Explain why a plant with damaged roots that cannot absorb magnesium ions will show reduced rates of photosynthesis.

Pearson Edexcel International GCSE Biology (4BI1) — 2.5 Nutrition: flowering plants · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Magnesium ions are required for the synthesis of chlorophyll.
Without Mg²⁺, chlorophyll production is reduced or absent.
Chlorophyll is the pigment that absorbs light energy for photosynthesis.
If less chlorophyll is present, less light energy is absorbed, so the rate of photosynthesis falls.

Examiner tips

  • Use the command word ‘explain’ – give a clear cause–effect chain. Mention Mg²⁺, chlorophyll, light absorption and the rate of photosynthesis. Keep the answer concise and use the exact terms from the mark scheme.

Common mistakes

  • Saying ‘roots can’t absorb water’ – the question is about Mg²⁺, not water. Using vague phrases like ‘less energy’ without linking to chlorophyll. Writing a long paragraph instead of a short, structured answer.

Mark scheme (4 marks)

  1. Magnesium ions are needed to make chlorophyll
  2. Without magnesium, less / no chlorophyll is produced
  3. Chlorophyll absorbs light energy for photosynthesis
  4. Less light energy absorbed means photosynthesis occurs at a lower rate

Key terms in this question

magnesium · photosynthesis

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