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A-Level Biology October/November 2024 Q10(c): Glycogen synthase catalyses the conversion of glucose to glycogen in liver cells. The p…
A-Level Biology · Paper 9700/42 · October/November 2024 · Question 10(c) · [3 marks]
Glycogen synthase catalyses the conversion of glucose to glycogen in liver cells. The production of glycogen synthase is coded for by the gene GYS2. A mutation in GYS2 leads to a condition called glycogen storage disease type 0 (GSD0) in which glycogen is not formed efficiently. Suggest what the consequences would be if a person with GSD0 has a meal rich in glucose.
A full-marks model answer with a mark-by-mark examiner breakdown is below.
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A person with GSD0 has non-functioning glycogen synthase, so they cannot efficiently convert glucose into glycogen for storage in the liver. After a meal rich in glucose, this will lead to a prolonged and significant increase in blood glucose concentration (hyperglycaemia). As the blood glucose level will exceed the renal threshold, glucose will be excreted in the urine. The excess glucose that cannot be stored as glycogen will be converted into lipids for storage, leading to increased fat synthesis.
How the marks are awarded
- B1 — Correctly identifying that there will be a high blood glucose concentration, as stated in 'increase in blood glucose concentration (hyperglycaemia)'.
- B1 — Stating the consequence for the kidneys, that glucose will be present in the urine, as mentioned in 'glucose will be excreted in the urine'.
- B1 — Describing an alternative metabolic fate for the excess glucose, such as increased lipid synthesis, as stated in 'converted into lipids for storage, leading to increased fat synthesis'.
Common mistakes
- Confusing glycogen (a storage polysaccharide) with glucagon (a hormone), for example by stating 'glucagon cannot be made'.
- Incorrectly describing the problem as being related to insulin, for example stating 'no insulin is produced' or 'cells are resistant to insulin', which describes diabetes mellitus, not GSD0.
- Giving vague answers like 'the person will get sick' without specifying the physiological consequences such as hyperglycaemia or glucosuria.
- Stating that no glycogen is formed at all; the question states it is not formed 'efficiently', so answers should reflect a reduction or impairment, not a complete absence of the process.
Examiner tip: For questions involving metabolic diseases, always trace the pathway step-by-step from the molecular defect (e.g., faulty enzyme) to the systemic, physiological consequences.
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