By the end of this lesson
- Explain why nitrate ions are needed for amino-acid synthesis.
- Explain why magnesium ions are needed for chlorophyll.
- Relate deficiencies to symptoms and growth.
BIG QUESTION
Why can a plant surrounded by carbon dioxide, water and light still fail to grow?
Photosynthesis supplies carbon skeletons, but mineral ions supply essential atoms and support molecules such as chlorophyll.
Nitrate supplies nitrogen
Plants absorb nitrate ionsMineral ions that supply plants with nitrogen to make amino acids and then proteins; a lack causes stunted growth and often yellowing, especially of older leaves. and use their nitrogen to make amino acids, then proteins. Proteins include enzymes, membrane carriers and structural components needed for growth.
Severe nitrate deficiency therefore causes poor growth and often yellowing, especially in older leaves as nitrogen is reallocated.
Magnesium is part of chlorophyll
magnesium ionsMineral ions that plants absorb from the soil and need to make chlorophyll; a lack reduces chlorophyll, causing chlorosis (often between the leaf veins) and less photosynthesis. are required to synthesise chlorophyll. Deficiency reduces chlorophyll concentration and causes chlorosisLoss or reduction of the green colour of leaves because less chlorophyll is made, for example between the leaf veins when a plant lacks magnesium ions., often between leaf veins.
Less chlorophyll can reduce light absorption and photosynthesis, so deficiency can also restrict biomass gain.
Roots absorb ions selectively
Root hair cells increase contact area with soil solution. Some ion uptake occurs against a concentration gradient by active transportMovement of substances across a membrane against a concentration gradient, from lower to higher concentration, using carrier proteins and energy released by respiration., using respiratory energy and membrane proteins.
Waterlogging can reduce oxygen around roots, restricting aerobic respiration and therefore active ion uptake.
Fertilisers solve one problem but may create another
Fertilisers can replace mineral ions removed in crops and increase yield when deficiency limits growth. Excess application can wash into waterways.
Nutrient enrichment can drive algal blooms and oxygen depletion during decomposition, so precise application matters.
Required mineral ions
Do not swap the element, product and symptom.
| Ion | Biological use | Likely deficiency effect |
|---|---|---|
| Nitrate, NO₃⁻ | Nitrogen for amino acids and proteins. | Stunted growth; older leaves may yellow. |
| Magnesium, Mg²⁺ | Component needed to make chlorophyll. | Chlorosis and reduced photosynthesis. |
| Phosphate, PO₄³⁻ (extension) | Phosphates in nucleic acids, ATP and membranes. | Poor root growth or dark foliage in some species. |
| Potassium, K⁺ (extension) | Enzyme activation and stomatal function. | Weak growth and leaf-edge damage in some species. |
Language that earns precision
- mineral ion
- A charged inorganic particle absorbed in dissolved form.
- nitrate
- An ion supplying nitrogen for amino-acid and protein synthesis.
- magnesium ion
- An ion required to make chlorophyll.
- chlorosis
- Loss or reduction of green chlorophyll pigmentation.
- active transport
- Energy-requiring movement against a concentration gradient through membrane proteins.
Common misconceptions
Claim: Nitrate is plant food.
Correction: Nitrate supplies nitrogen atoms; plants synthesise their own organic molecules.
Claim: Magnesium provides energy for photosynthesis.
Correction: Magnesium is needed for chlorophyll; light supplies the energy.
Claim: All yellow leaves prove magnesium deficiency.
Correction: Many stresses cause yellowing; pattern and controlled evidence are needed.
Claim: More fertiliser always means more growth.
Correction: Once another factor limits growth, excess ions may harm organisms and ecosystems.
Match the answer to the command word
Pair nitrate with amino acids/proteins and magnesium with chlorophyll.
Continue the chain to growth or photosynthesis rather than stopping at a molecule name.
Balance yield benefits against runoff and ecosystem consequences.
Recall, apply, then justify
Explain why nitrate deficiency can reduce plant growth.
Hint
Nitrate → amino acids → proteins.
Suggested marking points
- Nitrate supplies nitrogen for amino-acid synthesis.
- Amino acids are joined to make proteins including enzymes and structural proteins.
- Less protein synthesis restricts cell function and growth.
Explain why magnesium deficiency may reduce photosynthesis.
Hint
Link the ion to pigment and then energy capture.
Suggested marking points
- Magnesium is required to make chlorophyll.
- Deficiency causes less chlorophyll/chlorosis.
- Less light energy is absorbed, reducing photosynthesis.
Explain why waterlogging can reduce mineral-ion uptake.
Hint
Follow oxygen to ATP to transport.
Suggested marking points
- Waterlogged soil contains less oxygen for root respiration.
- Less aerobic respiration produces less ATP/usable energy.
- Active transport of ions is reduced.
Design a hydroponic investigation to identify which of two mineral ions causes a deficiency symptom.
Planning hint
Use complete solution, omission treatments and replication.
What a strong design includes
- Grow genetically similar plants under identical environmental conditions.
- Use plants in complete nutrient solution as the control group.
- Create separate solutions lacking nitrate or magnesium only.
- Keep ion concentration, pH, light, temperature and water volume otherwise controlled.
- Use replicates and randomise plant positions.
- Measure growth and chlorophyll-related colour quantitatively over time.