3.5.2Unit 3 — Life on Earth
Nitrates — SQA National 5 Biology Revision Notes & Topical Questions
5 min read 4 parts Lesson 25 of 31
Part 1 of 5What Are Nitrates?
Definition
- Nitrates are nitrogen-containing mineral ions, NO₃⁻, dissolved in soil water and taken up by plant roots.
Why plants need them
- Plants need nitrogen to make amino acids, which are joined together to form proteins.
Source
- Nitrates come from decomposing organic matter and from fertilisers added by farmers.
Shortage
- Without enough nitrate, growth is poor, so crop yield falls.
Part 2 of 5Nitrogen Through a Food Chain
1
Roots absorb nitrates dissolved in soil water.
2
Inside plant cells, nitrates are converted into amino acids and used to build plant proteins.
3
Animals eat the plants and digest the plant proteins into amino acids.
4
The amino acids are rebuilt into new animal proteins, which become available to the next feeder in the chain.
5
Nitrogen returns to the soil through excretion, or through death followed by decomposition.
Part 3 of 5Why Soils Lose Nitrate
Why Soils Lose Nitrate
Harvesting
- When crops are removed from a field, the nitrogen in their tissues is taken away instead of returning to the soil.
Leaching
- Rainwater washes soluble nitrates down through the soil and out of reach of plant roots.
Fertilisers
- Adding nitrate fertiliser replaces the lost nitrate and restores the supply for the next crop.
Recycling
- Decomposers break down dead matter and waste, which returns nitrogen compounds to the soil.
Part 4 of 5Key Ideas to Remember
A cycle
- Nitrogen compounds cycle through an ecosystem, passing from soil to plants to animals and back to the soil.
Detail not required
- You do not need the full nitrogen cycle, but must know that nitrogen compounds are passed on.
Role of decomposers
- Bacteria and fungi break down dead organisms so that nitrogen is available to plants again.
Link to farming
- Farming removes nitrogen in crops, which is why fertilisers are added.
Part 5 of 5Exam Checklist & Tips
3.5.2 – Key Points for the Exam
- Nitrate A nitrogen-containing mineral ion absorbed by plant roots.
- Amino acid A building block of protein, made in plants using nitrate.
- Protein A large molecule made of amino acids, needed for growth and repair.
- Leaching The washing of soluble substances out of soil by rainwater.
- Decomposer An organism, such as a bacterium or fungus, that breaks down dead material.
Common Mistakes
- Saying plants take in nitrogen gas through their roots; roots absorb dissolved nitrate ions.
- Writing that animals make proteins from nitrates; animals get amino acids by eating plants or other animals.
- Forgetting that harvesting removes nitrogen from the field.
- Leaving out decomposition when explaining how nitrogen returns to the soil.
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