SAJC 2017 H2-Bio-TYS-ANS

2017 UCLES ‘A’ Level H2 Biology Mark Scheme (SAJC) 1 2017 ‘A’ Level H2 Biology Mark Scheme PAPER 1 (MCQ)

2017 UCLES ‘A’ Level H2 Biology Mark Scheme (SAJC) 2 PAPER 2 (CORE) QUESTION 1 Fig. 1.1 shows a representation of a glycogen molecule. Glycogen molecules are made up of glycogen molecules surrounding a central core. Fig. 1.1 (a) (i) State the number of glycogen molecules shown in Fig. 1.1. ……………………………………………………………………………...……………….. [1] 1 Two / 2 Examiner’s comments: Most candidates were able to interpret the diagram correctly. (ii) Name the type of molecule forming the central core of the glycogen granule, in Fig. 1.1. ……………………………………………………………………………...……………….. [1] 1 Protein / Polypeptide Examiner’s comments: The majority of candidates identified the type of molecule forming the central core.

2017 UCLES ‘A’ Level H2 Biology Mark Scheme (SAJC) 3 (iii) Suggest the role of the molecule forming the central core of the glycogen granule. ……………………………………………………………………………...……………….. [1] 1 It allows glycogen molecules to attach / bind to it, so as to form the glycogen granule. Examiner’s comments: Most candidates correctly deduced the role of this molecule. (b) Explain how the structure of glycogen is related to its role in living organisms. ……………………………………………………………………………...……………….. [4] 1 Glycogen molecule has a very compact (helical) shape; this enables it to be stored in large quantities within a limited space. 2 Glycogen molecule is large in size and majority of its hydroxyl groups of glucose monomers project into the interior of helix; this makes it insoluble in water. 3 Glycogen molecule being insoluble in water; allows it to be stored in large quantities without greatly affecting the osmotic potential of the cell. 4 Glycogen molecule is highly branched (due to α (1,6) glycosidic linkages) and do not form inter-molecular cross-linkages; this enables it to be easily accessed by enzymes to be broken down into glucose. Examiner’s comments: All candidates were familiar with both the structure and role of glycogen in living organisms, but not all considered how the structure was related to its role. (c) Describe how the structure of cellulose is different from the structure of glycogen. ……………………………………………………………………………...……………….. [3] [Any three] Cellulose Glycogen 1 Linear/straight chains ; Helical molecule ; 2 Unbranched molecule ; Highly branched molecule; 3 Comprised of β-glucose monomers; Comprised of α -glucose monomers; 4 Adjacent monomers are orientated 180o to each other; Adjacent monomers are of same orientation / no 180o rotation; 5 Presence of β (1,4) glyosidic linkages; Presence of α (1,4) glyosidic linkages and α (1,6) glyosidic linkages; 6 Presence of cross-linking / hydrogen-bonding between chains; Absence of cross-linking / hydrogen-bonding between chains; Examiner’s comments: Candidates were familiar with the structural differences between cellulose and glycogen, with many good answers being given. [Total: 10]

2017 UCLES ‘A’ Level H2 Biology Mark Scheme (SAJC) 4 QUESTION 2 Fig. 2.1 is a diagram representing a cell surface membrane. Five proteins are labelled Q, R, S, T and V. Fig. 2.1 (a) (i) Explain why proteins are required for the transport of glucose across the cell surface membrane. ……………………………………………………………………………...……………….. [3] 1 Glucose is too large in size, so it is unable to fit through the phospholipids in the cell surface membrane / diffuse through the phospholipid bilayer. 2 Glucose is a polar / hydrophilic / water-soluble molecule, which prevents it from passing through the hydrophobic core of the phospholipid bilayer. [Reject: “Not lipid soluble”, too vague] 3 Protein transporters, such as protein channels, help to provide a passageway for glucose to enter the cell via facilitated diffusion. Examiner’s comments: Candidates were familiar with properties of the cell surface membrane and were able to give full accounts of why proteins are required to transport glucose across the membrane. (ii) Describe the structural features of a protein that enable it to transport glucose into a cell. ……………………………………………………………………………...……………….. [2] 1 It is a transmembrane protein, to connect both the extracellular and intracellular region across the cell surface membrane. 2 The exterior of the protein contain mostly amino acids with non-polar R- groups, for hydrophobic interactions with the non-polar hydrocarbon chains of phospholipids of the cell surface membrane. 3 The interior of the protein contain mostly amino acids with charge / polar R- groups, so it can serve as a hydrophilic passageway for glucose transport. 4 The channel of the protein is complementary in conformation for glucose, to selectively allow for only glucose to move through it. Examiner’s comments: Many candidates considered the significance of having both hydrophobic and hydrophilic regions for proteins that transport glucose into cells.

2017 UCLES ‘A’ Level H2 Biology Mark Scheme (SAJC) 5 (iii) State the letter of the protein in Fig. 2.1 that could transport glucose into a cell. ……………………………………………………………………………...……………….. [1] 1 Q Examiner’s comments: Nearly all candidates correctly identified the channel protein shown in Fig. 2.1. (b) Fig 2.2 shows the effect of increasing the external concentration of glucose on the rate of glucose uptake into a cell. Fig. 2.2 Using the information shown in Fig. 2.2, describe and explained the effect of increasing the external glucose concentration on the rate of glucose uptake into a cell. ……………………………………………………………………………...……………….. [4] 1 Low level description: The rate of glucose uptake increased when external glucose concentration increased. 2 Higher level description: The rate of glucose uptake increased rapidly / sharply at the start, but eventually levels off to reach a plateau. [Accept: “Increases at a decreasing rate” as alternative description] 3 Data Quoting: Specific and accurate reference to any pair of data (x-axis & y-axis), with correct units stated. E.g.: external concentration of glucose increased from 0 to 1 mmol dm-3, rate of glucose uptake increased from 0 to 180 arbitrary units (AU) ; rate of glucose uptake plateau off at 395 AU after external concentration of glucose increases beyond 10 mmol dm-3 [ACCEPT: 11 mmol dm-3] ;

2017 UCLES ‘A’ Level H2 Biology Mark Scheme (SAJC) 6 4 Explanation for 1st half of curve: The sharp increase in glucose occurred due to the increase in concentration gradient. 5 Explanation for 2nd half of curve: The ‘levelling off’ effect occurred as the transport proteins for glucose became saturated / fully occupied / AW. Examiner’s comments: Most candidates had little difficulty in describing the effect of increasing external glucose concentrations on the rate of glucose uptake. Better performing candidates provided sound explanations on why the effect was observed. Weaker responses stated that the rate of glucose uptake became constant when the concentrations of glucose on each side of the membrane were equal. In the absence of other mechanisms, this would have resulted in the net rate of glucose uptake falling to zero. [Total: 10]

2017 UCLES ‘A’ Level H2 Biology Mark Scheme (SAJC) 7 QUESTION 3 Fig. 3.1 shows model of a haemoglobin molecule and part of a collagen molecule, drawn to the same scale. Fig. 3.1 (a) Describe the main features of the molecular structures of haemoglobin and collagen, visible in Fig. 3.1. ……………………………………………………………………………...……………….. [5] Haemoglobin: 1 It has a globular structure. 2 It contains prosthetic heme groups. 3 It has a quaternary structure / comprises of four subunits. Collagen: 4 It has a fibrous structure. 5 It has a helical structure. 6 It has a quaternary structure / comprises of three polypeptide chains. Examiner’s comments: C