AQA A LEVEL BIOLOGY Paper 2 MS 2021

A-level BIOLOGY 7402/2 Paper 2 Mark scheme June 2021 Version: 1.0 Final *216a7402/2/MS* Mark schemes are prepared by the Lead Assessment Writer and considered, together with the relevantquestions, by a panel based on subject teachers. This mark scheme includes any amendments made at the standardisation events which all associates participate within and is the scheme which was used by them within this examination. The standardisation process ensures that the mark scheme covers the students’ responses to questions and that every associate understands and applies it within the same correct way. As preparation for standardisation each associate analyses a number based on students’ scripts. Alternativeanswers 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 arerequired to refer these to the Lead Examiner. It must be stressed that a mark scheme is a working document, within many cases further developed and expanded on the basis based on students’ reactions to a particular paper. Assumptions about future mark schemes on the basis based on one year’s document should be avoided; whilst the guiding principles based on assessment remain constant, details will change, depending on the content based on a particular examinationpaper. Further copies based on this mark scheme are available from Copyright information 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 thirdparty even for internal use within the centre. Copyright © 2021 AQA and its licensors. All rights reserved. 2 Level based on response marking instructions Level based on response mark schemes are broken down into levels, each based on which has a descriptor. The descriptor for the level shows the average performance for the level. There are marks within each level. Before you apply the mark scheme to a student’s answer read through the answer and annotate it (asinstructed) 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 based on 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 within 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 based on the mark scheme. When assigning a level you should look at the overall quality based on the answer and not look to pick holes within small and specific parts based on the answer where the student has not performed quite as well as the rest. If the answer covers different aspects based on different levels based on the mark scheme you should use a best fit approach for defining the level and then use the variability based on the response to help decide the mark withinthe level, ie if the response is predominantly level 3 with a small amount based on level 4 material it would be placed within level 3 but be awarded a mark near the top based on the level because based on 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 ananswer within the standardising materials which will correspond with each level based on 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 thenuse 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 andassure yourself that the level and the mark are appropriate. Indicative content within the mark scheme is provided as a guide for examiners. It is not intended to beexhaustive and you must credit other valid points. Students do not have to cover all based on the points mentioned within the Indicative content to reach the highest level based on the mark scheme. An answer which contains nothing based on relevance to the question must be awarded no marks. 3 Question Marking Guidance Mark Comments 1. Phosphorylation based on glucose using ATP; 2. Oxidation based on triose phosphate to pyruvate; 3. Net gain based on ATP; 4. NAD reduced; Accept all mark points withindiagrams. 2. Accept removal based on hydrogen from triosephosphate for oxidation. 01.1 4 max 3. Accept any description that indicates a net gain e.g., 4 produced, 2 used. 4. Accept NADH/NADH2/NAD H + H+ produced. Question Marking Guidance Mark Comments 01.2 1. Less/no reduced NAD/coenzymes OR Fewer/no hydrogens/electrons removed (and passed to electron transfer chain); 2. Oxygen is the final/terminal (electron) acceptor; 2 1. Accept less/no FAD reduced. 4 Question Marking Guidance Mark Comments 1. Higher concentration based on potassium ions inside and higher concentration based on sodium ions outside(the neurone) OR potassium ions diffuse out OR sodium ions diffuse in; 2. (Membrane) more permeable to potassium ions (leaving than sodium ions entering) OR (Membrane) less permeable to sodium ions (entering than potassium ions leaving); 3. Sodium ions (actively) transported out and potassium ions in; 1. Accept ‘more’ for ‘higher concentration’. 