HKDSE Biology Paper 2 Mock (2027): Four Electives, Answer Two, with Answers

Question 1

(a) Insulin (1 mark), produced by the islets of Langerhans (beta cells) in the pancreas (1 mark). (b) It promotes glucose uptake by liver and muscle cells for storage as glycogen, and speeds up the oxidation of glucose in cells to release energy (2 marks, one each). (c) Diabetes (1 mark).

Question 2

(a) Speed = 0.6 ÷ (6.0 × 10⁻³) = 100 m s⁻¹ (2 marks). (b) Neurotransmitter is stored only in the synaptic vesicles of the presynaptic terminal and released into the cleft when stimulated, while receptors sit only on the postsynaptic membrane, so transmission runs one way (2 marks). (c) Acetylcholine, which acts at the neuromuscular junction to make muscle contract (1 mark).

Question 3

(a) Arterioles in the skin dilate, so more blood flows near the surface and radiates and convects more heat away (1 mark); sweat glands secrete more, and evaporation of sweat removes latent heat (1 mark). (b) The hypothalamus (1 mark). (c) When a value drifts from its set point the system produces a response opposite to the drift – too hot means more heat loss – so the value returns to its normal range (2 marks).

Question 4

(a) Luteinising hormone (LH) (1 mark). (b) A steady, raised level of oestrogen and progesterone inhibits the secretion of FSH and LH (1 mark), so no follicle matures and ovulation does not occur (1 mark), and the endometrium and cervical mucus change so that fertilisation and implantation are unlikely (1 mark). (c) The oviduct (Fallopian tube) (1 mark).

Question 1

(a) About 10% (1 mark). (b) 25 000 × 10% × 10% × 10% = 25 kJ (2 marks). (c) Quadrat area = 0.25 m², so density = 12 ÷ 0.25 = 48 per m² and the estimate is 48 × 100 = 4 800 snails (2 marks).

Question 2

(a) E. coli (1 mark): its count reflects sewage or faecal contamination, and a higher count means worse pollution (1 mark). (b) Excess nitrogen and phosphorus make algae bloom; when the algae die, decomposers break them down and use up the dissolved oxygen, so fish suffocate (2 marks). (c) Use less fertiliser, keep a buffer strip between fields and streams, or treat animal waste before discharge (1 mark).

Question 3

(a) Conservation means using and managing natural resources sustainably so they last, while preservation means leaving them untouched to develop naturally (2 marks). (b) Habitat loss and fragmentation; over-hunting or over-collecting; pollution; invasive species; climate change (2 marks, any two). (c) A reserve keeps habitats and food webs intact so populations can breed with little human disturbance, preserving species and genetic diversity (1 mark).

Question 4

(a) Meeting the needs of the present without harming the ability of future generations to meet theirs (2 marks). (b) Renewable: solar and wind (or hydro, biomass) (1 mark); non-renewable: fossil fuels such as coal, oil and natural gas, or uranium (1 mark). (c) Recycling still costs energy, transport and reprocessing, whereas reducing use cuts waste and demand at source at the lowest environmental cost, so the order is reduce, then reuse, then recycle (1 mark).

Question 1

(a) Cells double each generation, so numbers rise exponentially and metabolism is at its most active (2 marks). (b) High temperature denatures proteins and enzymes and kills the cells; low temperature only slows metabolism and reproduction, and the cells survive (2 marks). (c) Autoclaving (about 121 °C, 15 psi, 15 minutes) or flaming in a Bunsen flame (1 mark).

Question 2

(a) Carbon dioxide (1 mark). (b) C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ (2 marks, ethanol and carbon dioxide both required). (c) The rate roughly doubles (1 mark), but sugar, oxygen, temperature or space becomes limiting and the alcohol produced inhibits the yeast (1 mark).

Question 3

(a) A virus has no cell structure – just nucleic acid in a protein coat – while a bacterium is a complete cell (1 mark). (b) Influenza and measles (or AIDS, hepatitis, dengue) (1 mark). (c) Antibiotics target bacterial structures such as the cell wall, ribosomes or specific enzymes, and viruses have neither the structures nor the pathways (1 mark). (d) A vaccine carries weakened or killed pathogens, or their antigens, and triggers antibodies and memory cells, so a later encounter with the same pathogen brings a faster, stronger response (2 marks).

Question 4

(a) To stop airborne microorganisms contaminating the culture and to protect the worker from pathogens (1 mark). (b) Flaming kills all microorganisms on the loop so no contaminants enter the medium and none leave the laboratory (2 marks). (c) It stops condensation dripping onto the agar and spreading the colonies, and reduces contamination (2 marks).

Question 1

(a) (i) Cut out the human insulin gene with a restriction enzyme, or make cDNA from insulin mRNA; (ii) cut open a plasmid vector with the same enzyme; (iii) join the gene into the vector with DNA ligase to make recombinant DNA; (iv) introduce the plasmid into bacteria (transformation), select the successful cells and mass-culture them to harvest insulin (4 marks, one per step). (b) A restriction enzyme cuts DNA at a specific base sequence to give matching ends, and DNA ligase seals the joins to make stable recombinant DNA (1 mark).

Question 2

(a) About 94-96 °C (1 mark). (b) Each cycle doubles the number of molecules: 1 × 2⁵ = 32 (2 marks). (c) Every cycle denatures the DNA at a high temperature, which would destroy an ordinary polymerase, whereas a heat-stable enzyme such as Taq stays active through the denaturation step so many cycles can run (2 marks).

Question 3

(a) Bt corn, given an insect-resistance gene from Bacillus thuringiensis (or herbicide-tolerant soybean, golden rice with beta-carotene) (1 mark). (b) Genetic engineering introduces a foreign gene into another organism to change a trait, whereas cloning introduces no foreign gene: the nucleus of a body cell is put into an enucleated egg to produce an individual genetically near-identical to the donor (2 marks). (c) Gene flow to wild relatives creating super-weeds; resistant pests; loss of biodiversity; patents and food equity; religious or moral objections (2 marks, any two).

Question 4

(a) Higher yield and lower cost; no animal pathogens carried over; the sequence matches human insulin exactly, reducing rejection; supply does not depend on animals (2 marks, any two). (b) Monoclonal antibodies for diagnosis and targeted therapy, recombinant vaccines, or lactase for lactose-free milk (1 mark). (c) To stop recombinant bacteria escaping into the environment, where the foreign gene could transfer to other microorganisms, and to protect public health (2 marks).