Explain why a mineral ion deficiency in the soil can reduce the rate of photosynthesis in a plant.

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).

Plants absorb mineral ions from the soil through their roots. One essential mineral ion is magnesium, which is needed to make chlorophyll. Without sufficient magnesium, the leaves of a plant turn yellow.

Model answer (4 marks)

Magnesium is required for the synthesis of chlorophyll.
A deficiency of magnesium means the plant produces less or no chlorophyll.
With less chlorophyll, the leaf absorbs less light energy.
Consequently, the rate of photosynthesis is reduced.

Examiner tips

  • Use the exact wording from the mark scheme (e.g., ‘chlorophyll’, ‘less light energy is absorbed’).
  • Show the logical sequence: magnesium → chlorophyll → light absorption → photosynthesis.
  • Keep the answer concise – one sentence per point.

Common mistakes

  • Confusing magnesium with other nutrients (e.g., nitrogen).
  • Failing to link chlorophyll to light absorption or photosynthesis.

Mark scheme (4 marks)

  1. Magnesium is needed to make chlorophyll
  2. A deficiency of magnesium means less / no chlorophyll is produced
  3. Less chlorophyll means less light energy is absorbed / captured
  4. Therefore less / no photosynthesis can occur, reducing the rate

Key terms in this question

photosynthesis

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