2022 TJC JC2 H2 Biology Prelim P2 ANS

1 [TURN OVER READ THESE INSTRUCTIONS FIRST Write your Center 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 on the Question Paper. The use of an approved scientific calculator is expected, where appropriate. You may lose marks if you do not show any working or if you do not use appropriate units. TEMASEK JUNIOR COLLEGE 2022 JC2 PRELIMINARY EXAMINATION Higher 2 CANDIDATE NAME CENTRE NUMBER S INDEX NUMBER BIOLOGY 9744/02 Paper 2 Structured Questions 23 AUGUST 2022 2 hours Candidates answer on the Question Paper. No Additional Materials are required. The number of marks is given in brackets [ ] at the end of each question or part question. For Examiner’s Use 1 / 9 2 / 9 3 / 11 4 / 7 5 / 8 Answer all questions. This document consists of 11 printed pages and 1 blank page.

2 Answer all questions. 1 Muscle cells are known to store large amounts of glycogen that act as glucose reserves when blood glucose concentration is lower than the homeostatic range. Fig 1.1 shows an electronmicrograph of a part of a muscle cell. Fig. 1.1 (a) Outline structural features shown in Fig 1.1 that identify G as the Golgi body and not the rough endoplasmic reticulum (rER). [2] Note: There are 2 parts to this question – (1) what distinguishing features of G are, AND (2) why NOT rER, which will require comparison with rER. 1. G consists of separate / not interconnected flattened, curved membraneous sacs whereas rER consists of flattened sacs connected to one another. ½ m – if no mention of separate vs interconnected Marker’s comments: Many students missed out the word “flattened” in point 1 or only mentioned the word ‘cisternae’ without describing what it means. 2. The surface of G is smooth whereas the surface of rER is rough. OR Ribosomes absent on G whereas ribosomes present on rER. AVP: The outer membrane of nucleus envelope is not continuous with G whereas it is continuous with the membrane of rER (idea of continuity with nucleus; R: connected to nucleus). Max 1 m if no mention / comparison with rER (basically merely stating the features of G and not addressing the second part of the question)

3 [TURN OVER (b) Describe how the function of G and rER are linked. [2] 1. Ribosomes located on the surface of rER synthesise polypeptide chains which are inserted into the lumen / both organelles involved in formation of lysosomes / secretory vesicles 2. Polypeptide chains will be packaged into ER vesicles and transported to Reminder: it is NOT rER vesicles 3. cis-face of G. (R: golgi) 4. At G, these polypeptide chains will go through chemical modification, sorted and transported / packaged in secretory vesicle. (MUST write chemical modification) Most students are unable to write point 4 in full with the necessary details ½ m: proteins / enzymes synthesised at rER, packaged into vesicles at G (Awarded if MP1 and MP4 not awarded) Fig. 1.2 shows the binding of glucagon to a cell surface receptor. Fig. 1.2 (c) Receptors for some hormones are found within their target cells. Explain why glucagon receptors are found on the cell surface membranes of target cells and never within the cells. [2] 1. Glucagon cannot enter the cell A: cannot pass through the membrane (if MP3 not given) 2. Glucagon is a polar and large molecule 3. Unable to cross the hydrophobic core of the phospholipid bilayer 4. Glucagon receptors have an extracellular ligand-binding site R: Glucagon receptors on the extracellular side of the cell 5. Allows for signal to be transduced / passed into the cell without glucagon entering the cell / cellular response to be triggered. Marker’s comments: Some students focused on the structure of the receptor being hydrophobic (7 alpha- helices) instead of the reason why the receptors have adapted to be found on the cell surface membrane instead of within the cell.

4 (d) Use Fig. 1.2 to explain how the presence of glucagon is able to trigger a signal transduction pathway inside the target cell. [3] Note: students are reminded to use their own contextual knowledge to help them interpret the diagram – the diagram is static and some understanding of G protein activation is required to fully answer the question! 1. Glucagon binds to the extracellular ligand-binding site of the GPCR 2. Triggers conformational change and activates GPCR Note: GPCR must be mentioned to be awarded the mark – learn to be specific in your answers. 3. Inactivated G protein binds to GPCR Note: always remember to mention the change from inactivated to activated. 4. GTP displaces GDP, activating G protein 5. QF: α-subunit of G protein and GTP molecule diffuse / travel / move along cell membrane Note: there is a difference between across and along 6. Bind to adenylyl cyclase and activates it, triggers signal transduction pathway Note: link back to question – no need to mention PKA and phosphorylation cascade when question asks for triggering of signal transduction pathway (NOT for cellular response) [Total: 9]

5 [TURN OVER 2 Collagen is the most common protein in the human body. It is a fibrous protein which is found in structures such as blood vessels, bones, cartilage, connective tissue, tendons, and skin. Fig. 2.1 Fig. 2.1 shows a molecular model of collagen. (a) Describe the primary structure of collagen polypeptide chains and how it contributes to the function of collagen. [3] 1. Each polypeptide chain has about 1000 amino acid residues 2. and consists mainly of repeated glycine-X-Y sequences. 3. This repeating organisation contributes to a stable helical structure (secondary structure). Note: students must state that it is the helical structure that is stabilized. 4. Every third amino acid in the polypeptide is glycine. 5. The R-group of glycine is a H atom and is the only R -group that is small enough to fit into the centre of the triple helix. 6. This allows close association of the three polypeptide chains. Pay attention to the use of ‘triple helix’ and ‘three polypeptide chains’ in MP5 and MP6. (b) Explain how three collagen polypeptide chains are associated with one another. [2] Note: This question is asking about the bonds within ONE tropocollagen molecule. Points regarding covalent cross links are not relevant to this question! 1. Three helical chains wound around each other to form a Note: Many students failed to state that each chain is helical 2. triple helix. 3. The chains are held together by interchain hydrogen bonds 4. between the -NH groups of glycine residues on one chain, and the -CO groups of proline residues of adjacent chain. Marker’s comments: About half of the cohort also mentioned about glycine being small enough to fit into the centre of the triple helix. This is a key feature of the primary structure (every third amino acid being glycine which allows for close association) and does not directly answer the question on how (bonds) the three chains can come together.

6 (c) Collagen molecules are built up into fibrils. Each of the lines in the diagram below represents one of the collagen molecules shown in Fig. 2.1. Complete the diagram to show the location of some possible covalent cross -links between the collagen molecules that make up part of a fib