2025 HKDSE Biology Practice Paper | DSE Mock

A newly developed conjugate vaccine against Streptococcus pneumoniae consists of a bacterial capsular polysaccharide linked to a carrier protein.

(a) Explain why coupling the polysaccharide to a carrier protein is necessary to stimulate a helper T cell response. (2 marks)
(b) Describe how the memory cells produced after vaccination lead to a rapid and robust secondary immune response upon subsequent infection by the actual pathogen. (3 marks)

An experiment was conducted to study the absorption of glucose and fructose by epithelial cells of the human small intestine. The cells were treated with a metabolic inhibitor that completely blocks mitochondrial ATP synthesis.

(a) The uptake rate of glucose dropped by 90%, whereas the uptake rate of fructose remained unaffected. Deduce the primary transport mechanisms of glucose and fructose across the apical membrane of these cells. (3 marks)
(b) Explain why the cellular consumption of oxygen would decrease when active transport of glucose is completely inhibited. (2 marks)

In the industrial production of citric acid using the fungus Aspergillus niger, the fermentation medium is formulated with an excess of sucrose but a strictly limited concentration of iron ions (\(Fe^{2+}\)/\(Fe^{3+}\)).

(a) Explain the biological significance of providing excess sucrose in this fermentation process. (2 marks)
(b) Iron is a cofactor for aconitase, the enzyme that catalyzes the conversion of citric acid to isocitric acid in the Krebs cycle. Explain how limiting iron concentration leads to the accumulation of citric acid. (3 marks)

Central diabetes insipidus is a condition characterized by a deficiency in the secretion of antidiuretic hormone (ADH) from the pituitary gland.

(a) Predict and explain the changes in urine volume and concentration in a patient with this condition. (3 marks)
(b) Suggest a hormone analogue that can be administered to treat this condition, and explain how it helps restore normal urine output. (2 marks)

A researcher treated the leaves of a well-watered plant with abscisic acid (ABA) under constant light.

(a) Explain how the application of ABA affects the transport of potassium ions (\(K^+\)) across the guard cell membrane and subsequently leads to stomatal closure. (3 marks)
(b) Although stomatal closure conserves water, explain why prolonged exposure to high levels of ABA would lead to a reduction in the dry mass of the plant. (2 marks)

An experiment was conducted to investigate the effect of surface area-to-volume (SA/Vol) ratio on the rate of diffusion. Three agar cubes containing phenolphthalein and sodium hydroxide (which makes the agar pink) were prepared with side lengths of 1 cm, 2 cm, and 3 cm. The cubes were immersed in 0.1 M hydrochloric acid (HCl) for exactly 10 minutes, after which they were cut in half to measure the depth of the colorless zone. (a) State the independent variable and the dependent variable of this experiment. [2 marks] (b) Explain why the agar blocks changed from pink to colorless when immersed in hydrochloric acid. [2 marks] (c) After 10 minutes, the depth of the colorless zone was found to be 2 mm in all three cubes. (i) Explain why the depth of the colorless zone was identical in all three cubes. [2 marks] (ii) Calculate the percentage of the volume of the 1 cm cube that remained pink. Show your working. [2.2 marks] (iii) Based on the relationship between organism size and diffusion rate demonstrated by this experiment, explain why large multicellular organisms require a specialized transport system. [1 mark]

A student set up a bubble potometer to study the effect of wind speed on the rate of water uptake by a leafy shoot. (a) State two precautions that must be taken when cutting and mounting the leafy shoot into the potometer, and explain how each precaution ensures the validity of the results. [4.2 marks] (b) The student measured the movement of the air bubble under two conditions: 'Still Air' and 'Windy' (using an electric fan). The air bubble moved 15 mm in 5 minutes under still air, and 45 mm in 5 minutes under windy conditions. (i) Explain the difference in the rate of bubble movement under the two conditions in terms of the water vapor concentration gradient. [3 marks] (ii) Why does the rate of water uptake measured by the potometer not exactly equal the actual rate of transpiration of the leafy shoot? [2 marks]

An ecological survey was conducted to study the distribution of two gastropod species, Patella vulgata (Species A) and Littorina littorea (Species B), along a rocky shore from the low tide mark to the high tide mark. (a) Describe how systematic sampling using a line transect and quadrats should be carried out along the rocky shore. [3 marks] (b) The results showed that Species A was predominantly found near the low tide mark, while Species B was found near the high tide mark. (i) Suggest one abiotic factor that changes from the low tide to the high tide mark, and explain how it affects the distribution of Species A. [2.2 marks] (ii) Species A is a stronger competitor for space but is highly susceptible to desiccation compared to Species B. Use this information to explain the distribution of Species B on the shore. [2 marks] (c) State one limitation of using a quadrat to estimate the abundance of highly mobile animals, and suggest an alternative method suitable for mobile animals. [2 marks]

A student investigated the rate of anaerobic respiration in yeast under different glucose concentrations. A yeast suspension was mixed with glucose solutions of 0%, 5%, 10%, and 25% concentrations in test tubes. Each tube was sealed with a layer of liquid paraffin. The volume of gas produced was measured using a gas syringe over 30 minutes. (a) State the role of the layer of liquid paraffin in this investigation. [2 marks] (b) The results showed that gas production was highest in the 10% glucose solution but decreased significantly in the 25% glucose solution. (i) Explain why the rate of gas production was higher in 10% glucose than in 5% glucose. [2.2 marks] (ii) Explain why the rate of gas production was significantly lower in the 25% glucose solution. [3 marks] (c) Design a suitable control setup for this experiment to show that the gas is produced by yeast respiration and not by the spontaneous decomposition of glucose. [2 marks]

An experiment was conducted to study primary and secondary immune responses in mice. Two groups of healthy mice were injected with Antigen X and Antigen Y according to the schedule below: - Group 1: Injected with Antigen X on Day 0, and injected with Antigen X again on Day 28. - Group 2: Injected with Antigen X on Day 0, and injected with Antigen Y on Day 28. The concentration of antibodies in the blood of both groups was measured weekly. (a) For Group 1, describe and explain the differences in the antibody level against Antigen X after the first injection (Day 0) and the second injection (Day 28). [4 marks] (b) For Group 2, compare the antibody concentration against Antigen Y on Day 35 with the antibody concentration against Antigen X in Group 1 on Day 35. Explain the difference. [3.2 marks] (c) Explain why this experimental setup should use mice of the same age, gender, and genetic background. [2 marks]

Compare and contrast the processes of natural selection and artificial selection. Your answer should cover their driving forces, the criteria for selecting traits, and their respective impacts on the genetic diversity and long-term survival of the populations. (9 marks content + 2 marks communication)