HSC Biology 2025

[2 marks]. Two named components with how each protects:

(Nasal hair trapping inhaled pathogens, or mucus lining trapping microbes, would also each earn a mark.)

Marker's note. Use the components shown in the stimulus and say how each one works, for example "nasal hairs trap pathogens inhaled in the air", rather than just naming a barrier.

(a) [2 marks]. Many fungi reproduce asexually by producing spores. The parent fungus releases large numbers of spores; when a spore lands in a favourable environment it germinates and grows into a new, genetically identical fungal organism. (Budding, as in yeast, is also acceptable.)

(b) [2 marks]. Cholera bacteria produce a toxin that causes watery diarrhoea. This flushes large numbers of bacteria out of the host into water sources where sanitation is poor, so other people who drink the water are infected. The adaptation increases transmission between hosts, not just survival inside one host.

Marker's note. In (a) link the process to its outcome (spores carried to a new site grow into a new organism). In (b) the adaptation must help the pathogen move between hosts, not simply benefit it inside a single host.

[3 marks].

Similarity: both are reproductive technologies in which a human deliberately transfers the male gametes to the female part of a chosen individual to control which parents reproduce.

Difference: artificial insemination is carried out in animals (sperm into the female tract), while artificial pollination is carried out in flowering plants (pollen onto the stigma).

Marker's note. Write a genuine comparison (a stated similarity and difference), not two separate descriptions. Do not confuse artificial insemination with IVF, and if you use a table, give it clear headings for "similarity" and "difference".

(a) [2 marks]. Mechanism A (cooling, when temperature rises): sweating. Mechanism B (warming, when temperature falls): shivering.

(b) [2 marks]. When body temperature drops below the normal range, the control centre triggers shivering, the rapid involuntary contraction of skeletal muscles. These contractions release heat, which raises body temperature back into the normal range, returning the body to homeostasis.

Marker's note. Read the flow chart so mechanism A increases heat loss and mechanism B generates heat. In (b) link shivering specifically to heat generation and the return to the normal range.

[3 marks]. The product statement is not adequate. From the graph, the UV level is at or above 3 from roughly 8 am to 6 pm, which is wider than the 10 am to 4 pm window the product recommends. So for about two hours in the morning (8 am to 10 am) and about two hours in the late afternoon (4 pm to 6 pm) the UV level already exceeds 3, yet a person following only the product advice would be unprotected. The product therefore underestimates the period needing protection and does not fully match the Cancer Council guidance.

Marker's note. Tie the data in the graph directly to both the Cancer Council level (3) and the product times, and reach a clear, evidence-based judgement rather than a general comment.

[4 marks]. Visual disorder: myopia (short-sightedness), where light focuses in front of the retina.

• How each treats it: LASIK uses a laser to reshape the cornea so it refracts incoming light less strongly, moving the focal point back onto the retina. Prescription glasses with concave (diverging) lenses also reduce the convergence of light before it reaches the eye, so the image again falls on the retina. Similarity: both correct myopia by adjusting how light is refracted so it focuses on the retina.
• Differences: LASIK is a surgical, permanent and more expensive procedure with a healing period; glasses are non-invasive, removable, cheaper and immediately reversible.

Marker's note. Keep the focus on treatment of a named disorder and how each technology refracts light to fix it, not just a general comparison of cost or healing time. Bring in a technology beyond the stimulus.

[6 marks]. Group A uses internal fertilisation; Group B uses external fertilisation.

• Similarity: in both groups fertilisation is the fusion of a male and a female gamete to form a zygote.
• Group A (internal): the gametes meet inside the female body, so the chance of an egg being fertilised is high and the gametes are protected from drying out. Far fewer eggs are needed, which matches the data (1 to 100 in snakes, 1 to 3 in monotremes). Internal fertilisation also allows reproduction on land.
• Group B (external): gametes are released into water and fertilise outside the body, so the gametes are spread through a large volume and the probability of any one being fertilised is low. To compensate, huge numbers of eggs are produced, which matches the data (100 000 to 2 million in sea urchins). External fertilisation must occur in water so the gametes do not dry out.

So the number of eggs is far higher in Group B because external fertilisation has a much lower success rate per egg than the protected, higher-probability internal fertilisation of Group A.

Marker's note. Identify the fertilisation type in each group, compare internal and external fertilisation (not fertilisation versus parental care, or external fertilisation versus asexual reproduction), and quote the egg-number data to support the comparison.

(a) [4 marks]. A macrophage engulfs the alpha-gal antigen and presents it.

