Ch 3 · Mineral Nutrition · O Level Biology 5090
Topic 3 of 3 in Plant Nutrition — O Level Biology 5090 — 48 lessons.
Introduction
Photosynthesis gives a plant carbohydrate, which contains only carbon, hydrogen and oxygen. To build proteins, chlorophyll and everything else it needs, the plant must also absorb mineral ions from the soil through its root hairs. This topic focuses on two ions Cambridge names explicitly, nitrate and magnesium, and on the water-culture experiments used to show what happens when one is missing.
Key ideas — Why plants need nitrate ions
Glucose provides carbon, hydrogen and oxygen, but proteins also contain nitrogen. Plants obtain nitrogen as nitrate ions, absorbed from the soil solution by root hair cells, largely by active transport. Nitrate is combined with carbohydrate to make amino acids, which are joined together to make proteins.
🧑🏫 Teacher's explanation
Think of a plant like a construction site. Glucose is the basic building material like bricks and wood, providing the bulk structure. However, to make the complex machinery (enzymes) and the specialized blueprints (DNA/proteins) that actually run the site, you need specialized components that glucose doesn't have. Nitrogen is that essential 'specialist' component.
📖 Definitions
🔍 Worked example — Easy
A plant is grown in a solution containing only glucose and water. Predict the effect on the plant's growth and state why.
🔍 Worked example — Standard
Compare the appearance of a plant grown in a nitrate-deficient solution versus a plant grown in a complete nutrient solution.
🔍 Worked example — Exam-style
A student observes that a plant grown in a solution lacking magnesium ions has pale yellow leaves (chlorosis). Explain this observation using your knowledge of mineral ions.
⚠️ Common errors
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🎯 Exam question
Key ideas — Why plants need magnesium ions
Magnesium ions are needed to make chlorophyll; every chlorophyll molecule contains one magnesium atom at its centre. Without magnesium the plant cannot make chlorophyll, so less light energy is absorbed, photosynthesis is reduced and less carbohydrate is made for growth and storage. Magnesium, like nitrate, is absorbed by the roots and carried to the leaves in the xylem.
🧑🏫 Teacher's explanation
Think of a plant like a high-tech solar panel system. To capture sunlight, the panels need a specific core component to function. In a plant, that core component is the chlorophyll molecule. Magnesium ions act as the central 'engine' or 'hub' of every single chlorophyll molecule.
📖 Definitions
🔍 Worked example — Easy
A student observes a plant with pale yellow leaves. State the most likely mineral deficiency and the specific part of the plant where the deficiency is most visible.
🔍 Worked example — Standard
Explain why a deficiency in magnesium ions leads to reduced growth in a plant.
🔍 Worked example — Exam-style
In an experiment, two sets of plants are grown in different solutions. Set A contains all essential minerals. Set B contains all minerals except magnesium ions. Predict the appearance of Set B after 4 weeks and explain why.
⚠️ Common errors
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🎯 Exam question
Key ideas — Deficiency symptoms
effects of deficiency. A plant lacking nitrate ions cannot make enough amino acids and proteins, so cell growth and division slow. It shows stunted growth, with a weak, spindly stem and small leaves, and the older, lower leaves turn yellow first because the plant moves its scarce nitrogen to the young leaves.
🧑🏫 Teacher's explanation
Think of a plant like a construction site. To build the structure (the plant body), you need bricks (amino acids/proteins) and solar panels (chlorophyll) to power the machinery. If the delivery truck for bricks (nitrate ions) stops coming, the building stops growing taller. If the delivery truck for solar panels (magnesium ions) stops coming, the site can't capture sunlight, and the whole operation slows down.
📖 Definitions
🔍 Worked example — Easy
A student observes a plant grown in a solution lacking magnesium ions. Describe the appearance of the leaves.
🔍 Worked example — Standard
Compare the effect of nitrate deficiency versus magnesium deficiency on a plant's growth.
🔍 Worked example — Exam-style
A plant is grown in a culture solution lacking nitrate ions. Calculate the percentage decrease in plant height if the height drops from 15 cm to 6 cm.
⚠️ Common errors
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🎯 Exam question
Key ideas — Interpreting water-culture experiments
The standard investigation grows seedlings in culture solutions of known mineral content dissolved in distilled water.
🧑🏫 Teacher's explanation
Think of a plant like a high-end factory. To build the factory itself (the structure and machinery), you need specific raw materials like steel and concrete. If you run out of steel, you can't build more machines. In a plant, nitrate ions are like that steel; they are needed to build amino acids, which are the building blocks of proteins. Without proteins, the plant cannot grow or repair itself.
📖 Definitions
🔍 Worked example — Easy
A student grows three sets of pea seedlings. Set A is in a complete culture solution. Set B is in a solution lacking magnesium ions. Set C is in a solution lacking nitrate ions. After 4 weeks, the student observes that Set B has pale yellow leaves, while Set C is stunted in height. Identify the missing ion for Set B and explain the effect on the plant.
🔍 Worked example — Standard
A plant is grown in a solution lacking nitrate ions. Calculate the percentage decrease in dry mass if the control plant has a dry mass of 0.50 g and the nitrate-deficient plant has a dry mass of 0.15 g.
🔍 Worked example — Exam-style
In an experiment to test the effect of nitrate ions on growth, a student uses 10 seedlings in the control group and 10 seedlings in the nitrate-deficient group. Explain why measuring 'dry mass' is more reliable than measuring 'height' when determining the effect of the mineral deficiency.
⚠️ Common errors
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🎯 Exam question
Set A: Grown in a complete culture solution. Set B: Grown in a culture solution lacking nitrate ions.
Worked example
Question. Four groups of tomato seedlings of the same age were grown for three weeks in culture solutions. Group A had all mineral ions; Group B lacked nitrate; Group C lacked magnesium; Group D was grown in distilled water. Mean dry masses were A 4.8 g, B 1.6 g, C 3.1 g, D 0.9 g. Group C leaves were yellow between the veins. Explain these results. [5]
🔍 Worked example — Easy
A plant is grown in a solution containing only water and potassium. It shows stunted growth and pale leaves. Identify the most likely missing mineral ion and state its biological role.
🔍 Worked example — Standard
Calculate the percentage decrease in dry mass from Group A (4.8 g) to Group B (1.6 g) in the provided experiment.
🔍 Worked example — Exam-style
Explain why a plant grown in distilled water (Group D) shows the least growth compared to all other groups.
⚠️ Common errors
Common mistakes
Practice
Answers (a) Nitrate ions are used to make amino acids [B1], which are joined to make proteins for growth and enzymes [B1]. (b) Magnesium ions are needed to make chlorophyll [B1], which absorbs light energy for photosynthesis [B1]. Leaves are yellow, especially between the veins [B1]; magnesium is needed to make chlorophyll [B1]. To keep light from the solution so algae cannot grow [B1]; algae would use up the mineral ions [B1]. Water content varies from plant to plant and hour to hour [B1]; dry mass measures the biological material actually made [B1]. Nitrate is needed to make amino acids [B1]; amino acids are needed to make proteins [B1]; without proteins, new cytoplasm, membranes and enzymes cannot be made, so fewer new cells form [B1].