These tasks cover selected ideas from the twelve revision chapters. They are original questions, with guidance for AQA 7402 and OCR A H420 where the topics overlap.
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.
AQA 3.2.1 · OCR A 2.1.1
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Write and improve an answerA 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.
AQA 3.2.3 · OCR A 2.1.5
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Write and improve an answerInterpreting 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.
AQA 3.1.4 · OCR A 2.1.4
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Write and improve an answerA 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.
AQA 3.4.2, 3.8.1 · OCR A 2.1.3, 6.1.1
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Write and improve an answerWhat 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.
AQA 3.5.1 · OCR A 5.2.1
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Write and improve an answerWhy 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.
AQA 3.5.2 · OCR A 5.2.2
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Write and improve an answerA 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.
AQA 3.7.1 · OCR A 6.1.2
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Write and improve an answerTransferred 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.
AQA 3.2.4 · OCR A 4.1.1
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Write and improve an answerMaintaining 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.
AQA 3.3.1, 3.3.2 · OCR A 3.1.1
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Write and improve an answerWhat 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.
AQA 3.6.2 · OCR A 5.1.3
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Write and improve an answerADH 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.
AQA 3.6.4 · OCR A 5.1.2
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Write and improve an answerExplaining 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.
AQA 3.7.4 · OCR A 6.3.1
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