HCI H2 Bio 2024 Prelim P2 Ans

HWA CHONG INSTITUTION / 2023-2024 H2 BIOLOGY / Preliminary Examination Paper 2 / MS 1 HWA CHONG INSTITUTION (COLLEGE SECTION) 2024 JC2 9744 H2 BIOLOGY PRELIMINARY EXAMINATIONS PAPER 2 MARK SCHEME QUESTION 1 (a) Describe one feature visible in Fig. 1.1 that identifies the amoeba as a eukaryotic organism.[1] 1 it contains a true nucleus / a membrane bound nucleus / membrane bound organelle like the food containing vacuole (b)(i) Explain how the structure of the contractile membrane enables it to perform its function. [ 4] 1 presence of protein pumps / carriers for ions embedded on the contractile vacuole membrane 2 allow for active transport / pumping of ions into the lumen of contractile vacuole 3 resulting in the solute potential of the lumen being high / the lumen of the vacuole being of more negative water potential than the cytoplasm 4 so that excess water can enter the contractile vacuole by osmosis and expelled from the cell by exocytosis 5 presence of hydrophobic core of phospholipid bilayer / membrane 6 results in the membrane of contractile vacuole being impermeable to ions 7 allows accumulation of ions in the / prevents ions from leaving the lumen of the contractile vacuole so that solute potential of the lumen is high 8 so that excess water can enter the contractile vacuole by osmosis and expelled from the cell by exocytosis 9 contractile vacuole is made by phospholipids that form a bilayer / phospholipid bilayer 10 presence of weak hydrophobic interactions between fatty acid tails of the phospholipids / within the hydrophobic core 11 allow for membrane fluidity 12 that helps in fusion of the contractile vacuole with the cell surface membrane for exocytosis of water 13 presence of water channels / aquaporin embedded on the contractile vacuole membrane 14 allow for facilitated diffusion of water into the lumen of the contractile vacuole 15 so that excess water can enter the contractile vacuole by osmosis and expelled from the cell by exocytosis ref. to water moving from region of less negative water potential to region of more negative water potential 16 ref. to fluidity of the membrane allows for fusion of many small vesicles to the contractile vacuole to allow the vacuole to expand / increase in size (b)(ii) Explain how organelles of the endomembrane system are involved in the synthesis of proteins embedded within the membranes of contractile vacuoles. [4] 1 mRNA coding for proteins are translated by rough endoplasmic reticulum (RER) bound ribosomes forming polypeptides 2 movement of polypeptide into lumen of RER is halted when the polypeptide is midway through and the polypeptide continues to fold, resulting in a protein that is embedded in RER membrane 3 Transport vesicles with such proteins bud off the RER

HWA CHONG INSTITUTION / 2023-2024 H2 BIOLOGY / Preliminary Examination Paper 2 / MS 2 4 these vesicles migrate to and fuse with cis face of Golgi apparatus (GA) where chemical modification occurs 5 GA releases these as vesicles that remain in the cytoplasm to serve as contractile vacuole / secretory vesicles containing these enzymes migrate to and fuse with contractile vacuoles [Total: 9] QUESTION 2 (a) State the level of protein structure found in β-keratin. [1] secondary (b) With reference to Fig. 2.1, describe features of keratin structure that contribute to its stability. [4] 1 β-keratin polypeptide + (Anti-parallel) β-pleated sheets + numerous intrachain hydrogen bonding between C=O and N-H groups of neighbouring segments 2 in a β -keratin dimer, there are two covalent / disulfide bonds between two β -keratin polypeptides 3 in a β -keratin fibril, there are two covalent/disulfide bonds between β-keratin dimers 4 in a β -keratin fibre, there are numerous / extensive disulfide bonds between neighbouring / adjacent β-keratin fibrils 5 disulfide / covalent bonds being, strong / not overcome by temperature / pH (c) Keratinase is a type of protease that catalyse the hydrolysis of α-keratin and β-keratin. Many proteases are able to hydrolyse more than one type of protein. Explain why it is possible for a protease to act on different types of protein. [3] 1 induced-fit hypothesis 2 substrates / proteins with similar 3D conformation / shape being able to fit / bind active site of enzyme 3 active site changes its 3D conformation slightly to fit the substrate more firmly/snugly 4 R groups of catalytic amino acids are brought into close proximity to the chemical bonds to be broken 5 straining of chemical bonds [Total: 8]

HWA CHONG INSTITUTION / 2023-2024 H2 BIOLOGY / Preliminary Examination Paper 2 / MS 3 QUESTION 3 (a)(i) Name molecules A and B. [2] A - messenger RNA B - aminoacyl tRNA (a)(ii) With reference to molecules A and B, discuss the role of hydrogen bonds in translation. [2] 1a hydrogen bonds between complementary segments of the tRNA 1b stabilising the 3D conformation of tRNA / tRNA folds into a specific 3D conformation for binding to specific aminoacyl tRNA synthetase 2a hydrogen bonds between anti-codon and codon 2b giving rise to specificity of amino acid residues added to the polypeptide 3a hydrogen bonds between tRNA and rRNA 3b holding it in the A-site and P-site (b) Describe the differences between transcription and DNA replication. [3] 1 RNA polymerase vs DNA polymerase as the catalyst 2 transcription produces RNA while DNA replication produces DNA daughter strands 3 only one of the two DNA strands is used as template for transcription while both strands are used as template for DNA replication 4 type of monomers being different 5 transcription starting at promoter while DNA replication begins at origin of replication 6 transcription copies selected genes while DNA replication copies the whole DNA / genome 7 RNA polymerase unwinding and unzipping DNA double helix in transcription vs helicase in DNA replication 8 RNA being synthesised continuously in transcription while lagging strand is synthesised discontinuously in DNA replication 9 RNA primers needed to provide free 3’OH group for DNA polymerase to extend in DNA replication but not needed in transcription 10 transcription being unidirectional vs DNA replication being bidirectional 11 proof reading (c) Explain how carbovir triphosphate may prevent the synthesis of viral DNA. [3] 1 absence of free 3’ OH group when carbovir triphosphate is added to a growing chain 2 DNA polymerase cannot form phosphodiester bond between incoming free deoxyribonucleotide and growing strand 3 complete (viral) DNA not made / chain does not elongate / no more nucleotides added 4 similar shape to, substrate / (activated / phosphorylated) nucleotide + acts as a competitive inhibitor 5 any further detail of how, inhibitor / carbovir triphosphate, may act 6 proofreading mechanism [Total: 10]

HWA CHONG INSTITUTION / 2023-2024 H2 BIOLOGY / Preliminary Examination Paper 2 / MS 4 QUESTION 4 (a) Identify structures A to D. [2] A: viral envelope B: RNA C: nucleoproteins / RNA-binding proteins D: glycoprotein (b) State the origins of structures A and D. [1] Structure A is from previous host cell and structure D is from virus ; (c) Both coronaviruses and influenza viruses rely on a similar class of enzyme for their replication processes. Describe how the coronavirus pr