2022 J2 ASRJC H2 Biology Prelims Paper 2 (Ans)

ASRJC BIOLOGY DEPT 9744//2022/J2PRELIM/P2 [Turn over H2 ANDERSON SERANGOON JUNIOR COLLEGE HIGHER 2 ANSWERS 2022 JC2 PRELIMINARY EXAMINATIONS CANDIDATE NAME CLASS INDEX NUMBER BIOLOGY 9744/02 PAPER 2 SHORT STRUCTURED QUESTIONS Candidates answer on the Question Paper. No Additional Materials are required. 13 SEPTEMBER 2022 TUESDAY 2 HOURS READ THESE INSTRUCTIONS FIRST Write your name and class on all the work you hand in. Write in dark blue or black pen. You may use an HB pencil for any diagrams or graph Do not use paper clips, highlighters, glue or correction fluid. Answer all questions. 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 13 printed pages and 1 blank page For Examiner’s Use 1 / 10 2 / 11 3 / 9 4 / 10 5 / 10 6 / 10 7 / 10 8 / 10 9 / 10 10 / 5 11 / 5 Total /100

ASRJC BIOLOGY DEPT 9744//2022/J2PRELIM//P2 Answer all the questions. 1 Chitin, the second most abundant organic polymer after cellulose on Earth, is found in the cell walls of fungi and the exoskeleton of insects. Similar to cellulose, chitin is a structural polysaccharide. Chitin consists of N-acetylglucosamine. Fig. 1.1 shows the structure of chitin. Fig. 1.1 (a) With reference to Fig. 1.1, (i) name the covalent bond between two monomers in a chitin molecule and describe how this bond is formed. 1. Monomers are linked by β (1,4) glycosidic bonds; 2. Which is formed via a condensation reaction, with loss of 1 water molecule per bond formed; 3. between a hydroxyl/OH group on carbon atom 1 of one N- acetylglucosamine and a hydroxyl/OH group on carbon atom 4 of another N-acetylglucosamine; 4. every alternate/successive N-acetylglucosamine monomer has to be rotated by 180°/ inverted with respect to one another , so that the –OH groups on carbon atoms 1 and 4 to line up alongside each other during condensation. [3]

3 ASRJC BIOLOGY DEPT 9744//2022/PRELIM/P2 [Turn over (ii) explain how the structure and property of chitin are related to its role as a structural polysaccharide in fungi and insects. 1. Alternate N-acetylglucosamine in chitin chain is inverted/ rotated 180o → results in a straight / linear chitin chain/molecule, allowing bundling of cellulose chains into microfibrils, macrofibrils and fibres; 2. Polar/hydrophilic –OH groups and/or N-acetyl groups in chitin chains are projected outwards in all directions; 3. This allows extensive cross-linking between chitin chains by formation of interchain hydrogen bonds between –OH groups and N -acetyl groups across parallel chains; 4. Parallel straight / linear chitin chains bundle/associate together to form microfibrils, which are arranged in larger bundles to form fibres/ Parallel chitin chains associate together to form a fibrous structure; 5. giving it high tensile strength; 6. each chitin chain/molecule is composed of large number of N- acetylglucosamine monomers → large molecule, thus is insoluble in water. [4] (c) Chitinase is an enzyme found in plants. It degrades chitin in fungal cell walls and exoskeletons of insects, protecting the plants against a range of pathogens. Describe the mode of action of chitinase. 1. Chitinase lowers activation energy + any one elaboration below 1. physically strains bonds within substrates to be broken / distorts the substrate; 2. Altering charges on substrates; 3. active site provides a favorable microenvironment for catalysis to occur, eg hydrophobic micro-environment for hydrophobic substrates to react; max 2 from above 2. [lock and key hypothesis ] The ( 3D conformation of) substrate (e.g., cellulose) is complementary to the active site of the enzyme, thus forming enzyme-substrate complexes; 3. [induced fit hypothesis] With ref. to change in 3D conformation of enzyme for a better fit/ tighter binding between substrate and enzyme; 4. Resulting in hydrolysis of glycosidic bond between N-acetylglucosamine residues Note to answer in context, presence of 1 or more than 1 types of substrate [3] Easy: 3, Moderate: 5, Challenging: 2 [Total: 10]

ASRJC BIOLOGY DEPT 9744//2022/J2PRELIM//P2 2 Fig. 2.1 is an electronmicrograph of a human cell during mitosis. Fig. 2.1 (a) Describe the events that take place in the stage of mitosis before that seen in Fig. 2.1. 1. During prophase, chromatin fibres condense to become discrete chromosomes; 2. nuclear envelope breaks down/disintegrates; 3. nucleolus gradually disappears (reject disintegrate nucleolus); 4. Centrioles migrate to opposite poles of the cells; 5. The mitotic spindle begins to form and spindle fibres begin to assemble/extend. Reject Interphase events The majority of candidates were able to identify the correct stage of mitosis. [3]

5 ASRJC BIOLOGY DEPT 9744//2022/PRELIM/P2 [Turn over (b) The normal diploid number of chromosomes for a human cell, such as that shown Fig. 2.1, is 46. The cell in Fig. 2.1 has 92 DNA molecules. Explain the presence of 92 DNA molecules in this cell and why it is important to have this number. Accounting of 92 ds DNA molecules: 1. DNA replication occurs during S phase of interphase before mitosis occurs; 2. each strand of parental DNA is used as template to synthesize a complementary DNA strand → doubling in number of DNA molecules from 46 to 92; 3. After DNA replication, each chromosome consists of two sister chromatids joined at the centromere; Importance of 92 ds DNA molecules: 1. ensures that after mitosis, each daughter nucleus is genetically identical (same type and number of chromosomes) to the parent nucleus; 2. for growth in multicellular organisms(role), - increase in the number of genetically identical cells, carrying out the same function; 3. to repair tissues by replacement of (NOT REPAIR cells) damaged or worn-out cells with genetically identical cells Most candidates demonstrated a good understanding of the underlying reason for DNA replication during mitosis, but few complete answers were seen. Details were often missing. Although the information in the question clearly stated that the context was mitosis, the most common error was for candidates to confuse mitosis with meiosis. A number of candidates referred to the formation of haploid gametes and the production of diploid zygotes following fertilisation. (WRONG!) [4]

ASRJC BIOLOGY DEPT 9744//2022/J2PRELIM//P2 (c) Fig. 2.2 shows DNA replication occurring in a human cell (A) and in an Escherichia coli (B). Diagrams are not shown to scale. A