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 b