Explain why starch is well suited to its role as a storage carbohydrate in plant cells.

AQA A-Level Biology (7402) — 3.1.2 Carbohydrates · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

Starch is insoluble in water, so it does not alter the cell’s water potential or interfere with osmosis.

It is a large polymer of many glucose units linked by α‑1,4‑ and α‑1,6‑glycosidic bonds, giving a compact structure.

Because of its size, a large amount of glucose can be stored in a small volume.

When energy is required, starch is hydrolysed by amylases to release glucose.

Thus starch efficiently stores glucose without affecting cell water balance and can be mobilised when needed.

Examiner tips

  • Use the word ‘insoluble’ to show understanding of water potential. Mention both α‑1,4 and α‑1,6 bonds to show polymer detail. Explain compact storage and hydrolysis to earn all 5 marks.

Common mistakes

  • Confusing starch with glycogen or saying it is soluble. Forgetting to mention the α‑1,6 branching. Not linking hydrolysis to glucose release.

Mark scheme (5 marks)

  1. Starch is insoluble (in water)
  2. Being insoluble means it does not affect the water potential / osmosis of the cell
  3. Starch is a large / complex molecule (polymer) made of many glucose units joined together
  4. This means a large amount of glucose can be stored in a compact / small space
  5. Starch can be broken down / hydrolysed to release glucose when (energy is) needed

Key terms in this question

starch

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