Explain how the structure of starch makes it suited to its function as an energy storage molecule in plant 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)

Starch is a polysaccharide made of many glucose units linked by glycosidic bonds, giving it a large store of chemical energy.
Starch is insoluble in water, so it does not disturb the cell’s water potential or osmotic balance.
Its structure is compact – amylose forms a tight helix and amylopectin is highly branched – allowing a large amount of glucose to be stored in a small volume.
The glucose units can be rapidly hydrolysed by enzymes such as amylase, releasing glucose for immediate use as an energy source.

Examiner tips

  • Use the exact terms ‘glycosidic bonds’, ‘insoluble’, ‘compact/coiled’, ‘hydrolysed by amylase’.
  • Show each point clearly and link structure to function.
  • Keep each point brief – one sentence per mark.

Mark scheme (4 marks)

  1. Starch is a polysaccharide composed of many glucose monomers joined by glycosidic bonds, providing a large store of chemical energy.
  2. Starch is insoluble in water, so it does not affect the water potential / osmotic balance of the cell.
  3. Starch is compact / coiled (amylose forms a helix; amylopectin is branched), allowing large quantities to be stored in a small volume.
  4. Starch can be rapidly hydrolysed by enzymes (e.g. amylase) to release glucose monomers when energy is needed, making it a readily mobilisable energy store.

Key terms in this question

starch

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