MARK SCHEME – AS BIOLOGY – 7401-2

AS BIOLOGY 7401/2 Paper 2 Mark scheme June Version: 1.0 Final MARK SCHEME – AS BIOLOGY – 7401/2 – JUNE 2 Mark schemes are prepared by the Lead Assessment Writer and considered, together with the relevant questions, by a panel of subject teachers. This mark scheme includes any amendments made at the standardisation events which all associates participate in and is the scheme which was used by them in this examination. The standardisation process ensures that the mark scheme covers the students’ responses to questions and that every associate understands and applies it in the same correct way. As preparation for standardisation each associate analyses a number of students’ scripts. Alternative answers not already covered by the mark scheme are discussed and legislated for. If, after the standardisation process, associates encounter unusual answers which have not been raised they are required to refer these to the Lead Assessment Writer. It must be stressed that a mark scheme is a working document, in many cases further developed and expanded on the basis of students’ reactions to a particular paper. Assumptions about future mark schemes on the basis of one year’s document should be avoided; whilst the guiding principles of assessment remain constant, details will change, depending on the content of a particular examination paper. Further copies of this mark scheme are available from 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. MARK SCHEME – AS BIOLOGY – 7401/2 – JUNE 3 Level of response marking instructions Level of response mark schemes are broken down into levels, each of which has a descriptor. The descriptor for the level shows the average performance for the level. There are marks in each level. Before you apply the mark scheme to a student’s answer read through the answer and annotate it (as instructed) to show the qualities that are being looked for. You can then apply the mark scheme. Step 1 Determine a level Start at the lowest level of the mark scheme and use it as a ladder to see whether the answer meets the descriptor for that level. The descriptor for the level indicates the different qualities that might be seen in the student’s answer for that level. If it meets the lowest level then go to the next one and decide if it meets this level, and so on, until you have a match between the level descriptor and the answer. With practice and familiarity you will find that for better answers you will be able to quickly skip through the lower levels of the mark scheme. When assigning a level you should look at the overall quality of the answer and not look to pick holes in small and specific parts of the answer where the student has not performed quite as well as the rest. If the answer covers different aspects of different levels of the mark scheme you should use a best fit approach for defining the level and then use the variability of the response to help decide the mark within the level, ie if the response is predominantly level 3 with a small amount of level 4 material it would be placed in level 3 but be awarded a mark near the top of the level because of the level 4 content. Step 2 Determine a mark Once you have assigned a level you need to decide on the mark. The descriptors on how to allocate marks can help with this. The exemplar materials used during standardisation will help. There will be an answer in the standardising materials which will correspond with each level of the mark scheme. This answer will have been awarded a mark by the Lead Examiner. You can compare the student’s answer with the example to determine if it is the same standard, better or worse than the example. You can then use this to allocate a mark for the answer based on the Lead Examiner’s mark on the example. You may well need to read back through the answer as you apply the mark scheme to clarify points and assure yourself that the level and the mark are appropriate. Indicative content in the mark scheme is provided as a guide for examiners. It is not intended to be exhaustive and you must credit other valid points. Students do not have to cover all of the points mentioned in the Indicative content to reach the highest level of the mark scheme. An answer which contains nothing of relevance to the question must be awarded no marks. MARK SCHEME – AS BIOLOGY – 7401/2 – JUNE 4 Question Marking Guidance Mark Comments 01.1 1. (Reference to) hydrolysis of peptide bonds; 2. Endopeptidase act in the middle of protein/polypeptide OR Endopeptidase produces short(er) polypeptides/ increase number of ends; 3. Exopeptidases act at end of protein/polypeptide OR Exopeptidase produces dipeptides/amino acids; 4. Dipeptidase acts on dipeptide/between two amino acids OR Dipeptidase produces (single) amino acids; 4 Accept chain/chain of amino acids/peptide for polypeptide Accept digest/breakdown/ break for ‘act’ Mark points 2, 3 and 4 reject answers where substrate or product is incorrect eg ‘Endopeptidase produces dipeptides’ Ignore references to source and location of enzymes 01.2 1. No significant difference (in protein absorption); 2. (because ± 2) SDs overlap; 3. (So mean) percentage absorbed not affected by percentage in diet; 4. Amount of protein (in diet) is not a limiting factor OR Something else is limiting factor eg amount of protease; 5. (But) small range of protein in diet OR (Should) Investigate wider range; 3 max Ignore reference to ‘significance’ unless qualified, eg ‘difference’ 2. Accept error bar for SD 01.3 1. More/remaining/undigested (protein) broken down; 2. (So more) amino acids absorbed; 3. (Because) protein/food passes again through stomach/ileum; 3 1. Accept all (protein) broken down TOTAL 10 MARK SCHEME – AS BIOLOGY – 7401/2 – JUNE 5 Question Marking Guidance Mark Comments 02.1 1. Increases/more oxygen dissociation/unloading OR Deceases haemoglobin’s affinity for O2; 2. (By) decreasing (blood) pH/increasing acidity; 2 1. Accept more readily 1. Accept releases more O2 2. Reject if reference made to active site 02.2 1. High(er) affinity for O2 (than haemoglobin) OR Dissociates oxygen less readily OR Associates more readily; 2. Allows (aerobic) respiration when diving/at low(er) pO2 OR Provides oxygen when haemoglobin unloaded OR Delays anaerobic respiration/lactate production; 2 1. Accept holds O2 at lower ppO2 2. Accept acts as an oxygen store 02.3 Correct answer for 2 marks 10.8 to 11 (mins) OR 10 minutes and 48 seconds = 2 marks;; Accept for 1 mark, 10.48 minutes OR Reference to 2057.7 to 2058 (10 700 ÷ 5.2, time oxygen would last if its mass was 1 kg) OR Reference to 56 to 56.3 (10700 ÷ 190, oxygen in 1 kg of seal) OR Reference to 988 (5.2 x 190, oxygen used min-1 by the seal) OR Incorrect answer with correct answer shown in working 2 TOTAL 6 MARK SCHEME – AS BIOLOGY – 7401/2 – JUNE 6 Question Marking Guidance Mark Comments 03.1 Watson and Crick OR Crick and Watson; 1 Correct spelling Ignore first/given/fore names 03.2 DNA polymerase; 1 Correct spelling 03.3 Role of single-stranded DNA fragments 1. Template; 2. Determines order of nucleotides/bases; Role of DNA nucleotides 3. Forms complementary pairs / A – T, G - C OR Forms complementary (DNA) strand; 3 3. Ignore forms complementary bases 3. Accept sequence/ chain for strand TOTAL 5