VS 2020 O Level Pure Biology P1+2 MS w Feedback

2020 Biology 6093 Suggested Answers Paper 1 1 2 3 4 5 6 7 8 9 10 D B B D C A C A B D 11 12 13 14 15 16 17 18 19 20 C B A C A A D B D D 21 22 23 24 25 26 27 28 29 30 C D B A D B B A C D 31 32 33 34 35 36 37 38 39 40 D D A C B C B D A D Feedback for Paper 1

2 Many candidates selected option D. This indicates they did not recognise that root hair cells have a need to absorb oxygen in order to carry out aerobic respiration and release energy. 7 Hardly any candidate selected options A or D, proving that the majority were aware that the liver does not excrete urea. Candidates who selected option B were unclear about the role of the liver in glycogen metabolism. They were aware that glycogen is broken down by the liver, but did not recognise that the liver converts excess glucose in the blood into glycogen, which it then stores. 12 Some candidates thought that option A was correct. This indicates that they hold two misconceptions: firstly, that plants do not expend energy in translocation and secondly, that transpiration needs energy from the plant in order to take place. 14 The vast majority of candidates selected either option C (correct) or option D. The candidates who selected option D did not appreciate that cells in the alveolus wall need to absorb glucose from the blood in the capillary in order to meet their own energy needs. 15 A minority of candidates selected options B, C or D. This indicates these candidates did not fully understand the sequence of events during a cardiac cycle, in particular there appears to be a lack of appreciation about pressure differences being responsible for whether the valves are open or closed. 21 Some candidates chose option B. These candidates did not appreciate that the kidney dialysis machine relies on the principle of diffusion. This being so, the dialysis fluid leaving the machine cannot have a higher concentration of urea than that of the blood entering it. 25 Option B was selected by some candidates. These candidates knew that the suspensory ligaments become slack when viewing a near object, so that the lens can assume a more convex shape. The difficulty of these candidates was in understanding how this was achieved. Some candidates thought the muscles in the ciliary body became longer, perhaps because they did not realise that the muscle fibres run in a circular direction. 30 Some candidates opted for option A, indicating that the approximate dates for the peak in oestrogen during the menstrual cycle are imprecisely known by these candidates. 36 Some candidates selected option D. This shows that they knew that human females possess two X chromosomes, but did not appreciate that these constituted part of the total chromosome number of 46. Paper 2 Section A

Q No Suggested Answers Marks Feedback 1(a) A: ovary B: fallopian tube C: cervix 1 1 1 Most candidates correctly identified A and B. Weaker responses labelled C as ‘vagina’ or ‘uterus’, and a few as ‘urethra’. 1(b) On fallopian tube 1 The vast majority of candidates could correctly identify where fertilisation normally occurs. 1(c) Fertilised egg, also called a zygote,will travel down fallopian tube to uterus by the peristaltic movement in the wall of the fallopian tube and sweeping of cilia of the fallopian tube Zygote will divide by mitosis to form a ball of cells called embryo Embryo becomes embedded in the thickened uterine lining in the process of implantation 1 1 1 Most candidates showed good knowledge of the early development of the fertilised egg. Weaker responses suggested that meiosis took place. They also described the implantation of a zygote or fertilised egg itself. Several candidates gave unnecessary extra detail of fertilisation, or hormone release, and subsequent fetal development. 1(d)(i) Cilia sweep the zygote; along the fallopian tube to the uterus. 1 Most candidates were able to give the correct role of cilia in structure B. A small proportion incorrectly described the role of cilia in the respiratory system, removing mucus or dust. 1(d)(ii) Respiratory system 1 The majority of candidates correctly stated that the respiratory system had cilia, but weaker responses only named an organ in the correct system, such as the trachea. A few responses confused cilia with villi and referred to the digestive system.

2(a) D - DNA replication , the mass of the DNA doubles from 8 au to 16 au. E - Cell division. [Cytokinesis] , a furrow develop s in the cell membrane which eventually separates the two daughter cells, each 8 au mass of DNA. 1 1 For stage D, weaker responses only described what they could see in the graph, in terms of DNA doubling or increasing, rather than suggesting what was happening. Most responses recognised that replication was happening. The majority of candidates correctly suggested that cell division was occurring during stage E. A minority of candidates incorrectly described the process as cytokinesis. 2(b)(i) anaphase 1 2(b)(ii) before: metaphase after: telophase 1 1 Most candidates were confident with the order of the stages of mitosis. 2(b)(iii) 3 During anaphase, the sister chromatids separate to form chromosomes. Since there are 12 of them, diploid number is 6 . (the cell is dividing mitotically.) Haploid number is always half the number of the somatic cells, 6/2 = 3 1 1 1 A significant number of candidates did not correctly determine the diploid or haploid numbers from the figure. Several responses did not realise that the cell shown in the figure had undergone DNA replication and gave the incorrect answer of six for the haploid number. 2(c) During the prophase I of the meiosis, crossing over occurs in the chiasmata of the homologous chromosomes, allowing segments to switch, resulting in a new combination of alleles. 1 Many candidates made correct reference to ‘independent assortment’ and ‘crossing over’. Although the stages were not required, some responses incorrectly assigned these processes to the wrong stage. Weaker responses made no reference to the

Random assortment of homologous chromosomes occurs in metaphase I of meiosis, resulting in variation. Fertilisation involves the fusion of the nuclei of a random haploid sperm and egg, allowing variation to occur in the combination of chromosome s in the zygote. 1 1 randomness of the fertilisation process. 3(a) I A I O X I O I O I A I O I A I O I A I O I O I O I O I O 1 1 1 Most responses correctly completed the genetic diagram. Weaker responses gave the father’s genotype as IA IA, or described the parental genotypes as ‘blood group A’ and ‘blood group O’. 3(b) Codominance: Occurs when two alleles are equally expressed in the heterozygous condition . I