GCE Human Biology. Mark Scheme for June Unit F221: Molecules, Blood and Gas Exchange. Advanced Subsidiary GCE

Transcription

1 GCE Human Biology Unit F221: Molecules, Blood and Gas Exchange Advanced Subsidiary GCE Mark Scheme for June 2017 Oxford Cambridge and RSA Examinations

2 OCR (Oxford Cambridge and RSA) is a leading UK awarding body, providing a wide range of qualifications to meet the needs of candidates of all ages and abilities. OCR qualifications include AS/A Levels, Diplomas, GCSEs, Cambridge Nationals, Cambridge Technicals, Functional Skills, Key Skills, Entry Level qualifications, NVQs and vocational qualifications in areas such as IT, business, languages, teaching/training, administration and secretarial skills. It is also responsible for developing new specifications to meet national requirements and the needs of students and teachers. OCR is a not-for-profit organisation; any surplus made is invested back into the establishment to help towards the development of qualifications and support, which keep pace with the changing needs of today s society. This mark scheme is published as an aid to teachers and students, to indicate the requirements of the examination. It shows the basis on which marks were awarded by examiners. It does not indicate the details of the discussions which took place at an examiners meeting before marking commenced. All examiners are instructed that alternative correct answers and unexpected approaches in candidates scripts must be given marks that fairly reflect the relevant knowledge and skills demonstrated. Mark schemes should be read in conjunction with the published question papers and the report on the examination. OCR will not enter into any discussion or correspondence in connection with this mark scheme. OCR 2017

3 Annotations Annotation Meaning of annotation Correct answer Incorrect response Benefit of Doubt Not Benefit of Doubt Error Carried Forward Given mark Underline (for ambiguous/contradictory wording) Omission mark Ignore Correct response (for a QWC question) QWC* mark awarded 2

4 Subject-specific Marking Instructions INTRODUCTION Your first task as an Examiner is to become thoroughly familiar with the material on which the examination depends. This material includes: the specification, especially the assessment objectives the question paper the mark scheme. You should ensure that you have copies of these materials. You should ensure also that you are familiar with the administrative procedures related to the marking process. These are set out in the OCR booklet Instructions for Examiners. If you are examining for the first time, please read carefully Appendix 5 Introduction to Script Marking: Notes for New Examiners. Please ask for help or guidance whenever you need it. Your first point of contact is your Team Leader. 3

5 Question Answer Marks Guidance 1 (a) (i) Any two from : clean needle and (should be a) sterile (needle) ; 2 max ACCEPT sterile syringe artery and (should be the) vein ; (a) (ii) (b) (c) dressing and (should be a) sterile dressing ; band and should be tourniquet idea that a screening test needs to be completed for, a virus / pathogen OR idea that need to check, RBC count / haemoglobin levels ; idea of an example of an issue arising with transportation as shown in Fig.1. e.g. maintaining temperature ; biological molecule protein AND feature globular / AW ; 1 CREDIT answers which suggest a sequence of dressings e.g. cotton wool ball followed by a second dressing. CREDIT named virus / pathogen e.g. HIV CREDIT any health check not linked to a disease CREDIT test for blood group 1 e.g. large distance between donor centre and hospital e.g. refrigeration units are not available OR cold boxes used e.g. ambient temperatures higher in tropical regions 1 Both terms needed for one mark ACCEPT a description of globular e.g. hydrophilic R groups on the outside CREDIT globular protein IGNORE polar as a description of the molecule 3

6 (d) (i) enzymes / proteins, denatured by extremes of ph ; hydrogen / ionic, bonds (in tertiary structure) are broken ; 3 max CREDIT proteins/enzymes begin to denature outside this ph range DO NOT CREDIT if list includes disulphide or peptide bonds enzyme s active site no longer complementary to substrate ; AVP ; ACCEPT no / fewer enzyme substrate complexes form e.g. low ph means more H+ ions which affect the charges in the protein / enzyme molecule (ii) use of a buffer ; 1 CREDIT description e.g. use of named buffer IGNORE sodium citrate Total 9 4

7 Question Answer Marks Guidance 2 (a) (i) named electrolyte e.g. sodium ions ; CREDIT correct symbols e.g. Na+ 1 CREDIT ref to an electrode for mp1 idea of measuring voltage between inner and outer, electrode / selectively permeable membrane ; idea of measurement being compared to reference electrode ; 1 max DO NOT CREDIT 'potentiometry' without some indication of how the method works (as the question asks for an outline of the method) 5

8 (a) (ii) increase in electrolytes will decrease water potential (in plasma) OR decrease in electrolytes will increase water potential (in plasma) ; 2 (so) blood cells will lose water by osmosis and crenate OR (so) blood cells will gain water by osmosis and burst / haemolyse ; (b) (water) is a polar molecule ; idea that it has oxygen atom with slight negative charge and hydrogen atoms with slight positive charges ; idea that, electrolytes / ions, surrounded by (polar) water molecules ; idea that, ions / electrolytes, dissolve in water / water is a universal solvent ; 3 max mp 2 must be linked to the correct detail in mp1 CREDIT a description of crenation e.g. cells shrink / shrivel ACCEPT idea of a shell of water molecules CREDIT electrolytes are soluble in water QWC ; 1 Two of the following terms, used in the appropriate context with correct spelling: polar negative positive Total 8 dissolve solvent 6