• X (helper T cell): recognises the presented antigen and releases cytokines that activate the matching B lymphocytes.
• Y (B lymphocyte): the specific B cell, once activated, proliferates and differentiates.
• Z (plasma cell): the differentiated B cell that secretes large quantities of antibodies against alpha-gal (some activated B cells instead become memory cells).

(b) [2 marks]. On the first exposure (the tick bite) antibody production is slow and reaches only a low concentration. On the second exposure (eating meat) antibodies are produced much faster and reach a far higher concentration, the pattern of a secondary response.

(c) [3 marks]. Memory cells are long-lived B and T cells made during the first (primary) exposure to a specific antigen. They remain in the body, for example in the lymph nodes, after the infection clears. On re-exposure to the same antigen they recognise it immediately and rapidly divide and differentiate into plasma cells (and cytotoxic T cells), producing antibodies faster and in greater amounts. This rapid secondary response usually neutralises the pathogen before disease develops.

Marker's note. In (a) label or name X, Y and Z explicitly and show the progression from antigen presentation to antibody production. In (b) describe distinct features of each exposure from the graph (speed and level), not a single broad trend. In (c) explain both how memory cells are made and their role on re-exposure.

(a) [2 marks]. The Varroa mite is a parasitic pathogen that lives on and harms honey bees, and it is transmitted from infected bees and hives to healthy ones through direct contact. Because it is caused by a pathogen and spreads between organisms, the infestation is classed as an infectious disease.

(b) [4 marks].

1. Early identification and isolation. Regular monitoring of colonies for mites allows infested hives to be detected early and quarantined, stopping bees from those hives mixing with healthy hives and carrying mites to them.
2. Destroying infected hives. Euthanising the bees in confirmed infested hives kills the mites along with their hosts, removing the source of infection so it cannot spread to surrounding healthy hives. (Washing and disinfecting shared equipment is also acceptable.)

Marker's note. In (a) tie the answer to "caused by a pathogen" and "transmitted between organisms". In (b) give procedures specific to the Varroa scenario and link each one to how it stops spread through the wider bee population, rather than generic biosecurity statements.

(a) [4 marks]. Use recombinant DNA technology to make a transgenic mosquito:

1. Use a restriction enzyme to cut the SERPINE1 gene out of human DNA, leaving sticky ends.
2. Cut a bacterial plasmid (vector) with the same restriction enzyme so the ends are complementary, then join the SERPINE1 gene into the plasmid using DNA ligase.
3. Transform bacteria with the recombinant plasmid and let them reproduce to amplify many copies of the gene.
4. Insert the gene into the mosquito's germline by micro-injecting it into mosquito egg cells (or using a viral vector), so mosquitoes that develop from those eggs carry SERPINE1, express PAI-1, and pass the trait to their offspring.

(b) [7 marks]. Genetic technologies bring real benefits but also genuine risks, so the statement is largely, but not unconditionally, supported.

Benefits. In recombinant DNA technology the human insulin gene has been transferred into bacteria, which then produce large amounts of human insulin cheaply, so people with diabetes have reliable, affordable treatment and longer, better-quality lives. Gene technologies such as CRISPR and the transgenic mosquitoes above can reduce vector-borne diseases like malaria and dengue, lowering disease incidence and easing pressure on health systems. GM crops (for example Bt cotton) raise yields and cut insecticide use, increasing food supply.

Costs and concerns. These technologies also carry unintended health effects, environmental risks (for example a released gene spreading through wild populations), and ethical concerns about patenting living things and equitable access, since poorer farmers and patients may be priced out.

Judgement. On balance the benefits to health, food supply and disease control are substantial and well evidenced, so genetic technologies are mostly beneficial for society, provided the environmental and ethical risks are regulated and monitored.

Marker's note. In (a) recognise this needs a transgenic organism (germline modification), name the steps (restriction enzymes, ligase, plasmid vector, micro-injection), and use precise terminology rather than "place into mosquito". In (b) name specific technologies with real examples, weigh benefits against risks at a societal level (health, environmental, ethical, economic), and keep one consistent judgement throughout.

(a) [3 marks]. Autosomal recessive. Unaffected parents in the pedigree have an affected child, so the allele must be recessive and was carried, unexpressed, by both parents. Because affected individuals are both male and female and the trait is not passed in a sex-linked pattern (an affected daughter is not explained by a single X from the father), the gene is autosomal. So each unaffected carrier parent is heterozygous (Bb) and an affected child is homozygous recessive (bb), having inherited one recessive allele from each parent.

