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A-Level Biology May/June 2025 Q10(a): Diving to hunt for fish has an effect on the respiration of the grey seal. A study was…
A-Level Biology · Paper 9700/42 · May/June 2025 · Question 10(a) · [4 marks]
Diving to hunt for fish has an effect on the respiration of the grey seal. A study was carried out to measure the blood lactate concentration of a grey seal before, during and after a dive in deep water. With reference to Fig. 10.2, suggest reasons for the change in blood lactate concentration of the seal: • • during the dive after the dive.
A full-marks model answer with a mark-by-mark examiner breakdown is below.
1 answer
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During the dive
Initially, before the dive, the seal uses aerobic respiration, so blood lactate concentration is low. During the dive, the seal holds its breath, so there is no or very limited oxygen available to its muscle cells. This forces the cells to switch to anaerobic respiration to continue producing a small amount of ATP.
In this process, pyruvate, the end product of glycolysis, is reduced to lactate. This reaction regenerates the NAD+ needed for glycolysis to continue in the absence of oxygen, causing lactate to accumulate in the blood.
After the dive
After the dive, the seal begins to breathe again, and a plentiful supply of oxygen becomes available to the cells, allowing normal aerobic respiration to resume. The high concentration of lactate is now removed from the blood.
Lactate is transported to the liver where it is oxidised back into pyruvate. This pyruvate can then enter the Krebs cycle for aerobic respiration or be converted into glucose. This process requires extra oxygen, known as repaying the oxygen debt (or EPOC).
How the marks are awarded
- MP2 — Explaining that the lack of oxygen during the dive forces the seal's cells to switch to anaerobic respiration.
- MP3 — Correctly stating the biochemical step where pyruvate is reduced to form lactate.
- MP4 — Linking the period after the dive to the renewed availability of oxygen, which allows aerobic respiration to occur again.
- MP5 / MP6 — Describing the removal of lactate after the dive by explaining that it is oxidised back to pyruvate, often mentioning this occurs in the liver.
Common mistakes
- Confusing anaerobic respiration in mammals with that in yeast, incorrectly stating that ethanol and carbon dioxide are produced.
- Providing a vague explanation, such as 'the seal runs out of air', without linking it to the lack of oxygen as the final electron acceptor in aerobic respiration.
- Incorrectly stating that lactate itself is used as an energy source directly, without mentioning its conversion back to pyruvate first.
- Failing to explain why lactate levels decrease after the dive, focusing only on its production during the dive.
Examiner tip: Always connect physiological data from a graph or table to the specific underlying biochemical pathways responsible for the observed changes.
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