GCSE ADDITIONAL SCIENCE / BIOLOGY

Transcription

1 GCSE ADDITIONAL SCIENCE / BIOLOGY BL2HP Report on the Examination 4408 / 4401 June 2013 Version: 1.0

2 Further copies of this Report are available from aqa.org.uk Copyright 2013 AQA and its licensors. All rights reserved. AQA retains the copyright on all its publications. However, registered schools/colleges for AQA are permitted to copy material from this booklet for their own internal use, with the following important exception: AQA cannot give permission to schools/colleges to photocopy any material that is acknowledged to a third party even for internal use within the centre.

3 General Particular problems which occurred quite frequently included: The inability to express ideas clearly and unambiguously Paying insufficient attention to information provided in the stem of a question in order to guide a reasoned response and avoid the inclusion of irrelevant information Repeating, rather than using, information given in the question, for which no marks are available The inclusion of extra, incorrect information in addition to the correct answer Careless reading of the question resulting in an inappropriate answer Limited ability to apply what had been learned to a novel situation Poor understanding of certain topics, such as the digestion of fat and limiting factors in photosynthesis. Question 1 (Standard Demand) This was the first of three standard demand questions common to both the Foundation and Higher Tier papers. (ii) The vast majority of students recognised the terms xylem and phloem as the names of the two tissues in the leaf that are used for transport. Just over half of the students were able to give a satisfactory definition of diffusion. Although most knew that a substance would move from high to low concentration by diffusion, far fewer stressed that it was molecules / particles of the substance that moved. Over two-thirds of students correctly gave oxygen as the gas that would diffuse out of the leaf on a sunny day. Question 2 (Standard Demand) This was the second of the three common questions. While three-quarters of students knew that to investigate the distribution of clover plants in a field the quadrats should be placed randomly, less than one-fifth could explain satisfactorily how this randomness could be achieved. In this section, students had to estimate, from a diagram, the percentage cover of clover plants within a quadrat. The first step involved an estimate of the number of squares covered from the 25 into which the quadrat was subdivided. Approximately two-thirds of students were able to do this and explain how they had counted the squares completely covered and added on an estimate from those partially covered. It was evident that some schools had taught their students that squares with coverage of half or more counted as a whole square while less than half coverage could be ignored. Other students tried to piece together the partially-covered squares the best they could. Either of these methods was considered satisfactory and each should have resulted in an answer of either 7 or 8 squares out of the 25. A common error was to include all the squares in which the clover was simply present rather than to give the cover specified in the question. The final stage was to scale this figure up to a percentage which resulted in answers between 28 and 32 per cent, although allowance was made for calculations 3 of 7

4 based upon incorrect estimates of the number of squares covered. Around two-thirds of students were successful in this exercise. (c) Approximately three-quarters of students were able to suggest an environmental factor that might have affected the distribution of the clover plants, the most common being light, water and nutrients in the soil. Some suggestions, such as carbon dioxide or oxygen levels, were considered unlikely to have varied sufficiently over short distances to have made any difference in the given situation. Question 3 (Standard Demand) This was the third of the three common questions. The vast majority of students read enzyme Z s optimum ph from the graph as 8.6. However, less than half knew that hydrochloric acid was the substance that generated the correct ph for enzyme action in the human stomach, with a little under two-thirds identifying enzyme X (showing an optimum ph of 1.9 in the graph) as the one that would work best in the stomach. In this question, students were required to describe how the human digestive system was able to break down molecules of fat so that they could be absorbed by the body. The quality of written communication was also taken into account in the awarding of marks. This question differentiated very well across the ability range and was generally well answered with 4 to 6 marks being scored by around half of the students. Most knew that lipase was involved and that it converted fat into fatty acids. Many knew that the lipase was produced in the pancreas and / or the small intestine (although the mouth and stomach were common errors). A large proportion knew that bile was involved and perhaps knew that this was made by the liver (rather than the gall bladder), explaining that the bile was alkaline and hence helped to neutralise the acid coming into the small intestine from the stomach (a common error was to suggest that bile entered the stomach to neutralise the acidity). Many knew that the bile emulsified the fat, or described this process as the production of tiny droplets of fat (the breakdown of fat molecules was a common error here). Many students also explained how initially the food could be broken down mechanically by the teeth and how eventually the small molecules of fatty acids (and perhaps glycerol) produced by digestion could be absorbed by the small intestine. However, many students were confused about which processes occurred where, about the name of the enzyme ( amylase was a common error) and about the products of digestion (e.g. amino acids were sometimes suggested). One major mistake, made by a large proportion of students was to misinterpret the question and not limit themselves to the remit of fat digestion. Consequently, these students wasted a great deal of time giving details of carbohydrate and protein digestion which were not required and for which no marks were available. The only penalty incurred was that of wasting valuable time which could have been better devoted to gaining marks elsewhere. Question 4 (Standard Demand) Around three-quarters of students knew that structure A in the yeast cell was the cytoplasm and that B was the cell membrane (cell wall being a common error). Here students were given the word equation for anaerobic respiration in yeast and had to give one way in which the same process in humans was different. Two-thirds of the students knew that humans produced lactic acid, or did not produce alcohol. 4 of 7

