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A-Level Biology May/June 2025 Q7(b)(ii): Suggest explanations for the changes in mean blood glucose concentration of the volunte…
A-Level Biology · Paper 9700/41 · May/June 2025 · Question 7(b)(ii) · [3 marks]
Suggest explanations for the changes in mean blood glucose concentration of the volunteers who ate meal B.
A full-marks model answer with a mark-by-mark examiner breakdown is below.
1 answer
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The mean blood glucose concentration initially falls as glucose is taken up from the blood by body cells and used for aerobic respiration to release energy.
When the blood glucose concentration falls below the normal set point, the α-cells in the islets of Langerhans in the pancreas are stimulated to secrete glucagon.
Glucagon travels in the blood to the liver and binds to receptors on liver cells, stimulating the breakdown of stored glycogen into glucose (glycogenolysis). This glucose is then released into the bloodstream, causing the mean blood glucose concentration to rise back towards the normal level.
How the marks are awarded
- M1 — Explaining that the fall in blood glucose is due to its use in respiration by cells.
- M1 — Stating that glucagon is secreted/released in response to the fall in blood glucose.
- M1 — Describing the conversion of glycogen to glucose (glycogenolysis) as the mechanism to raise blood glucose levels.
Common mistakes
- Confusing glucagon (the hormone) with glycogen (the storage polysaccharide).
- Stating that insulin is involved in raising blood glucose levels, thereby confusing the roles of the two pancreatic hormones.
- Only explaining why the blood glucose level falls, and failing to explain the subsequent rise back towards the normal level, which is part of the 'changes' mentioned in the question.
- Incorrectly stating that muscle glycogen is converted to glucose and released into the blood; muscle cells lack the necessary enzyme and use their glycogen stores for their own respiratory needs.
Examiner tip: When explaining homeostatic control from a graph, always describe the full negative feedback loop: the initial change (stimulus), the hormonal/nervous response, and the corrective action that restores the set point.
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