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Original practice tasks. The examples explain the reasoning; they are not official mark schemes.
Protein secretion in a gland cell
A gland cell secretes a protein enzyme. Its cytoplasm contains extensive rough endoplasmic reticulum, many transport vesicles and a large Golgi apparatus.
Explain how these structures support the production and release of the enzyme. Include the route from polypeptide synthesis to release outside the cell.
First answer
Review your reasoning
- Locate polypeptide synthesis at ribosomes bound to rough ER.
- Explain transfer to the Golgi and its role.
- Explain how the protein crosses the cell boundary.
Revised answer
A pump inhibitor and glucose absorption
In a mammalian ileum epithelial cell, a sodium–potassium pump on the blood-facing membrane maintains a low intracellular sodium concentration. Sodium–glucose co-transport occurs at the gut-facing membrane. An inhibitor reduces pump activity; other conditions initially remain comparable.
Explain why inhibiting the pump can reduce glucose uptake from the gut. Distinguish the energy source for pumping from the driving force at the co-transporter.
First answer
Review your reasoning
- Connect ATP use to sodium pumping.
- Follow the effect on the sodium electrochemical gradient.
- Explain coupled glucose uptake.
Revised answer
Interpreting a fall in enzyme activity
An enzyme preparation is heated for five minutes, cooled to 25°C, then tested at the same enzyme concentration, substrate concentration and pH as an unheated control. Its initial rate at 25°C is much lower.
Explain a mechanism consistent with the lower rate. Explain why measuring activity after cooling is more informative than measuring only while the enzyme is hot. Do not treat the rate result as direct observation of protein structure.
First answer
Review your reasoning
- Link heating to protein structure and the active site.
- Use the common assay temperature in the comparison.
- State what the observation supports and what it does not prove.
Revised answer
A base insertion in a coding region
A single nucleotide is inserted near the start of the protein-coding region of a nuclear gene. The insertion is retained in the mature mRNA. No compensating insertion or deletion occurs before the original stop position. Use the standard genetic code.
Explain how this insertion can affect the amino-acid sequence and protein function. Distinguish a predicted reading-frame effect from a guaranteed loss of function.
First answer
Review your reasoning
- Explain the change in the triplet reading frame.
- Connect altered codons to the polypeptide.
- Qualify the effect on function.
Revised answer
What a higher plateau shows
Two photosynthesis investigations use comparable plants at the same temperature. One has a higher carbon dioxide supply. At low light intensity the measured rates are similar. At high light, the higher-CO₂ treatment reaches a higher plateau. The measurement is net oxygen production.
Explain the low-light and high-light comparisons. State one limit on what these results establish about the limiting factor.
First answer
Review your reasoning
- Use the low-light comparison.
- Use the higher plateau at high light.
- Keep the inference tied to the experiment.
Revised answer
Why lack of oxygen affects earlier stages
A eukaryotic cell normally obtains much of its ATP through mitochondrial oxidative phosphorylation. Oxygen availability falls severely. Glycolysis does not use oxygen as a direct reactant.
Explain why low oxygen can nevertheless restrict mitochondrial ATP production and other respiratory stages. Explain how lactate formation can help glycolysis continue in a mammalian cell.
First answer
Review your reasoning
- Place oxygen at the end of electron transfer.
- Connect electron transfer to ATP and oxidised coenzymes.
- Explain what lactate formation regenerates.
Revised answer
A small family and an expected ratio
At one autosomal locus, A is completely dominant to a. Both parents are Aa. Assume normal allele segregation, equally likely gametes, independent offspring outcomes and no differential survival. Their four offspring all show the dominant phenotype.
Explain why this family does not contradict the Aa × Aa probability model. Calculate the probability that all four offspring show the dominant phenotype.
First answer
Review your reasoning
- Derive the probability for one offspring.
- Multiply for four independent outcomes.
- Distinguish probability from a quota.
Revised answer
Transferred antibodies and lasting protection
A recipient is given antibodies made by another individual. The supplied antibodies bind a toxin and reduce its attachment to host-cell receptors. Later, the supplied antibodies have been broken down. No independent recipient memory response has been demonstrated.
Explain the immediate protective mechanism and why it does not establish lasting antigen-specific memory in the recipient.
First answer
Review your reasoning
- Explain toxin neutralisation by binding.
- Identify passive immunity.
- Separate supplied antibodies from recipient memory.
Revised answer
Maintaining oxygen uptake at an alveolus
A mammalian alveolus has a thin exchange barrier and a large total surface area across the lungs. Ventilation and pulmonary blood flow continue. Oxygen partial pressure in alveolar air is higher than in the arriving capillary blood.
Explain how the barrier, surface area, ventilation and blood flow support net oxygen uptake. Distinguish ventilation from diffusion.
First answer
Review your reasoning
- Connect area and thickness to diffusion rate.
- State the partial-pressure direction and how it is maintained.
- Keep bulk air movement distinct from gas exchange.
Revised answer
What repolarises an axon?
An action potential has reached its peak in a typical axon. Voltage-gated sodium channels become inactivated and potassium conductance rises. The sodium–potassium pump continues to operate.
Explain the falling phase and why the pump is not the immediate cause of rapid repolarisation. Explain the role of the absolute refractory period.
First answer
Review your reasoning
- Use channel states and potassium movement.
- Distinguish fast voltage change from gradient maintenance.
- Connect sodium inactivation to absolute refractoriness.
Revised answer
ADH and collecting-duct water recovery
Blood water potential falls. The kidney medulla has a water-potential gradient that favours water movement out of the collecting duct. ADH release increases.
Explain how the signal can increase water recovery. Include where ADH is made and released, where it acts on a collecting-duct cell, and why a gradient is still necessary.
First answer
Review your reasoning
- Locate detection, synthesis and release.
- Explain receptor action and aquaporin placement.
- Use the existing water-potential gradient.
Revised answer
Explaining a mark–recapture bias
A first sample marks 40 animals. After mixing, a second sample contains 50 animals, 10 of them recognised as marked. The simple estimate is N = (40 × 50)/10 = 200. In fact, some marked animals lost their marks before the second sample. Assume the population remained closed and capture probabilities were otherwise comparable.
Explain the direction of bias caused by mark loss. Use the marked proportion and the denominator of the estimate, rather than merely saying the method is inaccurate.
First answer
Review your reasoning
- Explain what mark loss changes in the second sample.
- Follow the effect through the formula.
- Give the direction and its condition.
Revised answer