5 (c) (i) (c) (ii) Nearly all students could give at least one control variable in the given investigation of carbon dioxide production by yeast, and about three-quarters of these could give two. Suitable answers included the volume of the yeast suspension, the volume and concentration of the sugar solution, and the temperature, all of which were given in the description of the investigation. There was a similar level of success in this part which involved interpretation of four sets of data plotted on a graph. Most appreciated that fructose was the sugar that produced carbon dioxide at the highest rate and hence this was the sugar that was best to use in industry for alcohol production by the yeast. Question 5 (Standard and High Demand) The vast majority of students had no problem with completion of the word equation for photosynthesis by naming the three missing substances. Students were at liberty to use chemical formulae if they wished, but only correct formulae were credited e.g. for oxygen, O 2 is correct, but O and O 2 are not. In this section, students had to describe and explain the effects of two factors, apart from light intensity and the availability of water, that affect the rate of photosynthesis. Most students correctly selected temperature and carbon dioxide, but a minority included one of the factors that were excluded by the question, usually light intensity no marks were available for this. Students were informed that they were at liberty to include one or more sketch graphs in their answers. However, many did not include any graphs and had the much harder task of describing the effects of their chosen factors in a prose account. Those that did include simple, correct graphs often scored 4 out of the 5 marks available just from their graphs. Students found explanations of their described effects to be more challenging. For example, simply to state that carbon dioxide was needed for photosynthesis was regarded as inadequate, but if it was used as a raw material or converted into some organic substance, this was considered to be satisfactory. The explanation for the effect of temperature usually centred on the denaturation of enzymes at higher temperatures. Relatively few students mentioned the effect of speeding up chemical reactions at more moderate temperatures. A few students chose other factors, such as the ph of the soil, the availability of nutrients / ions, the wavelength of the light, the amount of chlorophyll in the leaves and the size of the leaves. These were given credit, but students often found description of the effect and its explanation rather more challenging for these factors. Question 6 (Standard and High Demand) (i) This question required knowledge and understanding of the relationship between the structure of a gene and the structure of the protein (here, an enzyme) for which it coded. As anticipated, only the better students were able to cope with this as they understood that a sequence of bases in the DNA coded for a sequence of amino acids in the protein and hence a change in the one caused a change in the other with a consequential change in the 3-dimensional shape of the enzyme protein causing it not to fit its substrate. In general, there was much confusion about various aspects of this, common errors being that DNA was made of amino acids and that the active site was part of the substrate rather than the enzyme molecule. Two-thirds of students knew that there are 46 chromosomes in human body cells. 5 of 7

