BIOLOGY · Stage 9 · Chapter 9: Plants and Photosynthesis
Biology 9.5 The need for minerals — Nitrates and Magnesium in Plants
5 min read 7 parts Lesson 5 of 9
Lesson summary
9.5 The need for minerals — Nitrates and Magnesium in Plants — Alhussan International Academy — Biology (Stages 7–9) notes: Explain why plants need nitrate and magnesium ions, where they obtain them and what deficiency symptoms show.Food — plants make carbohydrates by photosynthe…
- Without minerals
- Active transport
- Replacement
- Expected results
- Fertilisers
- Natural sources
Section 1 of 7Plants make food but need minerals
- Food
- plants make carbohydrates by photosynthesis, and use glucose in respiration or build it into starch, cellulose and lipids.
- Minerals = water-soluble substances that plant cells can absorb.
- Some substances cannot be made from carbohydrates alone.
- Without minerals
- a plant in light, water and air looks sickly and eventually dies.
Section 2 of 7Where plants get minerals
- Soil
- plants absorb mineral ions dissolved in soil water through root hairs.
- The minerals are carried around the plant in its transport systems with the water.
- Active transport
- nitrate ions are taken up by active transport, which needs energy from respiration.
- Replacement
- harvesting crops uses up soil minerals, so they must be replaced by fertilisers or natural recycling by decomposers.
Section 3 of 7Nitrate
- Source
- decomposers produce nitrates by breaking down waste and dead bodies of plants and animals.
- Use
- nitrates supply nitrogen to make amino acids and proteins, and also DNA and chlorophyll.
- Proteins control many reactions in plant cells, including photosynthesis, and are part of the cell structure.
- Plants cannot use nitrogen gas from the air directly.
- Deficiency
- growth is stunted because proteins for new cells cannot be made.
- Older leaves turn yellow because nitrogen is moved to the growing tips.
Section 4 of 7Magnesium
1Use2 points
- magnesium ions (Mg2+) are at the centre of every chlorophyll molecule.
- Plants need magnesium to make chlorophyll for new chloroplasts and to replace worn-out or damaged chlorophyll.
2Deficiency2 points
- without magnesium less chlorophyll is made, so less light is absorbed and photosynthesis falls.
- Leaves turn pale or yellow (chlorosis), often between the veins and on the older leaves first.
Section 5 of 7Mineral deficiencies compared
| Mineral | Used for | Deficiency |
|---|---|---|
| Nitrate | Amino acids, proteins, chlorophyll | Poor growth, yellow older leaves |
| Magnesium | Chlorophyll | Pale or yellow leaves with green veins |
| Phosphate | DNA, ATP, cell membranes | Poor root growth, purple leaves |
| Potassium | Enzymes, flowers and fruit | Yellow leaves with dead edges |
Section 6 of 7Investigating mineral deficiency
- Grow seedlings in culture solutions: one with all minerals (the control) and others each lacking one mineral, such as no nitrate or no magnesium.
- Use the same type of plant, volume of solution, light and temperature.
- Observe over two to three weeks and measure height, number of leaves and leaf colour.
- Compare each plant with the control to identify the effect of each mineral.
- Repeat with several plants and take means to improve reliability.
- Expected results
- the complete solution grows best, the no-nitrate plant is short and pale, and the no-magnesium plant has yellow leaves.
Section 7 of 7Legumes and fertilisers
- Legumes
- peas, beans and clover have root nodules full of bacteria that make nitrates from nitrogen in the air.
- The plants give the bacteria sugar from photosynthesis in return.
- Legumes grow well in nitrate-poor soil and add nitrates to the soil, making it more fertile.
- Farmers grow them in crop rotation to improve the soil for crops that need lots of nitrogen.
- Fertilisers
- they contain nitrogen, phosphorus and potassium (NPK) to replace minerals.
- Natural sources
- compost and manure return minerals as they decompose.
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