2025 JC2 TJC Prelim P2 ANS

1 [TURN OVER TEMASEK JUNIOR COLLEGE 2025 JC2 PRELIMINARY EXAMINATION Higher 2 CANDIDATE NAME ANSWERS CENTRE NUMBER S INDEX NUMBER BIOLOGY 9744/02 Paper 2 Part I 29 August 2025 2 hours Candidates answer on the Question Paper. READ THESE INSTRUCTIONS FIRST Write your Centre number, index number and 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. DO NOT WRITE IN ANY BARCODES. Answer all questions in the spaces provided in the Question Paper. The use of an approved scientific calculator is expected, where appropriate. The number of marks is given in brackets [ ] at the end of each question or part question. This document consists of 14 printed pages and 2 blank pages. For Examiner’s Use 1 / 10 2 / 10 3 / 10 4 / 11 5 / 9 6 / 10 7 / 11 8 / 8 9 / 11 10 / 5 11 / 5 Total / 100

2 Answer all questions. 1 Most membranes in a cell exist as phospholipid bilayers. Apart from the cell surface membrane, there are several organelles in the cell that are also enclosed by membranes. (a) The fluid mosaic model was devised by Singer and Nicolson in 1972 and was used to describe the membranes. Explain why membranes are described as fluid mosaic in structure. [2] 1. Fluid: a. Phospholipids and membrane proteins are always in constant motion [1/2] R: phospholipid bilayer b. and move laterally in the plane of the membrane. [1/2] 2. Mosaic: a. Membrane proteins are scattered in the sea of phospholipids. [1] R: embedded (there are extrinsic proteins as well) Fig. 1.1 shows a cross-section of the nuclear membrane. The arrow in Fig. 1.1 indicates the movement of substances towards a specific organelle Z. Fig. 1.1 (b) With reference to Fig. 1.1, match the letters A, B, and C to the following structures: (i) outer nuclear membrane B (ii) inner nuclear membrane C (iii) nuclear pore A [3] (c) (i) State the name of the organelle Z and briefly describe its structure. [2] 1. Rough endoplasmic reticulum [1] 2. Membrane-bound organelle consisting of a network of interconnecting membranous flattened sacs called cisternae [1/2] 3. Contains ribosome on its surface [1/2] Z A C B

3 [TURN OVER (ii) With reference to the function of organelle Z, suggest the advantage of the proximity of organelle Z to the nuclear membrane. [3] 1. mRNA molecules formed in nucleus by transcription 2. exit the nucleus via nuclear pores in the nuclear membrane 3. ribosomes near surface of rER can quickly bind to mRNA molecule 4. attach itself quickly to surface of rER for translation 5. polypeptide chain enters lumen of rER 6. and is packaged into ER vesicles for transport to Golgi apparatus. [Total: 10]

4 2 Collagen is the most abundant protein found in the human body. Fig 2.1 shows the primary structure of collagen, which consists of a repeated sequence. Fig 2.1 (a) (i) Name amino acid E. [1] Glycine (ii) With respect to collagen’s structure, explain the significance of amino acid E. [2] 1. Glycine’s R group is a H atom [1/2] 2. is small enough to fit into the centre of the triple helix. [1/2] 3. Allowing close association of the three polypeptide chains. [1/2] 4. To form a tropocollagen molecule. Collagen is an important structural protein found in our skin. Research has shown that the skin’s surface has a slightly acidic pH between 4.7 and 5.75. Various skincare products that contain acid derivatives can cause the skin surface to be more acidic. Certain skin conditions that cause inflammation or infection can also cause the skin’s surface pH to vary. Neutrophils and macrophages require oxygen when undergoing phagocytosis. (b) Suggest how inflammation can cause pH of the skin to be more acidic. [2] 1. Inflammation causes neutrophils and macrophages to migrate to site of infection. 2. Increased use of oxygen can result in anaerobic respiration if there is insufficient oxygen. 3. Lactic acid is released as a byproduct, which reduces pH. (c) Explain how a decrease in pH can affect the structure and stability of collagen. [3] 1. Decrease in pH is caused by an increase in the concentration of H+. 2. This changes the ionic charges on the basic and acidic R groups of the amino acid residues in collagen. 3. The hydrogen bonds between peptide linkages 4. C=O of proline and N-H of glycine of adjacent chains are disrupted. 5. Tropocollagen loses its triple helix structure (A: collagen) 6. and triple helix structure is unstable / tropocollagen molecule loses its stability.

5 [TURN OVER Aside from collagen, another structural molecule that can be found in nature is cellulose. (d) Compare the structures of collagen and cellulose. [2] Similarities [max 2M]: 1. Both are polymers / made up of many monomers. 2. Both are formed via condensation reactions between monomers. 3. Both contain interchain hydrogen bonds / hydrogen bonds within chains. 4. Both are fibrous structures. Differences [max 2M]: Point of comparison Cellulose Collagen monomer β glucose Amino acids (Reject: tropocollagen is the basic unit for collagen) variety in monomer Only β-glucose molecules. Mainly repeated glycine, proline and hydroxyproline residues. types of bonds between monomers β(1-4) glycosidic bonds Peptide bonds arrangement of monomers Alternate β-glucose molecule is rotated 180° to each other. Monomers are not rotated. structure adopted by each chain Each chain is straight / linear. Each polypeptide chain forms a helix / helical structure. (R: Collagen has a helical structure.) type of cross-links in the structure Only hydrogen bond cross - linking between neighbouring chains. interchain hydrogen bonds between neighboring chains and Covalent cross-links between C and N terminals between tropocollagen. Structure making up the macromolecule A straight / linear cellulose chain. Three helical chains to form a triple helix. arrangement of components in the macromolecule Chains are not in a staggered arrangement. The tropocollagen molecules / triple helix are in a staggered arrangement. [Total: 10]

6 3 Acetylcholine is an essential neurotransmitter. A neurotransmitter is a signalling molecule secreted by a neuron that affects other cells. They are important to the function of complex neural systems. Acetylcholine is involved in various physiological processes, including voluntary and involuntary muscle contraction, learning and memory. Acetylcholine is synthesised by the enzyme, choline acetyltransferase (CHAT). Fig. 3.1 shows the structure of CHAT. Fig. 3.1 (a) With reference to Fig 3.1, name the secondary structure of protein that is most observed. [1] alpha-helix/ α-helix

7 [TURN OVER Fig. 3.2 shows the mechanism involved in the synthesis of acetylcholine. Boxes have been drawn around the names of the molecules. Fig. 3.2 (b) With reference to Fig. 3.1 and 3.2, describe the mechanism of action of CHAT enzyme in terms of induced fit hypothesis. [2] 1. CHAT enzyme has two active sites, one for each substrate – choline and acetyl-coA A: one active site that has complementary shape to two substrates AVP: CHAT active site is a physically flexible structure. 2. Shape of substrates (choline and acetyl -coA) are still complementary to the shape of active site of enzyme but does not fit exactly. 3. Bindi