6 (ii) (iii) Answers in this part were often poorly expressed, hinting that the student understood, but often gave insufficient information to be worthy of the marks, with less than one-third scoring both marks. It was essential for students to assimilate the information given in the stem of the question, that the boy had actually inherited the mutated allele from his father, in order to deduce that, if the boy did not develop the genetic disorder, then the faulty allele must have been recessive and he must have inherited the dominant, normal allele from his mother. Many students described the boy as being a carrier, which was taken to infer heterozygosity. Nearly all students made some headway with the genetics problem in this part and just over half of students scored full marks. The vast majority drew a Punnett square, which is to be encouraged as this is the clearest presentation. The most common error was to fail to identify which genotype in the offspring caused which phenotype almost onethird of students scored 2 out of the 3 available marks, primarily because of this omission. Question 7 (High Demand) (c) Sufficient information was provided in the stem of this question, and in the diagram, to enable students to deduce that the immobilised enzyme remained attached to the beads in the glass column and thus would not contaminate the product and would still be available for re-use. Many students pointed out that it was a continuous flow process for fructose production, rather than a stop-start batch process. However, fewer than half of the students were able to suggest even one advantage of using the immobilised enzyme. Nearly all students scored 1 mark in this section for deducing from the table that there was no improvement in fructose production at flow rates in excess of 5 dm 3 per minute. Less than one-eighth could amplify upon this by pointing out that higher flow rates would have left unchanged glucose contaminating the fructose in the product, or would simply waste the extra glucose. Many thought a higher flow rate would be a waste of energy, despite the diagram showing that flow was by a gravity feed, hence requiring no energy input. Having been told that fructose was much sweeter than glucose, just over two-thirds of students were able to state that less fructose would be required for the same effect on taste. However, further amplification on the merits of fructose was generally financial rather than biological, with only about one-fifth of students pointing out that a lower sugar content in the slimming food would make it less fattening / lower in calories. Question 8 (Standard and High Demand) The diagrams of the chromosomes for each of the two plant species showed that each chromosome consisted of two strands (i.e. chromatids). A little over a quarter of students were able, in part (i), to explain this in terms of DNA or chromosome replication or copying; for most, each chromosome had simply split in two. The purpose of this process was a little better understood in part (ii), indicating that answers to part (i) were often marred by poor communication skills. Most appreciated that, during cell division, copies of the chromosomes had to pass to each offspring cell so that each cell contained a full set or were genetically identical. Many students forgot the context of the question (the chromosomes had come from root cells in the two plants) and included irrelevant material in their answers about gamete production. 6 of 7

7 (i) (ii) (iii) (c) (i) (c) (ii) The type of cell division used for gamete production, meiosis, was given correctly by two-thirds of students. Some, inevitably, had confused meiosis and mitosis while others spoiled their answers with hybrid spellings. Despite two diagrams showing that species A contained 8 chromosomes and species B 16, many students appeared to see 46; hence the correct numbers for the gametes, respectively 4 and 8, were given by only a little over a quarter of students. The answer accepted in part (iii), for the number of chromosomes in the cells of the hybrid plant, was the sum of the figures given in (ii), which resulted in a doubling of the success rate from the previous section. The evidence given by students for species A and B having evolved from a common ancestor frequently lacked sufficient detail e.g. the flowers look similar instead of a more specific reference to the shape of the petals or the arrangement of the reproductive structures. Similarly, to state that species B had twice the chromosome number of species A gave insufficient evidence as any number of species would have twice the chromosome number of a great variety of other, unrelated species. The diagrams of the chromosomes had been specially constructed using duplicate images so that students could see that they had the same shapes and sizes of chromosomes. Despite these shortcomings, over half the students were able to make at least one valid point. This question was designed to be more demanding, but sufficient information was given in the stem of the question to make it accessible to students who understood the principles of meiosis and could apply them. Thus, having been reminded that chromosomes must pair up for successful gamete production, better students realised that, with three copies of each chromosome this would not be possible and hence viable gametes would not be formed and this would mean that sexual reproduction could not occur. Even with the clues given in the question, only a small minority of students were able to give a complete answer. Some merely repeated that different species cannot reproduce sexually. Others, recognising this as the speciation question, gave an inappropriate answer they had prepared previously about natural selection and isolation. Some even believed that plants do not reproduce sexually. Mark Ranges and Award of Grades Grade boundaries and cumulative percentage grades are available on the Results Statistics page of the AQA Website. Converting Marks into UMS marks Convert raw marks into Uniform Mark Scale (UMS) marks by using the link below. UMS conversion calculator 7 of 7