2026 NJC H2 Bio Prelim P3 (MS)

©NJC 2026 9477/03/SH2 Preliminary Examination [Turn over NATIONAL JUNIOR COLLEGE, SINGAPORE Senior High 2 Preliminary Examination Higher 2 CANDIDATE NAME MARK SCHEME BIOLOGY CLASS 2bi2_____ REGISTRATION NUMBER Biology Paper 3 Long Structured and Free-response Questions Candidates answer on the Question Paper. No Additional Materials are required 9477/03 25 August 2026 2 hours READ THESE INSTRUCTIONS FIRST Write your name, Biology class and registration number on all the work you hand in. Write in dark blue or black pen. You may use an HB pencil for any diagrams or graphs. Do not use staples, paper clips, glue or correction fluid. Section A Answer all questions in the spaces provided on the Question Paper. Section B Answer any one question in the spaces provided on the Question Paper. The use of an approved scientific calculator is expected, where appropriate. You may lose marks if you do not show your working or if you do not use appropriate units. The number of marks is given in the brackets [ ] at the end of each question or part of question. For Examiner’s Use Section A 1 35 2 12 3 8 Section B 4 or 5 20 Total 75 This document consists of 18 printed pages and 2 blank pages.

2 ©NJC 2026 9477/03/SH2 Preliminary Examination Section A Answer all the questions in this section. 1 Glucose-6-phosphate dehydrogenase (G6PD) deficiency is the most common genetic disorder worldwide. (a) G6PD is a enzyme present in all cells . It catalyses the oxidation of glucose-6-phosphate (G6P) and reduces NADP+ to NADPH simultaneously. Fig. 1.1 shows the structure of G6PD which is composed of two identical polypeptide chains, each with an active site for G6P and NADP+, as well as a binding site for a cofactor NADP+. Fig. 1.1 (i) State and explain the highest level of protein structure of G6PD. quaternary; composed of two polypeptides / more than one polypeptide; [2] (ii) With reference to Fig 1.1, explain why enzymes like G6PD must be made from a hundred amino acids or more in order to fulfil their functions. Ref to contact amino acids in active sites and binding sites enable binding to substrates (G6P and NADP+) and cofactor NADP+ respectively; Ref to sites are complementary in shape and charge to the substrates and cofactor; Ref to contact / binding amino acids position substrates in correct orientation; Ref to active site contains catalytic amino acids that act on / bend / weaken the bonds in the substrates to catalyse the conversion to products;

3 ©NJC 2026 9477/03/SH2 Preliminary Examination [Turn over Ref to structural / scaffold amino acids allow folding of polypeptide into a three-dimensional structure of the active site and binding site; Ref to hydrophilic surface amino acids allows enzyme to be soluble in aqueous medium; AVP: acidic amino acids (amino acids with negatively charged R groups) need to bind to NADP+; Ignore generic protein structure e.g. amino acids needed to form secondary structures like alpha helix and beta pleated sheets, bonds between R-groups [4] Fig. 1.2 shows the biochemical pathway involving G6PD. The letters P and Q represent the enzymes that catalyse a reaction in the pathway. Fig. 1.2 (iii) In red blood cells, G6PD is the only source of NADPH, which is required for generating reduced glutathione, an important antioxidant. Using Fig. 1.2 and the information given, describe how G6PD deficiency can lead to haemolysis (rupturing of red blood cells). G6PD deficiency results in insufficient / absence of NADPH in red blood cells; (insufficient / absence of NADPH) results in insufficient / absence of reduced glutathione; (insufficient / absence of reduced glutathione) results in inability to break down H2O2 (hydrogen peroxide); reject less water produced accumulation of H2O2 causes cell damage, resulting in haemolysis; [3] (b) Researchers have identified over 230 variants of the G6PD gene. (i) Table 1.1 shows the classification of G6PD variants with varying G6PD activity relative to normal and the associated symptoms. Table 1.1 G6PD variant class G6PD activity relative to normal activity / % symptoms A <20 chronic haemolytic anaemia

4 ©NJC 2026 9477/03/SH2 Preliminary Examination B <45 acute haemolytic anaemia triggered by certain medicines, fava beans or infection C >60 no haemolysis Justify the claim that the G6PD deficiency phenotype is the result of genotype and the environment acting together. Genotype: ref. to 230 variants produced by mutation in the G6PD gene resulting in different levels of activity; OR severity of symptoms depends on variants e.g. class A more severe than class B, class C has negligible effect; Accept: class A and C are purely due to genotype causing different G6PD activity Environment: for class B, symptoms are triggered by diet (fava beans) / medication / infection ; Reject continuous variation / additive effects as there is only one gene involved [2] (ii) Researchers investigated the effect of substrate concentration on the activity of two G6PD variants, J and K. Variant J is the normal form of the enzyme, while variant K is resulted from a gene mutation. The results are shown in Fig. 1.3.

5 ©NJC 2026 9477/03/SH2 Preliminary Examination [Turn over Fig. 1.3 With reference to Fig. 1.3, describe the effect of the mutation on the activity of G6PD. K compared with J 1. rate of reaction is lower at all substrate concentrations ; 2. ref. to rate at any substrate conc e.g. at 0.4 mmol dm-3 rate for K is 41 µmol min-1 while J is 47 µmol min-1; 3. lower Vmax ; 4. Vmax is 10 µmol min-1 less OR Vmax for K is 50 µmol min-1 while J is 60 µmol min-1; (award once only if point 2 quote Vmax) 5. Less steep increase in rate as substrate increases ; 6. Ref. to gradient / quote rate increase for specific range of substrate conc.; 7. Similar/no change in affinity to substrate OR lower affinity / slightly higher Km ; 8. Both Km is approximately 0.2 mmol dm-3 OR Km is 0.02 mmol dm-3 higher / 0.2 vs 0.22 mmol dm-3 ; units must be correct when quoting values [3]

6 ©NJC 2026 9477/03/SH2 Preliminary Examination (c) Fig. 1.4 is a pedigree showing the inheritance of G6PD deficiency in a family. Fig. 1.4 (i) Explain how Fig. 1.4 provides evidence that the inheritance of G6PD deficiency is X-linked recessive. USE MOST CONCLUSIVE EVIDENCE! Justification for X-linked recessive: (at least 1) [Describe] when fathers are unaffected (I-2, II-4, III-2), all daughters will not be affected; [Explain] as every daughter will receive the X chromosome with dominant allele from their father; [Describe] affected mothers (III-1) results in all sons being affected; [Explain] mother is homozygous recessive and will pass an X chromosome carrying recessive allele to every son; Accept (note that the following points are weak to justify) (max 2): as data is statistically insignificant: [Describe] carrier mothers (I-1 and II-3) produce both affected and unaffected sons; [Explain] as every son has a 50% chance of receiving the X chromosome with recessive allele; [Describe] males display disease phenotype more often than females ; [Explain] as males only need to inherit one disease allele to be affected; only justify for recessive:

7 ©NJC 2026 9477/03/SH2 Preliminary Examination [Turn over [Describe] parents who are not affected (I-1 and I-2 / II-3 and II-4) can give rise to affected children (give one example of unaffected child); [Explain] the allele that cause the disease must be recessive and masked by a dominant allele in the parents; max 2 for description deduct 1m if no ref to specific individuals in fig. ignore all explanation of why not other mode of inheritance, DO NOT explain by elimination. [3] (ii) In Kilifi C