1. Accept ‘sodium ions can’t diffuse in (due to alternative explanation). 2. Accept for ‘less permeable to sodium ions’ is ‘impermeable to sodium ions’ or ‘sodium gates/channels are closed’ (alternative explanation). 02.1 3 1, 2 and 3 reference to ions or Na+ and K+ is required. If mentioned once allow for all mark points. 1, 2 and 3. If an answer provides two or three based on these mark points without any reference to ions – award one maximum mark. 3. Accept 3 Na+ out and 2 K+ within but reject if numbers used are incorrect. Question Marking Guidance Mark Comments 02.2 1. Myelination provides (electrical) insulation; 2. (In myelinated) saltatory (conduction) OR (In myelinated) depolarisation at nodes (of Ranvier); 3. In non-myelinated depolarisation occurs along whole/length (of axon); 3 1. Reject thermal insulation. 1. Accept description based on (electrical) insulation. 3. Accept action potentials for depolarisation. 2 and 3. ‘Messages’ or ‘signals’ disqualifies first based on these marks credited. 5 Question Marking Guidance Mark Comments 02.3 1. No/less ATP produced; 2. No/less active transport OR Sodium/potassium pump inhibited; 3. Electrochemical gradient not maintained OR (Facilitated) diffusion based on ions causes change to0 mV OR (Results in) same concentration based on (sodium and potassium) ions (either side based on membrane) OR No net movement based on (sodium and potassium)ions; 3 2. Accept Na+ not/fewer moved out and K+ not/fewer moved in. 3. Accept reaches electrical equilibrium/balance. 3. Accept concentration gradientbasedonsodium and potassium ions not maintained. 6 Question Marking Guidance Mark Comments 1. Tip produces IAA; 2. IAA diffuses (into shoot); 3. (More) elongation based on cells on one side (thanother); 1. Accept source/release for produces but ignore contains/stores IAA. 1 and 2. Accept auxin for IAA. 03.1 3 2. Accept IAA diffuses down. 3. Accept (more) elongation based on cells onleft side. 3. Reject any reference to shaded/dark side or away from light. Question Marking Guidance Mark Comments 03.2 1. Size based on shoot/tip; 2. Number based on shoot tips; 3. Size/type based on agar (block); 4. (Shoots) at same stage based on growth/development; 5. Time (period) tips kept on agar OR Time (period) agar/block kept on (cut shoot) OR Time (period shoots) kept within dark; 6. Temperature; 7. (Repeat several times and) calculate a mean; 8. Compare/read degree based on curvature (oncalibration curve) to determine (IAA) concentration OR Higher the degree based on curvature the higher theIAA concentration; 5 max Mark points 1 to 6 = max 3. 1 to 6. Ignore pH, species, carbon dioxide, humidity, nutrients, water and light. 3. Accept ‘amount based on agar’. 4. Accept (Shoots/plants) are same age. 7 Question Marking Guidance Mark Comments 03.3 1. (IAA) is not broken down by light OR (IAA) is produced within the dark OR Light/dark does not affect (IAA) production; 2. (IAA) moves away from light OR (IAA) moves to shaded side; 2 2. IAA accumulates on shaded side is not enough on its own, idea based on movement is required. Question Marking Guidance Mark Comments 1. Males have one allele; 2. Females need two recessive alleles OR Females must be homozygous recessive OR Females could have dominant and recessivealleles OR Females could be heterozygous/carriers; 1 Accept males only need one allele. 1 and 2. Ignore references to X and Y chromosomes. 04.1 2 1 and 2. Accept r as recessive allele and Ras dominant allele. If no reference to allele, accept for one mark male needs one recessive gene whereas females need two recessive genes. Question Marking Guidance Mark Comments 04.2 1. Box 2. All females red-eyed, all males white-eyed. 1 Reject if more than one box with tick. Ignore crossed-out ticks. Question Marking Guidance Mark Comments 04.3 1. The (two) genes are linked OR Autosomal linkage; 3 1. Accept that the genes are on the same chromosome. 8 2. No crossing over (occurs) OR (Linked) genes are close together; 3. No Gl and no gL (gametes produced) OR No Ggll and no ggLl (offspring produced) OR Only GL and gl (gametes produced); 1. Accept ‘Alleles are linked’ (accept symbols for alleles) but reject if context suggests alleles based on the‘same gene’. 2. Accept crossing over less likely to occur. 1 2, and 3. Ignore reference to independent assortment. Question Marking Guidance Mark Comments 04.4 1. Correct answer based on 8 × 1010 = 3 marks;;; 2. Correct answer not within standard form = 2 marksOR 1.6 × 1013 = 2 mark OR 1.6 × 1011 = 2 mark OR 6.4 × 1011 = 2 mark OR Shows 8 × 1010 w i t h i n the working = 2 marks;; 3. 