2025 RI H2 Bio Prelim P3 Answers 9477

© RI 2025 Preliminary Examination 9744/03 [Turn over RAFFLES INSTITUTION 2025 Year 6 Preliminary Examination Higher 2 BIOLOGY 9744/03 Paper 3 Long Structured and Free-response Questions 17th Sept 2025 2 hours Candidates answer on the Question Paper. Additional Materials: Writing paper. READ THESE INSTRUCTIONS FIRST Write your index number, CT group & name in the spaces at the top of this page. 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 writing paper provided. 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. At the end of the examination, hand in your essay question SEPARATELY. The number of marks is given in brackets [ ] at the end of each question or part question. This document consists of 15 printed pages. CIVICS GROUP CANDIDATE NAME INDEX NUMBER 5 S 0 3 2 Raffles Institution Internal Examination For Examiner’s Use Section A 1 / 31 2 / 8 3 / 11 Section B 4 or 5 / 25 Total / 75

2 © RI 2025 Preliminary Examination 9744/03 For Examiner’s Use Section A Answer all the questions in this section 1 Liver cells, or hepatocytes, are highly speciali sed and metabolically active. These cells are responsible for a vast array of vital functions, including the metabolism of carbohydrates, lipids, and proteins, detoxification of harmful substances, synthesis of plasma proteins, and bile production. Their constant activity makes them susceptible to damage from various factors such as viral infections (e.g., hepatitis), excessive alcohol consumption, and metabolic disorders like non-alcoholic fatty liver disease (NAFLD), which can impair their function and lead to serious health issues. Fig. 1.1 is a transmission electron micrograph showing a section of a human liver cell. Fig. 1.1 (a) (i) Name organelles A and B shown in Fig. 1.1. [1] A Mitochondrion (R: mitochondria) B Rough endoplasmic reticulum (R: rER) (ii) In liver cells, enzymes are attached to the membrane of smooth endoplasmic reticulum (SER). With reference to the functions of smooth endoplasmic reticulum, suggest the advantages of having enzymes attached to the membrane rather than free in the lumen of the SER. [2] 1. More efficient synthesis (R: metabolism) of lipid/ phospholipid/ steroid (R: other biomolecules); 2. Synthesis of products is in a pathway/ requires more than one enzyme thus allows enzymes to be close together/ products of one reaction are the substrates of next reaction/ in sequential arrangement of enzymes of a catalytic pathway R: compartmentalisation) 3. By embedding enzymes in membrane, enzymes are more stable/ greater protection from degradation; 4. AVP with sufficient elaboration;

3 © RI 2025 Preliminary Examination 9744/03 [Turn over For Examiner’s Use e.g. idea of greater efficiency as enzyme held down so that active site faces substrates in lumen; allows reactions to occur on cytoplasmic side of membrane; allows reactions to continue in SER transport vesicles; Neutral lipids or non-polar lipids such as triglycerides and cholesterol esters are synthesised in the membrane of SER of the liver cell. Fig. 1.2 shows the formation of a lipid droplet (LD) from the endoplasmic reticulum membrane. Fig. 1.2 (b) (i) Describe the formation of the lipid droplet shown in Fig. 1.2. [3] 1. Neutral lipids accumulate between the two layers of phospholipids (R: in the membrane of SER); A: lipids accumulate within phospholipid bilayer A: lipids accumulate within hydrophobic core of phospholipid bilayer 2. Continuous accumulation of neutral lipids leads to formation of a growing bulge A: Neutral lipids pushing two layers of phospholipids of SER membrane apart R: there is lens formation 3. S ingle phospholipid layer (R: single phospholipid bilayer, R: single membrane) pinches off from ER membrane into cytosol to form a lipid droplet; (ii) Lipid droplets serve as energy storage within the cell. Based on the properties of the biomolecules involved, explain how a lipid droplet maintains its stable structure in the cytoplasm. [2] 1. Lipid droplet core is composed of hydrophobic* neutral lipids (R: lipids), which interact (R: face) with hydrophobic hydrocarbon tails (R: fatty acids) of a surrounding single phospholipid monolayer; 2. Amphipathic phospholipids in monolayer orient phosphate heads* outwards, which are charged and hydrophilic*, interacting with aqueous cytoplasm (R: cytoplasm alone/ outside cell); 3. while their hydrophobic* hydrocarbon tails* face inwards, effectively shielding neutral lipid (R: lipids) core from water and preventing its dispersion;

4 © RI 2025 Preliminary Examination 9744/03 For Examiner’s Use (c) Prostaglandins are another type of lipid synthesised at the SER. Prostaglandins are small lipids produced in many tissues of the body. One role of prostaglandins is to cause inflammation at the site of an injury or infection. Inflammation is the normal first response of the immune system to injury or infection. Cyclooxygenase (COX) is an enzyme that catalyses one of the steps in the reaction pathway for the formation of prostaglandins from phospholipids. The reaction pathway occurs in the SER of cells. Part of the reaction pathway is shown in Fig. 1.3. Fig. 1.3 (i) Suggest an advantage for this reaction pathway occurring in the smooth endoplasmic reticulum of a cell rather than in the cytoplasm. [1] 1. SER is membrane-bound organelle so it can provide phospholipid/ synthesised phospholipid/ arachidonic acid which is a substrate in the reaction; 2. SER is involved in lipid transport thus prostaglandins can be transported (from SER) to Golgi Apparatus since that is the next immediate member of the endomembrane system or stored (in SER); 3. Compartmentalisation/ separated from other reactions in cytoplasm/ provides optimum conditions for enzyme reactions/ higher concentration of enzymes; (ii) Inflammation may have side-effects, such as pain. Aspirin is a drug that can be used to reduce these side-effects by reducing the catalytic activity of the COX enzyme by modifying the R-group of one of the amino acids. Explain how modifying the R-group of an amino acid in the COX enzyme can reduce the catalytic activity of the enzyme. [3] 1. A change in R group of a structural amino acid may result in different interactions between R groups such as ionic bond, hydrogen bond or hydrophobic interactions; 2. A change in R group can r esult in a change in conformation of enzyme active site*; 3. Active site is not completely complementary in shape /conformation* to the arachidonic acid or substrate ; 4. Enzyme–substrate* complex, not formed / formed at reduced rate; 5. Catalytic R group is absent that cannot catalyse the reaction/ contact R group is absent and cannot hold the substrate well at active site; 6. Ref. to effect of activation energy not being reduced ; e.g. ref to changed charges (so no electron transfer) no longer provides hydrophobic regions for reaction to occur;

5 © RI 2025 Preliminary Examination 9744/03 [Turn over For Examiner’s Use A number of chemicals can disrupt mitochondrial