause denaturation. Some had the idea that a lower temperature was necessary to prevent the growth of pathogenic bacteria, without reference to body temperature.

Results Plus: Examiner Comments
Typical answer gaining one mark for suggestion of a suitable temperature. However, the second mark cannot be awarded without referring to the temperature at which the growth of pathogenic bacteria would be encouraged.

Results Plus: Examiner Comments
This response gained 2 marks, appreciating the need for a temperature that would allow bacteria to grow, whilst remaining below 35ºC as that temperature would be close to human body temperature which would encourage the growth of harmful bacteria.

Results Plus: Examiner Tip
Read the question - if asked to provide an explanation, make sure you do - it's worth marks that won't be awarded unless an explanation is given.

Results Plus: Examiner Comments
This candidate has thought of the need for a temperature at which enzymes in the garlic would be active, so they haven't really considered the idea of incubation of bacteria. Also the temperature suggested of 121ºC would not only denature the enzymes, but also kill the bacteria. This response gained no marks.

Results Plus: Examiner Tip
Understand the range of temperatures encountered by living systems - it is basic knowledge that water freezes at 0ºC and boils at 100ºC -and as living organisms are mostly made of water, it applies to them too.

(b) (v) Most candidates correctly identified the clear area or correctly referred to the zone of inhibition and then went on to state that the garlic had inhibited bacterial growth or killed the bacteria. Many said that garlic had antimicrobial properties - as stated within the information provided in the question - but didn't explain that it had killed or inhibited the growth of the bacteria. Fewer pointed out that no bacteria were in the clear zone and only the best responses referred to diffusion. Most candidates were able to gain at least one mark.

Results Plus: Examiner Comments
A good example - this gained full marks. One mark for reference to 'inhibition zone', one for diffusion of extract out of the disc, one for extract being responsible for killing or inhibiting the growth of the bacteria. Another mark could also have been given for stating that there were no bacteria around disc A.

Results Plus: Examiner Comments
An example of a candidate writing a lot, but not saying very much.
This response gained no marks. Nothing could be given for reference to 'antimicrobial ring'.
The last sentence referring to bacteria being produced from disc A is confusing and suggests that this candidate did not re-read their answers to make sure they made sense.

Q6.
 
(i)
Most responses focussed on spruce being less effective than pine or larch. Few found it worthy of note that pine and larch seemed to have equal properties. A few candidates misinterpreted the data and rated spruce as the highest antimicrobial properties as growth continued to fall throughout the experiment.
(ii)
Candidates who appreciated the non-linear time scale were able to pick up marks here. Many candidates ignored the data and instead gave general responses about lack of repeats, lack of information regarding sample size or an explanation of how variables were controlled.
(iii)
This question was well answered by A grade candidates although few marks were awarded to many candidates. Although asked for improvements, too many responses described a different investigation based on measuring zones of inhibition.
There were few responses seen that explained the reason for their suggested improvement.

Results Plus: Examiner Comments
This response gains no marks as it refers to making the results more reliable. The question asks to provide additional data.

 

Mark Scheme

Q1.

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Q4.

Q5.

Q6.