What you need to know
- 4.8Importance of water and of nitrateNO₃⁻, a mineral ion taken up by plant roots and used to make amino acids, proteins and nucleic acids., calcium and magnesiumA mineral ion (Mg²⁺) that plants need to make chlorophyll. to plants.
The recommended additional practical on mineral deficiencies is not a core practical. Other ions such as phosphate and potassium matter in agriculture but are not the named 4.8 list. Do not treat RHS symptom photographs as official mark-scheme descriptions.
By the end, you should be able to…
- Give biologically specific roles for water, nitrate, calcium ions and magnesium ions.
- Link a named deficiency to a process, not just a colour change.
- Identify variables and limitations in a mineral-deficiency comparison.
Build your understanding
Water is more than a solvent
Water is the medium in which ions and sugars dissolve and move. It is a reactant in photosynthesisThe process in which plants use light energy to make glucose from carbon dioxide and water, releasing oxygen.. Evaporation from leaves can cool tissues, but “plants sweat to cool down” is too animal-like: state transpiration and latent heat if you use that idea.
turgorThe pressure of a plant cell’s contents pushing outwards against the cell wall., from water in the protoplast and vacuoleA fluid-filled sac. In plant cells, the large permanent vacuole contains cell sap and keeps the cell turgid. pressing on the wall, supports herbaceous tissues. If water potentialThe tendency of water to move out of a solution. Pure water is 0; adding solutes makes it negative. Water moves from higher to lower water potential. outside falls, water leaves, turgor drops and wilting follows. Support from lignifiedDescribes a cell wall strengthened and waterproofed by lignin. xylemPlant tissue that carries water and mineral ions up from the roots. Its vessels are dead, hollow and lignified. is different: woody tissue does not rely on turgor in the same way.
Three named ions
nitrateNO₃⁻, a mineral ion taken up by plant roots and used to make amino acids, proteins and nucleic acids. is a source of nitrogen for amino acidThe monomer of proteins: a central carbon with an amine group, a carboxyl group, a hydrogen and an R group., proteins and nucleic acids. Without it, protein synthesis and growth slow; leaves may be pale because chlorophyllThe green pigment in chloroplasts. It absorbs mainly red and blue light. production also needs nitrogen. magnesiumA mineral ion (Mg²⁺) that plants need to make chlorophyll. is a component of chlorophyll, so a shortage impairs light capture and can produce yellowing between veinA vessel that carries blood back to the heart at low pressure. It has a thin wall, a wide lumen and valves., often on older leaves because magnesium can be moved from them.
Calcium ions contribute to the middle lamellaThe layer of pectin, mainly calcium pectate, that sticks neighbouring plant cell walls together. as calcium pectate and to membrane stability. New tissues that are laying down walls are sensitive to shortage. Calcium is not highly mobile in the phloemPlant tissue that transports sucrose and other organic substances, in sieve tubes with companion cells., so symptoms often appear in young growing regions rather than as a simple copy of magnesium deficiency.
Interpreting a deficiency culture
A recommended investigation grows plants in solutions lacking one named ion, with a complete-nutrient control. The independent variable is which ion is omitted. Dependent variables might include height, dry mass or a described leaf symptom. Water volume, light, temperature, pHA measure of how acidic or alkaline a solution is, based on its H⁺ ion concentration. Below 7 is acidic, above 7 is alkaline. and all other ions should be standardised.
A pale leaf is not automatic proof of magnesium shortage: nitrate shortage and other stresses can also reduce chlorophyll. Ion interactions matter; high potassium supply can reduce magnesium uptake. Hydroponic symptoms are not always identical in soil, where pH and microbes change availability. Use plants of the same speciesA group of organisms that can interbreed to produce fertile offspring. and starting size, and do not treat a single pot as a general law.
NOT: “the plant needs minerals” does not name nitrate, magnesium or calcium.
- Nitrate deficiency: reduced growth and chlorosis
- Magnesium deficiency: yellow leaves, chlorophyll short
- Calcium deficiency: weak tissues, poor middle lamella
- Phosphate and potassium are not the named list here
Two traps this diagram blocks
- Do not swap magnesium and nitrate. Only magnesium is part of chlorophyll.
- Do not treat yellow leaves as proof of one ion unless the investigation isolates it.
