2026 DHS H2 Biology Prelim Paper 3 answers with marker's comments

1 Name: Centre/Index Number: Class: DUNMAN HIGH SCHOOL Preliminary Examination Year 6 H2 BIOLOGY Paper 3 Long Structured and Free-response Questions You must answer on the question paper. No additional materials are needed. 9477/03 15 September 2026 2 hours INSTRUCTIONS ● Section A: answer all questions. ● Section B: answer one question. ● Use a black or dark blue pen. You may use an HB pencil for any diagrams or graphs. ● Write your name, centre number/index number and class in the boxes at the top of the page. ● Write your answer to each question in the space provided. ● Do not use an erasable pen. Do not use correction fluid or tape. ● You may use an approved calculator. INFORMATION ● The total mark for this paper is 75. ● The number of marks for each question or part question is shown in brackets [ ]. For Examiner’s Use Section A / 55 1 24 2 31 Section B / 20 3 / 4 20 Total 75 This document has 20 pages.

2 © DHS 2026 9477/03 Section A Answer all the questions in this section. 1 Some types of snakes kill their prey and defend themselves by means of a poisonous bite. Fangs (hollow teeth) inject venom from specialized glands into the victim. The venom contains a protein, which is a toxin. Different species of snake have toxins that act in different ways. H aemolytic toxins are enzymes that hydrolyse phospholipids. They damage tissues, including heart muscle, which lead to cardiac arrest. Neurotoxins, such as the one produced by green mamba snakes, bind to receptor proteins on the surface membranes of nerve cells or muscle fibres. This interferes with the transmission of nerve impulse, leading to muscle paralysis and heart failures. Fig. 1.1 shows the molecular structure of fasciculin- 2, a neurotoxin produced by the green mamba snake. Fig. 1.1 (a)(i) With reference to Fig. 1.1, describe the molecular structure of fasciculin-2. Any one 1. Five + β-pleated sheets 2. antiparallel + β-pleated sheets 3. β-pleated sheets as the secondary structures, held by hydrogen bonds between C=O and NH of the peptide region / polypeptide backbone and 4. The single polypeptide further folded into a tertiary/globular structure, held by ionic bonds , hydrogen bonds, disulfide bonds and hydrophobic interactions between R groups (name at least 2 bonds). [Award 1m if point 1 and 2 are mentioned without bonds] Marker’s comments: Many students still need to revise on structure of protein. H bond in secondary structure is poorly described. Some students do not recognise β-pleated sheets. Always give at least 2 e.g. of bonds. Question is “describe the molecular structure” and not just describe secondary structure. Many students overwrite about secondary structure. 1 mark for secondary structure+bond, 1 mark for overall 3D shape+bond. [2]

3 © DHS 2026 9477/03 [Turn over (ii) With reference to haemolytic toxins and neurotoxins, explain why snake venom, which has been heated to 100 °C for several minutes, would likely lose its toxicity. [General effect of heat on proteins] 1. Heat breaks hydrogen bonds and hydrophobic interactions that hold the toxin in its intact 3D structure 2. Loses tertiary / globular / 3D structure, hence denatures the toxin [Effect of heat on hemolytic toxins] 3. Hemolytic enzymes loses the shape of its active site, hence no longer complementary in shape [mark once] to phospholipids, hence unable to bind and hydrolyze phospholipids [Effect of heat on neurotoxins] 4. Shape of neurotoxin is no longer complementary to the receptors on nerve/muscle cells, hence will not interfere with nerve impulse transmission. Marker’s comments: Q says haemolytic toxins and neurotoxins, so you need to address both. There are standard steps of descriptions for how heat, pH and changes in concentration affect enzymes. Please redo the enzyme tutorial if you need revision on the steps. [4] (iii) State how enzymes which hydrolyse phospholipids can lead to tissue damage. Disrupts cell membrane integrity leading to lysis of cells. [1]

4 © DHS 2026 9477/03 Some antibodies bind to toxins and inactivate them. These antibodies are known as anti - toxins. The human immune response is far too slow to be effective in making anti -toxins against snake venom. Injecting a very small, non-lethal quantity of venom into a horse produces anti-toxin. The horse produces anti -toxins that can be extracted from horse blood and used as an emergency treatment for those bitten by the same species of snake. Each time the hor se is injected with venom, it is able to tolerate larger doses and the concentration of the specific anti -toxin in its blood is greater. (b)(i) Explain why the human immune response is too slow to protect a person from a snake bite. 1. Toxin acts too fast for anti-toxin to develop 2. Time is needed for [any 2] • Internalisation of toxin by B cells and macrophages • Antigen presentation in complex with MHC class II by B cells and macrophages to helper T cells • Proliferation and differentiation of B cells into plasma cells • Antibody synthesis and secretion from plasma cells 3. Human unlikely to have been bitten before, hence no prior exposure to the toxin, thus has no memory B and T cells to immediately respond to the toxin Marker’s comments: Q is not asking why response takes time. So cannot just spam pt2. [3] (ii) Explain why a horse is injected more than once with a small amount of venom when it is being prepared for use as a source of anti-toxin. • First injection stimulates a primary immune response to produce anti -toxin- specific memory B cells and plasma cells, where only a small amount of anti- toxin are produced. • Second injection stimulates secondary immune response , where many memory B cells differentiate into many plasma cells, hence a large amount of anti-toxin is secreted. • AVP e.g. a single large dose would kill the horse Marker’s comments: Q asks why more than once. So, need to answer once vs more than once. Many students do not have the complete phrasing – primary/secondary, memory B cell (focus on B because question is about anti- toxin which is antibody) and plasma cells produce anti-toxin. [2]

5 © DHS 2026 9477/03 [Turn over (iii) It was observed that upon injection with the toxin, different types of anti -toxins are produced. Each type of anti-toxin is different at the variable region, but they are equally effective against the toxin. Suggest why different anti-toxins are produced. 1. Toxin is an antigen with different epitopes. 2. Each (naïve) B cell only recognize a single epitope , t hus, each B cell differentiate into plasma cells that produce anti-toxin specific to that epitope Marker’s comments: Q is asking why inject one toxin, and different anti-toxins of equal effectiveness are produced. This is only possible because of immune response to different epitopes on the toxin. So cannot accept somatic recombination [random joining of gene segments (V, D, and J) to build a basic, functional antigen receptor before an immune cell ever encounters a foreign invader], cannot accept somatic hypermutation, and affinity maturation [both work together to help your body make stronger, more targeted antibodies against after infection] [2] (iv) Explain why treatment with the horse anti -toxin will not produce long -term protection against snake bites. 1. Ref to. Passive immunity 2. Anti-toxin remains in the blood only for a short time / eventually destroyed

6 © DHS 2026 9477/03 3. The person’s immune system is not stimulated to