9 Question Answer Marks Guidance 3 (a) (i) bronchus (is lined with) ciliated epithelium / AW ; alveolus (is lined with) squamous epithelium / AW ; 2 ACCEPT AW e.g. epithelial tissue lining bronchus has cilia / goblet cells ACCEPT AW e.g. tissue lining alveolus has flattened cells (a) (ii) Any two from: 2 max MARK THE FIRST TWO ANSWERS ciliated epithelium AND traps and removes (mucus containing) bacteria / pathogens / dust particles ; cartilage AND keeps bronchi open / prevents bronchi collapsing ; smooth muscle AND relaxes to widen airways / contracts to narrow airways ; ACCEPT airway as bronchus is given in the question ACCEPT idea of restricting the airway for narrowing elastic fibres AND idea of recoil to return bronchi to original size ; (a) (iii) 41 (x) ; ; 2 Award one mark for: answer not given to whole number i.e / or correct working e.g. 6.5 cm or 65 mm CREDIT ecf for incorrect measurement of diameter but correct calculation. 7

10 (b) idea that breathing rate was greater in trace shown in Fig. 3.2(b) ; 3max CREDIT frequency for breathing rate CREDIT ORA for trace (a) supporting data e.g. comparative figures with units ; Fig (a) ACCEPT breaths per minute (5 6 breaths in 20 seconds) Fig (b) ACCEPT breaths per minute (19 20 breaths in 20 seconds) comparative comment on breathing depth / tidal volume ; e.g. idea that depth / tidal volume is more consistent in (b) idea that depth / tidal volume is similar in both idea that smaller depth / tidal volumes in (a) ORA supporting data e.g. comparative figures with units ; Range of tidal volume in (a) 0.64 to 0.88 dm 3 Range of tidal volume (b) 0.8 to 1.0 dm 3 (c) (i) idea that person has stopped breathing into apparatus OR named cause of respiratory arrest e.g. asthma attack ; (ii) tidal volume increases /AW ; idea that oxygen (concentration) in alveoli is too low AND carbon dioxide (concentration) is too high ; idea that diffusion gradient maintained ; 1 2max ACCEPT a description e.g. larger intake and exhale A reference to both oxygen and carbon dioxide is required for mp 2 Total 12 8

11 Question Answer Marks Guidance 4 (a) (i) V and W ; 1 Both needed for 1 mark (a) (ii) Y ; 1 (a) (iii) Y ; 1 (a) (iv) V and X ; 1 Both needed for 1 mark (b) similarities idea that have same molecular formulae of C 6 H 12 O 6 / are hexose sugars OR idea that both have hexagonal ring structure containing an O atom ; 2 max CREDIT any reference to number of individual atoms from the formula e.g. both have 12 H atoms difference idea that position of OH on carbon 2 is different ; DO NOT CREDIT references to carbon 6 as a difference 9

12 (c) role of membranes (protein synthesis) at ribosomes on / attached to rer / rough endoplasmic reticulum ; 4 max rer / rough endoplasmic reticulum packages the proteins ready for transport (to Golgi) ; ACCEPT idea that vesicles containing protein bud off rer (to Golgi) carbohydrate is added / protein is modified / glycoprotein is produced in Golgi (body) ; idea that vesicles / lysosomes bud off (Golgi body) ; DO NOT CREDIT in context of exocytosis max 3 Importance of transporting correctly idea that membranes separate /compartmentalise proteins that could damage the cell e.g. digestive enzymes ; max 1 QWC ; 1 Two of the following terms, used in the appropriate context with correct spelling: ribosome(s) rough endoplasmic reticulum Total 11 Golgi carbohydrate vesicle(s) 10

13 Question Answer Marks Guidance 5 (a) 5 One mark per row Statement Phospholipids are a type of triglyceride molecule. Intrinsic proteins span both sides of the phospholipid bilayer. Membranes with small amounts of cholesterol are less stable than membranes with large amounts of cholesterol. Protein chains attach to the phospholipid molecules to form glycolipids. Extrinsic proteins form carrier proteins to allow active transport to occur across the membrane ( ) or ( ) ; ; ; ; ; (b) (i) leucocytes have membrane bound organelles / named membrane bound organelle; 2 CREDIT ora for erythrocyte idea that the cell surface membrane would be contaminated with organelle membranes ; (b) (ii) forms cytoskeleton ; 2 helps to maintain biconcave shape / maintains higher surface area to volume ratio ; (c) (i) alpha / helix ; 1 IGNORE ref to polypeptide CREDIT secondary structure ACCEPT Beta (pleated) sheet (as diagram is not shaded) (c) (ii) disrupts / interacts with / penetrates, the phospholipid 1 max bilayer ; idea that SDS can dissolve the hydrophobic part of the 11

14 protein (in bilayer); Total 11 Question Answer Marks Guidance 6 (a) elastic ; smooth muscle ; endothelium ; sphygmomanometer ; 5 kpa ; ACCEPT kilopascal (b do not use thumb OR 1 use second and third fingers ; ACCEPT use two fingers (c) (i) The volume of blood pumped out of the left ventricle in 1 one minute ; (c) (ii) reduced thickness of ventricle wall(s) ; (so) heart muscle contracts, with less force / less strongly ; decreased stroke volume ; 2 max IGNORE ref to left or right Total 9 12

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