Q1.
No Examiner's Report available for this question

Q2.
 
(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.

Q3.
 
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.

 

Q4.
 
In this experimental design question the investigative context related to the tolerance of tomato plants to toxic lead ions. Generally the responses showed a good understanding of the importance of controlling variables and of repeats, and sensible practical setups were described involving tomato plants grown with different concentrations of lead. In many cases the method involved growing the plants in soil, but use of mineral ion solutions (a Core Practical technique) would allow for tighter control of variables and aid the measurement of the dependent variable (DV). In many cases a specific and measurable DV was not identified, perhaps because candidates were unsure how to measure whether the plant was alive; the examiners were willing to accept any reasonable measurements suggested by candidates, length or mass being typical examples (with a decrease taken to indicate the death of the plant). Most answers focused on the control of variables and the practical details of the method, placing most candidates at level 2.
Many candidates lost sight of the fact that the objective of the investigation was very specific: to determine the tolerance category of the tomato plants. Numerical data regarding lead concentrations was provided so it was expected that these be taken into account, firstly in the experimental method devised and secondly in the clear description of how the results would be used to determine the tolerance category. Those who attempted to refer to the lead concentrations given did not always do so successfully: methods involving four lead concentrations of "2000" were frequently seen, with candidates failing to specify which precise concentration within these ranges should be used. Few candidates produced level 3 answers in which they recognised that testing the boundary concentrations (300, 999 and 2000) was necessary to determine the category. It was also rare to see a clear and specific explanation of how the experimental results would be used to determine the category.

Results Plus: Examiner Comments
This is a level 1 answer, scoring 2 marks. The candidate makes sensible use of tomato plants and lead ions, showing consideration of the context of the question. There is also an appreciation of the need to control variables and the role of repeats. However, there is no clear practical technique described, no indication of appropriate concentrations of lead to be used, and no specific dependent variable identified (measuring 'growth' is not clearly defined). Inclusion of at least one of these features would have helped this answer move to level 2.

Results Plus: Examiner Tip
In an experimental design question, ensure that you give a clear method and identify a measurable dependent variable.
The Core Practicals you carry out during the course allow you to build up a 'toolkit' of practical techniques that you should be able to apply and adapt to new situations.

Results Plus: Examiner Comments
This was a level 3 answer, scoring 5 marks. Tomato plants and lead ions are used appropriately and several variables are controlled. There is an appreciation that an adequate concentration of other necessary ions should be provided, although this would be better controlled by growing the plants in solution rather than in soil. The length of the plant is identified as a measurable dependent variable. Most importantly, the candidate has analysed the data provided and recognised that the concentrations at the boundaries between the tolerance categories (ie. 300, 999 and 2000 ppm) are important in classifying the plant. This is a key feature that identifies this as a level 3 answer, albeit at the weaker end of level 3.

Results Plus: Examiner Tip
Remember that you are expected to be able to analyse and interpret scientific information: you must make good use of data provided.

 

Q5.
Question (a)
Although this seemed to be a straightforward question asking candidates to identify stages of mitosis from a photomicrograph, only just over half of all responses gained full marks.
The main problem appeared to be that many candidates did not check which cells were actually labelled X and Y.
Cell X was often claimed to be undergoing prophase, even though the chromosomes were quite clearly gathered along the equator of the cell.

Results Plus: Examiner Comments
A correct response – gaining 2 marks.

Results Plus: Examiner Tip
Double check the letters on the diagram when asked to label structures or name stages in mitosis.

Question (b)
The vast majority of candidates scored highly on this question, suggesting that the stages of mitosis are generally well known. Most candidates were able to state at least four of the events of prophase and were thus able to achieve full marks. Although most candidates managed to express themselves using appropriate vocabulary, occasionally a lack of precision in language caused problems, e.g. 'nucleus breaks down', rather than 'nuclear membrane breaks down'. The spelling of nucleolus was a challenge to many, with the hybrid 'nucleous' making several appearances. Some candidates did not appear to be able to differentiate between the words 'chromatin', 'centrioles' and 'centromeres'.
There were also some instances of marks being lost due to describing the 'supercoiling' of the DNA.
However, it was obvious that many candidates had thoroughly learnt all the events that occur during the different stages of mitosis.

Results Plus: Examiner Comments
A good answer gaining marks for condensing of chromosomes, break down of nuclear membrane and nucleolus, migration of centrioles to the poles of the cell and the formation of the spindle.
5 possible marks for a maximum of 4/4

Results Plus: Examiner Comments
This is a jumbled answer, indicating a muddled knowledge of the events at the separate stages of mitosis.
Marks can be given for condensing of chromosomes, break down of nucleolus (but not envelope without more detail), and formation of spindle.
However, had this been a QWC question where either spelling or logical sequencing were being assessed, this would not have scored 3 marks – nucleolus is misspelt and the chromosomes are not pulled to the poles during prophase.

Question (c)
A substantial number of responses correctly identified interphase, but very few were able to give a coherent reason; some candidates wrote quite a lot without saying anything useful. Several said the nuclear membrane was present although it wasn't realistically visible on the photo or went on to describe the details of DNA replication or organelle synthesis rather than the visible clues.
Cytokinesis was the most common incorrect response as they thought the cell was splitting. Many candidates were under the impression the cell had only just formed the cell wall between it and the one next to it in the image. Other incorrect responses listed other stages of the cell cycle having looked at the wrong cell in the picture.
There was a clue in the stem of the question, where it was clearly stated 'Cell Z is not undergoing mitosis' and then asked for suggestions for 'which stage of the cell cycle it is undergoing', this really doesn't leave much option if the cell cycle and stages of mitosis are known.

Results Plus: Examiner Comments
One mark for interphase, but nothing for 'all the DNA and organelles appear to be synthesising' – these details are not visible and would not provide a reason for choosing this stage.

Results Plus: Examiner Comments
No marks for telophase.

Results Plus: Examiner Tip
Read the instructions with care – the question states 'cell Z' implying a single cell and not 2 cells.

Results Plus: Examiner Comments
No marks here – again, this candidate has not clearly read the question which refers to a single cell.

Q6.
No Examiner's Report available for this question