Explain why plants need both nitrate ions and magnesium ions to grow healthily.

AQA A-Level Biology (7402) — 3.1.8 Inorganic Ions · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

Nitrate ions are the source of nitrogen, which is required for the synthesis of amino acids. Amino acids are the building blocks of proteins, so a lack of nitrate leads to reduced protein production and stunted plant growth.
Magnesium ions are the central atom in the chlorophyll molecule. Chlorophyll is essential for photosynthesis; without sufficient magnesium the plant cannot photosynthesise efficiently, causing chlorosis (yellow leaves) and poor growth.

Examiner tips

Use the exact terms "nitrate", "amino acids", "proteins", "chlorophyll", "chlorosis". Show the causal chain: nitrate → amino acids → proteins → growth. Similarly, magnesium → chlorophyll → photosynthesis → growth. Keep the answer concise and directly linked to the marks.

Common mistakes

Confusing nitrate with ammonium or other nitrogen sources. Forgetting to mention chlorophyll as the key role of magnesium. Using vague phrases like "nutrients" instead of the specific ions.

Mark scheme (5 marks)

Nitrate ions are needed to make / synthesise amino acids
Amino acids are used to make proteins
Without nitrate ions, plant growth is stunted / poor / plant cannot grow properly
Magnesium ions are needed to make chlorophyll
Without magnesium ions, the plant cannot photosynthesise efficiently / leaves turn yellow (chlorosis)

Key terms in this question

nitrate ions · magnesium ions