Explain why starch is described as an energy storage molecule in plants, and describe the chemical elements that make up a starch molecule.

Pearson Edexcel International GCSE Biology (4BI1) — 2.3 Biological molecules · 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 up of many glucose units linked by α‑1,4‑glycosidic bonds.

When plants need energy, enzymes break these bonds to release glucose, which is then oxidised in cellular respiration to give ATP.

Each glucose unit has the formula C₆H₁₂O₆, so starch contains the elements carbon, hydrogen and oxygen.

Examiner tips

Use the term ‘polysaccharide’ and ‘glucose units’ to show understanding of structure.
Explain the role of enzymes in breaking down starch for respiration.
Mention the specific elements (C, H, O) that make up glucose.
Keep the answer concise and to the point to fit the 4‑mark limit.

Common mistakes

Confusing starch with sucrose or cellulose; they are different polysaccharides.
Failing to mention that glucose is released and used in respiration.
Omitting one of the elements (e.g. hydrogen) from the list of starch components.

Mark scheme (4 marks)

Starch is made up of many glucose molecules joined together
Glucose can be released from starch and used in respiration to release energy
Starch contains the element carbon
Starch contains the elements oxygen and hydrogen

Key terms in this question

starch · energy storage