SAJC_2012_H2-Bio-TYS-ANS

2012 UCLES ‘A’ Level H2 Biology Mark Scheme 1 2012 ‘A’ Level H2 Biology Mark Scheme PAPER 1 (MCQ) 1 D 21 B 2 A 22 A 3 D 23 A 4 D 24 B 5 A 25 C 6 B 26 D 7 B 27 C 8 C 28 A 9 B 29 A 10 D 30 D 11 D 31 A 12 D 32 B 13 A 33 C 14 D 34 B 15 B 35 B 16 A 36 B 17 C 37 C 18 C 38 A 19 D 39 C 20 C 40 C

2012 UCLES ‘A’ Level H2 Biology Mark Scheme 2 PAPER 2 (CORE) QUESTION 1 (a) 1 Metaphase. (b) 1 Centromeres divide during anaphase ; 2 Sister chromatids separate to form independent chromosomes ; 3 Chromosomes pulled to opposite poles of the cell by the shortening of kinetochore microtubules. (c) 1 Centrioles are found in pairs at each pole and their position is important in determining the polarity of the cells ; 2 Centrioles play a role in organising the assembly of the spindle fibres, which aid in the separation of sister chromatids during nuclear division. Examiner’s comment: Most candidates correctly outlined the role of the centrioles in organising the synthesis of the spindle fibres, which then led to the separation of the chromatids. Fewer candidates mentioned that there was a pair of centrioles at each pole and that their position was important in determining the polarity of the cells. (d) 1 During prophase, the nuclear envelope disintegrates and disappears ; 2 This allows free movement of chromosomes to align at the metaphase plate ; 3 During telophase, the nuclear envelope reappears and encloses the separated chromosomes at opposite poles of the cell ; 4 This prevents entanglement of the de-condensing chromosomes and prepares the cell for cytokinesis. Examiner’s comment: While the majority of the candidates correctly described what happens to the nuclear envelope during mitosis, very few provided a full response to the question by explaining the changes that they had described. (e) Structures in region A: chromosomes (consisting of replicated sister chromatids) ; Structures in region B: spindle microtubules.

2012 UCLES ‘A’ Level H2 Biology Mark Scheme 3 QUESTION 2 (a) 1 Chloride ions are charged ; REJECT polar / hydrophilic 2 Unable to diffuse through the hydrophobic core of the phospholipid bilayer of the membrane ; 3 Transmembrane CFTR protein has a water-filled pore lined with hydrophilic amino acids which allows the diffusion of the chloride ions down their concentration gradient ; Examiner’s comments: Some candidates incorrectly referred to chloride ions as polar. (b) 1 Hydrophobic interactions between the non-polar tails of the phospholipids and the hydrophobic R-groups of the non-polar amino acids in contact with the phospholipids ; 2 Hydrophilic interactions between the polar phosphate heads and the hydrophilic R-groups of the polar amino acids on the exterior of the protein. (c) 1 ATP binds to the ATP binding domain of the CFTR protein ; 2 This changes t