1.28 × 1012 = 1mark OR 3.2 × 1011 = 1 mark OR 8 × 1011 = 1 mark OR 8 × 109 = 1 mark OR Shows 1.6 × 1011 w i t h i n the working = 1 markOR Shows 2004 within the working = 1 mark; 3 If no other mark is credited accept for one mark working which shows multiplication by 200 for 4 generations. Thiscould be shown within a variety based on ways e.g. multiplied by 400 divided by 2 for 4 generations. 9 Question Marking Guidance Mark Comments 1. Changes tertiary structure; 2. No longer complementary (to receptor); 1. Reject change within tertiary structure based on receptor. 05.1 2 2. Reject ‘active site’or reference to enzyme or substrate. Question Marking Guidance Mark Comments 05.2 1. Less/no AKT activated; 2. Fewer/no vesicles move to membrane OR Fewer/no (channel) proteins within membrane; 3. Less/no glucose diffuses into cell (so high blood glucose); 3 2. Accept ‘fuse with membrane’. Question Marking Guidance Mark Comments 05.3 1. High concentration based on glucose within blood/filtrate; 2. Not all the glucose is (re)absorbed at the proximal convoluted tubule; 3. Carrier/co-transport proteins are working at maximum rate OR Carrier/co-transport proteins/ are saturated; 3 1. Accept tubule for filtrate. 2. Reject no glucoseis (re)absorbed. 3. Accept all carrier/co-transport proteins are ‘in use’ but reject all carriers are ‘used up’. 3. Accept symport for carrier protein. 3. Accept not enough carrier proteins to absorb all the glucose. 10 Question Marking Guidance Mark Comments 1. Produce healthy (blood) cells; 2. No MDS/faulty/cancerous (blood) cells; 1 and 2. Produce only healthy/normal (blood) cells = two marks. 3. Stem cells divide/replicate; 1. Accept produce ‘normal’ /non-MDS cells. 06.1 3 2. Accept no (cancerous) tumour. 1 and 3. Ignore reference to totipotent/pluripotent/ multipotent/unipotent 3. Accept ‘clone’ for divide. Question Marking Guidance Mark Comments 06.2 1. (AZA) reduces methylation (of DNA/cytosine/gene); 2. (Tumour suppressor) gene is transcribed/expressed; 3. Prevents rapid/uncontrollable cell division OR Cell division can be controlled/stopped/slowed; 3 1. Reject any referenceto mutation. 2. Accept mRNA produced for transcription/transcribed. 2. Ignore gene is ‘switched on’ or activated but allow protein is formed. 3. Ignore growth. Question Marking Guidance Mark Comments 06.3 1. Effect based on AZA can be compared; 2. Unethical not to treat (control group); 2 1. Comparison on its own is not enough for amark. Question Marking Guidance Mark Comments 06.4 1. Correct answer based on 29/28.8 = 2 marks;; 2. Working shows 0.74 and 0.58 = 1 mark OR 58/57.6 = 1 mark OR 28 = 1 mark; 2 11 Question Marking Guidance Mark Comments 07.1 1. Correct answer based on 625 = 2 marks;; 2. Shows 625 but decimal point incorrect = 1 markOR Working shows 40 = 1 mark OR 1600/1.6 = 1 mark OR 667/666.6 = 1 mark; 2 Question Marking Guidance Mark Comments 07.2 (Cell/membrane has a) phospholipid bilayer OR No channel/carrier protein (for uptake) OR No need for channel/carrier protein (for uptake); 1 Question Marking Guidance Mark Comments 07.3 1. Both are more effective than the control; 2. Differences within the means not (likely to be) due tochance OR Significant difference (in effectiveness betweenboth types); 3. (As) SDs do not overlap; 4. HBsAg (reduced), not zero OR Replication (reduced), not zero; 5. Not (investigated in) humans OR (Investigated in) mice; 6. shRNA (more effective as) 7.5% (of control) compared with 50% for lhRNA; 7. No indication based on sample size/number; 5 max Mark points 4 to 10 = 4 max. 1. Accept both (results) are below thecontrol. 2. Reject ‘results are significant’. 2. Accept significantly higher or significantly lower within correct context. 3. Accept error bars do not overlap. 6. Accept 42.5% difference. 6. Accept (mean) concentration for %. 12 8. Long term effects not known OR Side effects not known; 9. No statistical test to determine significance; 10. (Investigated) within vitro OR Not (investigated) within vivo; 8. Accept ‘could be toxic’ for side effects not known. 10. Accept not done inside an organism or not done within liver (organ) but ‘only tested within liver cells’ is insufficient unless qualified. Ignore only ‘one study’ or ‘no repeats’. Question Marking Guidance Mark Comments 08.1 1. (Requires DNA fragment) DNA polymerase, (DNA) nucleotides and primers; 2. Heat to 95 °C to break hydrogen bonds (and separate strands); 3. Reduce temperature so primers bind to DNA/strands; 4. Increase temperature, DNA polymerase joins nucleotides (and repeat method); 4 1 and 4. Accept Taq polymerase for DNA polymerase. 