Explain how the structure of glycogen makes it well suited to its function as an energy storage molecule in animal cells.

IB DP Biology Standard Level (2023 syllabus) — B1.1 Carbohydrates and lipids · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Glycogen is a polysaccharide of many glucose units linked by α‑1,4‑glycosidic bonds, storing a large amount of chemical energy in a compact form.

Its highly branched structure gives many α‑1,6‑branch points, creating many free chain ends from which glucose can be released rapidly by hydrolysis.

Because it is relatively insoluble in water, glycogen does not alter the cell’s osmotic potential.

Glucose released by hydrolysis of the glycosidic bonds enters cellular respiration, producing ATP, so glycogen is an immediately accessible energy reserve.

Examiner tips

Use the exact terms ‘α‑1,4‑glycosidic bonds’, ‘α‑1,6‑branch points’, ‘hydrolysis’, ‘cellular respiration’, and ‘ATP’. Show the link between branching and rapid glucose release. Mention insolubility and osmotic potential as a separate point. Keep each point short and to the point to hit all four marks.

Common mistakes

Confusing glycogen with starch or using ‘β‑glucose’ instead of ‘α‑glucose’. Forgetting to mention the insolubility or the rapid release due to branching. Writing a long paragraph instead of separate concise points, losing marks for lack of structure.

Mark scheme (4 marks)

Glycogen is a polysaccharide made of many glucose monomers joined by glycosidic bonds, storing a large amount of chemical energy in a compact form.
Glycogen is highly branched, which means it has many free ends from which glucose can be rapidly released by hydrolysis when energy is needed.
Glycogen is insoluble / relatively insoluble in water, so it does not affect the osmotic potential of the cell.
Glucose monomers are released by hydrolysis of glycosidic bonds and can enter cellular respiration to release ATP, making glycogen an immediately accessible energy reserve.

Key terms in this question

glycogen