(b) [3 marks]. The template strand GTC ATA CAG CTG transcribes to the mRNA codons CAG UAU GUC GAC, which the chart reads as Glutamine - Tyrosine - Valine - Aspartic acid in the unaffected individual. In the CAMT sequence the third amino acid is Isoleucine, coded by AUC. So the GUC codon has become AUC: a single base (G to A) has changed. This is a point mutation (a substitution), and because it swaps one amino acid for another it is a missense mutation.

Marker's note. In (a) use the precise term "autosomal recessive" and justify with named individuals from the pedigree (unaffected parents with an affected child), drawing a Punnett square if it supports the case. In (b) use the codon chart to translate the unaffected sequence, identify the single changed base, and name it a point/substitution mutation.

[7 marks]. A long-term cohort (prospective) study tracking groups by age at exposure.

• Sample and groups. Recruit large numbers of people from regions A, B and C, sorted into cohorts by age at first exposure (about 10, 20 and 30 years old) to match the three lines on the graph. Large samples and equal numbers of males and females improve validity by reducing the effect of chance and confounding by sex.
• Comparison and control. Communities A and B are connected by the bridge and mix freely, so both can be exposed; isolated community C serves as an unexposed control to show the background rate of the disease. Comparing exposed and control groups lets you attribute risk to the exposure.
• Duration. Follow the cohorts for about 55 years, as shown on the x-axis, recording new cases over time so the rising risk for each age group can be plotted.
• Controlling confounders. Collect data on diet, lifestyle, occupation and other exposures, so any difference in disease risk can be linked to age at exposure rather than to another variable. Correlating these data identifies the true risk factor.
• Justification overall. Large samples, a control region, a long follow-up and recording confounders together make the results reliable and valid, and reproduce the age-related risk curves shown.

Marker's note. Use the stimulus (the three regions, the bridge, the age groups and the timescale) to justify each design feature, and show how the features support reliability and validity, rather than describing epidemiology in general.

(a) [3 marks]

Cell R (a somatic cell) undergoes mitosis, producing two daughter cells that are genetically identical to the parent and to each other, with the full (diploid) chromosome number. Cell S (a germline cell) undergoes meiosis, producing gametes with half the chromosome number (haploid) and, through crossing over and independent assortment, genetic variation. Similarity: both are preceded by DNA replication, in which the genetic content is doubled before division.

(b) [2 marks]

Individual 1 is a parent of Individual 2: a gamete produced by Individual 1's germline cells fused with another gamete at fertilisation to form the zygote that developed into Individual 2, who therefore inherited a set of chromosomes from Individual 1.

(c) [4 marks]

Mutation A is a germline mutation and mutation B is a somatic mutation.

• Cell U (the early cell line before either mutation) carries neither mutation A nor B.
• Because A occurs in a germline cell, it can be passed to offspring: cell V could carry mutation A but not B.
• Cell W, formed after both mutations, could carry both A and B.
• Cell X, in the germline line, could carry mutation A (heritable) but not the somatic mutation B.

A germline mutation (A) can be inherited by future generations, while a somatic mutation (B) is confined to the body cells in which it arose and is not passed on.

Marker's note. In (a) compare the division processes themselves (chromosome number, identical versus varied daughters), not just the outcomes. In (b) state the parent-offspring link via a gamete. In (c) use the key to track which of U, V, W and X carry the heritable germline mutation A versus the non-heritable somatic mutation B.

(a) [2 marks]. A smaller population has a smaller gene pool and lower genetic diversity, so each individual makes up a larger share of the allele frequencies. Random events (genetic drift) therefore change the frequencies by a large proportion, producing the pronounced fluctuations, whereas in the large population the same random events are averaged out across many more individuals.

(b) [4 marks]. Gene flow (here, the introduced alleles) adds genetic variation to a gene pool.

• Small population (20): the introduced allele is a large fraction of a small, low-diversity gene pool, so its frequency swings sharply. If the new allele is favourable it can spread quickly and improve the population's ability to adapt; if unfavourable, selection removes it rapidly, as seen in the steep changes in the small-population graph.
• Large population (2000): the same introduced allele is a tiny fraction of a large, already diverse gene pool, so it changes allele frequencies only slightly and gradually, as the flatter lines show.
• Judgement: gene flow generally has a positive effect by increasing genetic diversity, but its impact is far greater in the small population, while in the large population the effect is small but still adds variation. So the effect of gene flow depends on population size and environmental conditions.

Marker's note. In (a) link small population size to a small gene pool and to a larger effect of drift. In (b) recognise that the same gene flow was introduced into both populations, that gene flow changes the gene pool (not the reverse), and that population size governs how strongly it affects allele frequencies; reach a clear evaluation.