2025 RI H2 Bio Prelim P2 Answers 9477

© RI 2025 Preliminary Examination 9744/02 [Turn over RAFFLES INSTITUTION 2025 Year 6 Preliminary Examination Higher 2 BIOLOGY 9744/02 Paper 2 Structured Questions 24th September 2025 2 hours Candidates answer on Question Paper. No Additional Materials are required. READ THESE INSTRUCTIONS FIRST Write your index number, CT group & name in the spaces at the top of this page. Write in a dark blue or black pen. You may use a HB pencil for any diagrams or graphs. Do not use staples, paper clips, glue or correction fluid. Answer all questions 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 At the end of the examination, fasten all your work securely together. The number of marks is given in brackets [ ] at the end of each question or part question. This document consists of 25 printed pages. CIVICS GROUP CANDIDATE NAME INDEX NUMBER 5 S 0 3 2 For Examiner’s Use 1 / 10 2 / 8 3 / 9 4 / 10 6 / 9 7 / 11 8 / 9 9 / 8 10 / 8 11 / 8 Total / 90 Raffles Institution Internal Examination

2 © RI 2025 Preliminary Examination 9744/02 For Examiner’s Use [Turn over Section A Answer all the questions in this section. 1 All cells are surrounded by a cell surface membrane. All cell membranes have a similar basic structure. However, the proportions of the various molecules present vary between different membranes and even between the inner and outer layers of a membrane. Fig. 1.1 shows a generalised cell surface membrane. Fig. 1.1 (a) Molecule A is a glycolipid. State a major role of glycolipids in the outer layer of the cell surface membrane. [1] 1. Act as markers / antigens for cell-cell recognition* to distinguish cells as ‘ self’/’non-self’ as basis of immune system; or 2. Act as markers for cell-cell recognition* resulting in cell adhesion allowing cells to be attached to one another to form tissues and organs; or 3. Act as receptor* for certain pathogens and toxins to bind to host cells and gain entry into cell (A: specific ligands for signal transduction); (b) In certain circumstances, additional phospholipid or glycolipid molecules are added to the cell surface membrane. These additional molecules are synthesised inside the cell. (i) State where lipids are synthesised inside the cell and where they are converted to glycolipids. [2] lipids synthesised smooth endoplasmic reticulum (R: SER); converted to glycolipids Golgi apparatus; (ii) Suggest one reason why additional phospholipid or glycolipid molecules are added to the cell surface membrane. [1] Actively dividing cells preparing to undergo cytokinesis may add additional phospholipids and/ or glycolipids in the cell surface membrane to replenish membrane/ ensure sufficient membrane available before it divides into two daughter cells; A: cell may lose membrane during processes such as endocytosis and phagocytosis; A: cell is growing; Molecule A

3 © RI 2025 Preliminary Examination 9744/02 [Turn over For Examiner’s Use In bacteria cells, it is important to maintain ion concentration gradients across the cell surface membrane. These gradients, particularly electrochemical gradients of ions like protons and sodium, are essential . They are needed in various cellular functions, including nutrient uptake, ATP synthesis, and maintaining proper cellular pH. The antibiotic Gramidicin A is used to treat bacterial infections. Two molecules of the polypeptide Gramidicin A form the pore as shown in Fig. 1.2. Fig. 1.2 (c) Explain how Gramidicin A is used to treat bacterial infections. [3] 1. Gramidicin A has a hydrophilic pore, which allows ions to diffuse/ move down a concentration gradient; 2. Resulting in equilibrium/ same concentration of ions on either side (A: cannot generate gradient) of the membrane being established); 3. Loss of ion concentration gradient will disrupt important cell processes e.g. ATP synthesis and nutrient uptake/ protons which will lead to cell death; (d) Another antibiotic used to treat bacteria l infections is penicillin. Describe the mode of action of penicillin. [3] 1. Penicillin acts as a competitive inhibitor * which binds to active site * of enzyme transpeptidase; 2. This prevents transpeptidase from forming cross-links between peptidoglycan chains resulting in weakened bacterial cell walls in actively dividing cells; 3. When bacteria take in water by osmosis , there is increased turgor pressure which causes bacteria to swell and lyse; [Total: 10]

4 © RI 2025 Preliminary Examination 9744/02 For Examiner’s Use [Turn over 2 Glycolysis is the initial stage of respiration. It involves hydrolysis of glucose and the production of ATP. Fig. 2.1 shows part of glycolysis. Fig. 2.1 Control of glycolysis is by end-product inhibition: • High levels of ATP inhibit the enzyme PFK in the liver thus lowering its affinity for its substrate. • PFK is the main regulatory enzyme in glycolysis, but it is not the only one. • HK, the enzyme catalysing the first step of glycolysis, is inhibited by its product, glucose- 6-phosphate. (a) Fig. 2.2 shows the effect of the concentration of substrate on the activity of the enzyme PFK. Fig. 2.2 (i) Explain the shape of the graph shown in Fig. 2.2. [2] 1. PFK is an allosteric enzyme * with 2 or more subunits held together by weak intermolecular bonds, allowing movement that influences affinity for substrate, 2. Allowing cooperative binding* of substrate hence sigmoidal/ S shape; 3. The enzyme exists in two conformation states and binding of substrate at active site * stabilises active conformation; 4. Binding of a substrate molecule to first subunit induces a conformational change in other subunits such that their affinity for substrate increases; PFK activity/ arbitrary units Substrate concentration/ arbitrary units

5 © RI 2025 Preliminary Examination 9744/02 [Turn over For Examiner’s Use (ii) Draw on Fig. 2.2, a graph to show PFK activity when ATP concentration is increased. [1] (iii) Explain the graph you have drawn. [2] 1. ATP is an allosteric inhibitor which binds to allosteric site*, stabilising PFK in its inactive conformation which slows down/ inhibits PFK activity; 2. The active site* of PFK has a lower affinity for its substrate; 3. Same maximum rate of reaction can be reached when substrate concentration is high; (b) With reference to Fig. 2.1, s uggest why the cell uses inhibition of PFK as the main method of slowing glycolysis, rather than using the inhibition of HK. [1] Product of HK, glucose 6 -phosphate, can also be stored as glycogen (A: glycogenesis), hence inhibiting PFK is the most effective at inhibiting only glycolysis; AVP: PFK catalyses the first committed step in glycolysis; AVP: PFK catalyses the reaction where the glucose is activated so that glycolysis can take place; (c) Genetic mutation can result in PFK deficiency in humans. As a result, affected individuals display muscle weakness and fatigue. Explain why these individuals will display such symptom