Explain how the structures of starch and glycogen make them suitable for energy storage in plant and animal cells respectively.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Both starch and glycogen are polymers of α‑glucose linked by α‑1,4‑glycosidic bonds, making them chemically inert and suitable for storage.
Starch is a compact, coiled (helical) molecule, while glycogen is highly branched; both structures allow large amounts of glucose to be stored in a small volume without causing osmotic stress.
Glycogen’s greater branching gives many non‑reducing ends, so it can be hydrolysed rapidly by enzymes, providing quick glucose release when energy demand rises.
Glucose is released by hydrolysis of the glycosidic bonds, allowing controlled, regulated release of glucose for cellular respiration.
Starch is a compact, coiled (helical) molecule, while glycogen is highly branched; both structures allow large amounts of glucose to be stored in a small volume without causing osmotic stress.
Glycogen’s greater branching gives many non‑reducing ends, so it can be hydrolysed rapidly by enzymes, providing quick glucose release when energy demand rises.
Glucose is released by hydrolysis of the glycosidic bonds, allowing controlled, regulated release of glucose for cellular respiration.
Examiner tips
- Use the exact terms ‘α‑glucose’, ‘α‑1,4‑glycosidic bonds’, ‘compact/coiled’, ‘highly branched’, ‘non‑reducing ends’, and ‘hydrolysis’ to match the mark scheme.
- Show the link between structure and function (storage vs rapid release) to earn all 4 marks.
Common mistakes
- Confusing α‑glucose with β‑glucose; both polymers use α‑glucose.
- Over‑emphasising only one feature (e.g., only branching) and ignoring the compactness or inertness aspects.
Mark scheme (4 marks)
- Both starch and glycogen are polymers of alpha-glucose, linked by glycosidic bonds, making them chemically inert / stable for storage
- Both molecules are compact / coiled (helical in starch; highly branched in glycogen), allowing large amounts of glucose to be stored in a small volume / without osmotic effects
- Glycogen is more highly branched than starch, providing more non-reducing ends for rapid enzymatic hydrolysis / quick release of glucose when energy demand is high
- Glucose is released by hydrolysis of glycosidic bonds for use in respiration, so storage as a polymer allows controlled / regulated release of glucose on demand
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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