2. Accept temperature withinrange 90 to 95 °C. 3. Accept temperature withinrange 40 to 65 °C. 4. Accept temperature withinrange 70 to 75 °C. Question Marking Guidance Mark Comments 1. (Initially) number (of molecules) doubling is low OR Doubles each cycle to produce exponentialincrease; 2. Plateaus as no more nucleotides/primers; 1. First alternative relates to idea based on lownumbers i.e., 2, 4, 8,16, 32 etc. 08.2 2 2. Accept ‘levels out’ or ‘flattens’ for plateaus. 2. Accept enzyme/polymerase (eventually) denatures. 13 Question Marking Guidance Mark Comments 09.1 1. Method based on randomly determining position(of quadrats) e.g. random numbers table/generator; 2. Large number/sample based on quadrats; 3. Divide total percentage by number based on quadrats/samples/readings; 3 1. Ignore line/belt transect. 2. Accept many/multiple/lots but ignore several. 2. Ignore point quadrat. 2. Accept squares/frames (ofa grid) for quadrats. 2. If a specified number is given, it must be 10 or more. Question Marking Guidance Mark Comments 09.2 1. Increase within variety/diversity based on species/plants/animals; OR Increase within number based on species/populations; 2. Provides more/different habitats/niches OR Provides greater variety/types based on food; 2 1. Accept increase within biodiversity or speciesrichness. 2. Ignore shelter/homes/environments. 2. Ignore ‘more food’ but accept ‘more food sources’. 2. Accept ‘less hostile’ (environment). Question Marking Guidance Mark Comments 1. Significant (difference/decrease) with C (compared with A); 2. No significant (difference/decrease) with B and D (compared with A); 3. Reference to less than 5%/0.05 probability that difference is (less likely) dueto chance OR Reference to more than 95%/0.95 probability that difference is not due to chance; 4. Species based on algae not known OR Species based on algae may differ (on other reefs); 5. Only done off (coast of) Florida OR Not done on other reefs; Mark points 4 to 9 = 4 max. 1 and 2. Accept names based on fish species present as alternatives to sets B, C and D. 09.3 5 max 1 and 2. Award both marksif answer states only C is significantly (different/lower). 1 and 2. ‘Results are significant/not significant’ disqualifies first based on these marks credited. 3. Accept equal to specified probabilities. 14 6. Only done at 16 to 18 metres OR Not done (on reefs) at other depths; 7. Only 34 weeks; 8. Concrete/artificial reef could affect results/growth OR Natural reef results/growth may differ; 9. Cage may allow other fish/animals to enter; 8. Accept any reference to composition based on reef being different (from natural). Question Marking Guidance Mark Comments 10.1 1. Variation/differences due to mutation/s; 2. (Reference to) allopatric (speciation); 3. Smaller/different lakes have different environmental conditions OR Smaller/different lakes have different selection pressures; 4. Reproductive separation/isolation OR No gene flow OR Gene pools remain separate; 5. Different alleles passed on/selected OR Change within frequency based on allele/s; 6. Eventually different species/populations cannot breed to produce fertile offspring; 4 max 2. Ignore sympatric speciation. 3. Accept different populations for different lakes. Question Marking Guidance Mark Comments 10.2 1. Correct answer based on 10/10.4 = 2 marks;; 2. Working shows 14,112 = 1 mark OR 13.09/13.1 = 1 mark; 2 1. Ignore any numbers after 10.4 15 Question Marking Guidance Mark Comments 10.3 1. (Growth/increase of) algae/surface plants/algal bloom blocks light; 2. Reduced/no photosynthesis so (submerged) plants die; 3. Saprobiotic (microorganisms) aerobically respire OR Saprobiotic (microorganisms) use oxygen within respiration; 4. Less oxygen for fish to respire; 4 3. Accept: Saprobiont/saprophyte/ saprotroph 3. Neutral: decomposer Question Marking Guidance Mark Comments 10.4 1. Capture/collect sample, mark and release; 2. Ensure marking is not harmful (to fish) OR Ensure marking does not affect survival (of fish); 3. Allow (time for) fish to (randomly) distribute before collecting a second sample; 4. (Population =) number within first sample ×number within second sample divided by number based on marked fish within second sample/number recaptured; 4 2. Accept examples e.g., marking should not be toxic. Question Marking Guidance Mark Comments 10.5 1. Less chance based on recapturing fish OR Unlikely fish distribute randomly/evenly; 1 Accept ‘harder to capture marked fish’ (recaptured fish) but ignore ‘harder to capture fish’. Accept that fish may remain within one area. Accept fish may congregate. 16