2025 VJC H2 Biology Prelim P3 Answer

VICTORIA JUNIOR COLLEGE JC 2 PRELIMINARY EXAMINATION 2025 HIGHER 2 NAME : ……………………………………………………….………………. CT CLASS: ………………..……… BIOLOGY Paper 3 Long Structured and Free-response Questions 9744 / 03 18/09/2025 2 hour Candidates answer on the Question Paper. No Additional Materials are required. READ THESE INSTRUCTIONS FIRST Write your name and CT class 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. Section A Answer all questions in the spaces provided on the Question Paper. Section B Answer any one question in the space provided on the Question Paper. 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. The number of marks is given in bracket [ ] at the end of each question or part question. Question Marks Section A 1 2 3 Section B Total This document consists of 20 printed pages. [Turn over]

1 (a) Outline the roles of oxygen in aerobic respiration. [3] • S erves as the final electron acceptor at the end of the electron transport chain in oxidative phosphorylation; • A llows continuous flow of electrons along the electron transport chain for continuous production of ATP in oxidative phosphorylation; • A llows the regeneration of NAD and FAD for link reaction and Krebs cycle to continue in the mitochondria; A ir contains a mixture of gases, which include approximately 78% nitrogen, 21% oxygen and 0.03% carbon dioxide. Fluctuations in these levels have occurred multiple times throughout the evolution of earth’s atmosphere. It is known that Earth was once without oxygen, but the gas began to accumulate in the atmosphere around 2.3 billion years ago. The increased presence of oxygen produces a more efficient energy source in the form of aerobic metabolism, producing 16-18 times more adenosine triphosphate (ATP) per hexose sugar than anaerobic metabolism. Multicellular organisms require efficient oxygen transport systems because the simple process of diffusion is too slow and inefficient to reach all cells within their large bodies, which have a small surface-area-to-volume ratio. Invertebrates like molluscs and arthropods make use of a protein called haemocyanin to transport oxygen throughout their bodies. It is an extracellular protein not contained within cells, but is instead dissolved freely in the haemolymph, which is the invertebrate equivalent of blood. Haemocyanin is made up of a dimer or hexamer of subunit proteins depending on the species, with each subunit containing two copper ions to bind to one oxygen molecule. Fig. 1.1 shows the structure of a haemocyanin subunit, as well as the arrangement of the oxygen binding site in haemocyanin made up of six histidine amino acid residues (His) and the two copper ions. Fi g. 1.1 (b) With reference to the information provided, (i) state the highest level of organisation of haemocyanin. [1] • Q uaternary structure; ( ii) explain how the oxygen binding site of each haemocyanin subunit is formed. [2] • E xtensive folding of the polypeptide to bring the six histidine residues close together in the correct orientation;

• Forming a specific shape of the oxygen binding site that allows two copper ions to be bound; (c) The amino acid sequences of a portion of the core domain of the haemocyanin subunit from four different invertebrate species have been compared. Fig. 1.2 shows the results. Fig. 1.2 Using i nformation in Fig. 1.2, (i) Explain how the r esults shown support Darwin’s theory of evolution. [3] • Ref. to descent with modifications; • All four spec ies have haemocyanin (homologous protein), thus have descended from a common ancestor with haemocyanin; • The haemocyanin of each s pecies accumulated mutations independently after the four species diverged from the common ancestor; (ii) Draw a phylogenetic tree that repres ents the evolutionary history of the four invertebrate species. [2] • Species 1 and 3 are most closely related. [diff by 0,1 amino acid] • Species 4 differs slightly is closely related to species 1 and 3. [4 amino acids] • Species 2 shows more differences, so it is the most distantly related. [7 amino acids] • Start drawing from those most closed related (iii) Recent studies have concluded that species 1 an d 2 diverged 2.6 million years ago. Calcul ate the mutation rate of haemocyanin. Show your working in the space below. Leave your answer in 3 significant figures. …………… x 10-6 mutations per year [1] • 7 mutations / 2.6 m illion years = 2.69 x 10-6 mutations per year (3 s.f.); (iv) With reference to the g enetic code, suggest how the answer in (c)(iii) m ay be an underestimate. [2] 7 different amino acids universal? degeneracy? which more relevant here? it explains that species change over time and share a common ancestor through a process called natural selection

• Ref. to degeneracy of the genetic code and silent mutations; • Total number of mutations may be higher than 7 as some mutations may not result in changes in the amino acid sequence; (d) Haemoc yanin has demonstrated anti-cancer properties as it has been shown to stimulate apoptosis in tumour cells in experiments, reducing the mass of tumours in mouse models, as well as the number of viable cells in cancer cell cultures. Cancer formation involves the accumulation of different mutations. Some mutations are described as dominant mutations, while others are described as recessive mutations. (i) Outline a type of mutation in cancer formation that is described as do minant. [1] • Gain-of-func tion mutation of proto-oncogenes t o oncogenes; (ii) Explain why this type of mutation is described as dominant. [ 2] • Heterozygotes with 1 mut ated allele and 1 normal allele have higher level of stimulation of cell cycle; • When both mutated and normal alleles are expre ssed, heterozygotes have more proteins / higher protein activities that stimulate the cell cycle; • Mutation results in over-expression / expression of constitutively active proteins; (e) In hu mans, the protein that transports oxygen is haemoglobin. Haemoglobin A (HbA) is the most abundant type of haemoglobin in adults, making up 95-98% of total haemoglobin. A variant of haemoglobin known as foetal haemoglobin (HbF) is found in human foetus. It is produced in the foetus at around six weeks of pregnancy, and its levels remains high after birth until the baby is roughly two to four months old. Fig. 1.3 shows the affinity of the two types of haemoglobin. indicate concentration of oxygen mom blood oxygen --> baby blood to decide if is dominant, need to see normal higher affinity lower affinity

Fig. 1.3 With reference to Fig. 1.3, (i) compare the affinity of the two types of haemoglobin at 30 mm Hg of oxygen. [1] • HbA – 56-58% saturation vs. HbF – 80-81% saturation; (ii) suggest an advantage of the foetus expressing HbF. [1] • Able to pull oxygen from HbA in mother’s blood / red blood cells to HbF to be transported to tissues in the foetus; (f) H bA is made up of two α-chains and two β-chains, while HbF is made up of two α-chains and two ү-c hains. After birth, the expression of the ү -chains is switched off, and the baby’s body then begins to express the β-chains. The r egulation of expression of the ү -chains involves two proteins, BCL11a and ZBTB7a. They bind to promoter sequence of the ү-chain during the transition from foetal to adult haemoglobins. Disrupting the binding of these proteins is shown to increase HbF levels and is being developed as a therapeutic strategy for genetic blood disorders like sickle cell disease and β-thalassemia. (i) S tate the level of control of gene expression involving BCL11a and ZBTB7a proteins.