NJC 2023 H2 Biology Prelim P2 Ans

©NJC 2023 9744/02/SH2/H2/Biology/Preliminary Examination [Turn over NATIONAL JUNIOR COLLEGE, SINGAPORE Senior High 2 Preliminary Examination Higher 2 CANDIDATE NAME BIOLOGY CLASS 2bi2_____ REGISTRATION NUMBER Biology Paper 2: Structured Questions Candidates answer on the Question Paper. No Additional Materials are required. 9744/02 24 August 2023 2 hours READ THESE INSTRUCTIONS FIRST Write your name, Biology class and registration number on all the work you hand in. Write in dark blue or black pen. You may use an HB 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 workings or if you do not use appropriate units. The number of marks is given in the brackets [ ] at the end of each question or part of question. For Examiner’s Use 1 /10 2 /9 3 /11 4 /11 5 /9 6 /12 7 /10 8 /8 9 /10 10 /5 11 /5 Total /100 This document consists of 27 printed pages and 1 blank page.

©NJC 2023 9744/02/SH2/H2/Biology/Preliminary Examination 2 Section A Answer all the questions in this section. 1 Triglycerides are transported via lipoproteins such as low-density lipoproteins (LDL). LDL are taken up by target cells as shown in Fig. 1.1. Fig. 1.1 (a) Name the structures labelled A in Fig. 1.1. Ribosomes; [1] (b) Explain the roles of B in the synthesis of LDL receptor. chemically modifies the LDL receptors (e.g. glycosylation); packages LDL receptors the into transport vesicles to be targeted to the plasma membrane for insertion into membrane; [2]

©NJC 2023 9744/02/SH2/H2/Biology/Preliminary Examination [Turn over 3 (c) With reference to Fig. 1.1, describe how an LDL particle is taken up by a target cell. Ligands on LDP particle bind to LDL receptor on surface of target cell; Target cell forms an invagination of the cell surface membrane; An endosome / vesicle containing the LDL particle is formed within the cell; [3] Lipoproteins are made up of proteins and lipids. Their function is to transport cholesterol, triglycerides, and other lipids in the bloodstream. Fig. 1.2 represents the structure of a lipoprotein. Fig. 1.2 (d) (i) With reference to Fig. 1.2, describe the arrangement of phospholipids in lipoproteins. Phospholipids are arranged in a monolayer / single layer; Hydrophobic hydrocarbon tails face inwards while the hydrophilic phosphate heads face outwards; [2] (ii) Suggest why lipoproteins are needed to transport cholesterol, triglycerides, and other lipids in the bloodstream. Triglycerides are hydrophobic in nature and cannot interact with the aqueous medium in the blood; Ref. to phospholipids as amphipathic; Their tails interact with triglycerides within its interior and its heads interact with the aqueous medium in the blood; [2] [Total: 10]

©NJC 2023 9744/02/SH2/H2/Biology/Preliminary Examination 4 2 Collagen serves as the main structural protein in various connective tissues in animals. These tissues include bone cartilage, blood vessels, gum, and skin. (a) Describe the primary structure of collagen. Primary structure refers repeating Gly-X-Y triplet amino acid sequence joined together by peptide bonds; X is usually proline and Y is usually hydroxyproline; Ref. to every third amino acid in the polypeptide sequence is glycine; [2] (b) Explain how the structure of collagen differs from cellulose. Collagen Cellulose  Collagen is a polymer made up of amino acids.  Cellulose is a polymer made up of β-glucose monomers.  Compose of three repeat units of amino acids.  Compose of one repeat unit of glucose. (only β-glucose)  Monomers of collagen (amino acids) are linked via peptide bonds.  Monomers of cellulose (β-glucose) are linked via glycosidic bonds.  Collagen is composed of three helical polypeptide chains twisted together to form a triple helix / tropocollagen.  Cellulose chain is made up of a single straight and unbranched chain of β-glucose polymer.  Inter-chain H-bonding maintains the stability of within a single triple helix / tropocollagen.  H-bonding is between - C=O groups of proline residues on one chain and –NH groups of glycine residues on an adjacent polypeptide chain.  Inter-chain H -bonding maintains the stability between parallel chains of cellulose molecules → this cross-linking by hydrogen bonds form microfibrils.  H-bonding is between OH / hydroxyl groups attached to carbon atom 3 on one chain and carbon atom 6 on a β-glucose on an adjacent chain.  Each tropocollagen crosslinks with a neighbouring tropocollagen running parallel to it → via covalent bonds on adjacent tropocollagen forming collagen fibrils  No covalent cross -links exist between the cellulose microfibrils. [3]

©NJC 2023 9744/02/SH2/H2/Biology/Preliminary Examination [Turn over 5 Fig 2.1 show the main steps involved in the synthesis and assembly of collagen. Fig 2.1 (c) Suggest why the assembly of collagen takes place outside the cell. Cleaving of the ends of the tropocollagen bef ore assembly into collagen fibres is performed by enzymes found only outside of the cell; Collagen molecule is too large to pass through the cell surface membrane and has to be assembled outside the cell; Assembly of collagen outside the cell allows alignment of microfibrils / formation of bonds between the microfibrils; [2] (d) Vitamin C is necessary in aiding the hydroxylation of amino acids to form hydroxyproline and hydroxylysine in Step 2. A deficiency in Vitamin C can lead to scurvy, a disease associated with symptoms such as loss of teeth and easy bruising. Suggest why vitamin C deficiency can lead to scurvy. Hydroxylation / addition of OH groups on proline or lysine allow for formation of hydrogen bonds to further stabilize tropocollagen structure; It also allows for formation of covalent bonds between tropocollagen molecules forming collagen fibril; Step 1 Step 2 Step 3 Step 4 Step 5

©NJC 2023 9744/02/SH2/H2/Biology/Preliminary Examination 6 As a result, basic units of tropocollagen slide against each other as they are unable to form stable bundles of collagen fibres; leading to low tensile strength of collagen, thus causing scurvy; [2] [Total: 9] 3 In eukaryotes, transcription of a gene produces RNA transcripts that must be successfully processed before they can be exported out of the nucleus into the cytoplasm for translation. This process is illustrated in Fig. 3.1. Fig. 3.1 (a) With reference to Fig. 3.1, (i) describe the process occurring at stage X that results in the formation of a continuous coding sequence in mRNA. RNA splicing carried out by the spliceosome; Introns are excised/removed and exons are ligated/spliced/joined to each other to form a mature mRNA molecule; [2] (ii) suggest why the cap-binding complex and poly -A-binding proteins are essential for stage Y. Proteins are recognised by nuclear pore which facilitate the export of mRNA; To ensure that the mRNA is stable for export into the cytoplasm; [1] (b) Describe three differences between eukaryotic and prokaryotic translation. In prokaryotes small ribosomal subunit recognises and binds to the Shine-Dalgarno sequence on the mRNA, while in eukaryotes it recognises and binds to the 5’ cap; In prokaryotes the initiator amino-acyl -tRNA is fMet-tRNA, while in eukaryotes the initiator amino-acyl-tRNA is Met-tRNA / ref to first amino acid added; Prokaryotic translation uses polycistronic mRNA as a template, while eukaryotic translation uses monocistronic mRNA;

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