A student places a green aquatic plant in a solution containing magnesium ions and carbon dioxide. The plant is kept in bright light. After several weeks, the student notices the leaves have turned yellow-green. Explain why the leaves changed colour, linking your answer to the role of magnesium ions in photosynthesis.

AQA A-Level Biology (7402) — 3.5.1 Photosynthesis · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

Magnesium ions are a key component of the chlorophyll molecule. Without sufficient Mg²⁺ the plant cannot synthesise enough chlorophyll, the green pigment that gives leaves their colour. As chlorophyll levels fall the leaves become paler, appearing yellow‑green. Because chlorophyll absorbs light energy for photosynthesis, a reduction in its amount also lowers the rate of photosynthesis.

Examiner tips

  • Use the word ‘magnesium ions’ and ‘chlorophyll’ – these are the key terms the scheme rewards.
  • Show the causal chain: Mg²⁺ → chlorophyll synthesis → leaf colour → photosynthesis rate.
  • Mention that the plant was in bright light to emphasise the role of chlorophyll in light absorption.

Common mistakes

  • Confusing magnesium with calcium or other ions.
  • Saying the leaves turned green instead of yellow‑green.
  • Omitting the link between chlorophyll loss and reduced photosynthesis.

Mark scheme (5 marks)

  1. Magnesium ions are needed to make chlorophyll
  2. Without sufficient magnesium, the plant cannot produce enough chlorophyll
  3. Chlorophyll is the green pigment in leaves / gives leaves their green colour
  4. Less chlorophyll means the leaves appear yellow-green / paler / less green
  5. Chlorophyll absorbs light energy for photosynthesis, so less chlorophyll reduces the rate of photosynthesis

Key terms in this question

magnesium ions · photosynthesis

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