From the diagram alone
Question
Explain why a shortage of magnesium and a shortage of nitrate can both reduce growth, in different ways. [4]
Show a model answer
Magnesium is part of chlorophyll, so a shortage reduces photosynthesis and the sugars available for growth. Nitrate is needed to make amino acids and therefore proteins, so a shortage limits protein synthesis even if sugars are available. The symptoms can overlap, but the missing molecules are different.
| Requirement | Role to recall | If it is scarce |
|---|---|---|
| Water | solventA liquid in which other substances (solutes) dissolve. Water is a good solvent for ions and polar molecules., photosynthesis reactant, turgor | Wilting; reduced transport and cooling by transpiration |
| Nitrate | Nitrogen for amino acids, proteins, nucleic acids | Poor growth; reduced protein and chlorophyll synthesis |
| Magnesium ions | Chlorophyll | Impaired photosynthesis; interveinal yellowing, often older leaves |
| Calcium ions | Middle lamella and wall/membrane stability | Weak new wall cohesion; symptoms often in young tissues |
Make your reasoning visible
Name the molecule
“magnesiumA mineral ion (Mg²⁺) that plants need to make chlorophyll. is in chlorophyllThe green pigment in chloroplasts. It absorbs mainly red and blue light.”; “nitrateNO₃⁻, a mineral ion taken up by plant roots and used to make amino acids, proteins and nucleic acids. supplies nitrogen for amino acidThe monomer of proteins: a central carbon with an amine group, a carboxyl group, a hydrogen and an R group.”; “calcium pectate in the middle lamellaThe layer of pectin, mainly calcium pectate, that sticks neighbouring plant cell walls together.”.
Do not diagnose from colour alone
Several shortages and other stresses can yellow a leaf. Use the omitted ion plus a control.
Key explanations
- A deficiency explanation must name the ion, the molecule or structure it is used in, and the symptom that follows.
Avoid this misconception
The claim: Plants only need water and sunlight; minerals are optional extras.
The correction: Nitrate, calcium and magnesium ions are required for proteins, walls and chlorophyll. Water cannot replace those roles.
Check your understanding
Try each question before opening the suggested answer.
Practice question
Retrieval and explanation · suggested answer
Explain two reasons why water is important to a herbaceous plant.
Show suggested answer
It is the solvent for transport and a reactant in photosynthesis. Turgor from water in the protoplast supports non-woody tissues.
Practice question
Retrieval and explanation · suggested answer
Why does magnesium deficiency impair photosynthesis?
Show suggested answer
Magnesium is a component of chlorophyll, so shortage reduces chlorophyll and light capture.
Practice question
Retrieval and explanation · suggested answer
A complete-nutrient plant is green and tall; a no-nitrate plant is small and pale. What can you conclude, and what can you not?
Show suggested answer
Omitting nitrate reduced growth and greening under those conditions. You cannot conclude that colour always means magnesium shortage, or that soil-grown plants would show identical symptoms.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
Two cultures omit calcium or magnesium. Young leaves are distorted in one; older leaves yellow between veins in the other. Match the ions and explain the pattern, stating an assumption about mobility.
Show a hint
Think where the plant can move the ion from.
Explore the reasoning
Magnesium is mobile, so it can be withdrawn from older leaves, producing interveinal yellowing there. Calcium is poorly mobile, so new walls in young tissues suffer first. Assume no other ion is co-limiting and that the plants are otherwise comparable.
Recall this lesson
Lesson recall questions
Question 1 of 5: Why do plants need nitrate ions?
Water and mineral ions in plants
Practice question
Retrieval and explanation · suggested answer
Why do plants need nitrate ions?
Lesson: 4A 4: The Importance of Water and Minerals in Plants
Try this from memory first.
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
Reading a leaf as a nutrient clue, not a diagnosis app
4A · The Importance of Water and Minerals in PlantsThe Royal Horticultural Society describes magnesium deficiency as yellowing between veins, often on older leaves, and notes that high potassium feeds can make magnesium harder to take up.…Specification 4.8Read & use the scienceClose article
Why this matters
The Royal Horticultural Society describes magnesium deficiency as yellowing between veins, often on older leaves, and notes that high potassium feeds can make magnesium harder to take up. Calcium problems in fruit, such as blossom-end rot, are often about distribution inside the plant after drying, not a total lack of calcium in the compost. Those horticultural details are enrichment. Nitrate supplies nitrogen for amino acids and proteins; magnesium is part of chlorophyll; calcium has roles in wall and membrane function. Water carries these ions through the plant and helps maintain turgor. Garden symptoms can suggest a problem but do not isolate a cause without checking the growing conditions. The RHS example shows why agronomists inspect both the leaf and the root environment. A useful exam response states the named ion and its function before interpreting an unfamiliar symptom.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 2, pp. 21–24. The application extends beyond required recall.
- 4.8 understand the importance of water and inorganic ions (nitrate, calcium ions and magnesium ions) to plants
Use the science
Why might adding more calcium fertiliser fail to stop blossom-end rot if the compost already contains calcium?
Explore a possible response
If the ion cannot be delivered to the growing fruit because of irregular watering or poor transpiration flow, extra calcium in the medium may not reach the tissue that needs it.
Explore the source
- RHS: nutrient deficienciesRoyal Horticultural Society