IGCSE Biology: Plant Nutrition Practice Questions
Photosynthesis is the process by which plants make carbohydrates from carbon dioxide and water, using light energy absorbed by chlorophyll. The balanced equation is 6CO2 + 6H2O giving C6H12O6 + 6O2.
Topic 6 of Cambridge IGCSE Biology 0610 combines a fixed equation, a required practical and a graph interpretation skill. Limiting factor graphs appear repeatedly and reward a precise answer about which factor is limiting in each region of the curve. The questions below cover the equation, the practical, the graphs and mineral deficiencies.
What you need to know for Plant Nutrition
- Photosynthesis equationWord equation: carbon dioxide + water produces glucose + oxygen, in the presence of light and chlorophyll. Balanced equation: 6CO2 + 6H2O gives C6H12O6 + 6O2.
- Role of chlorophyllChlorophyll is the green pigment inside chloroplasts. It absorbs light energy and transfers it into chemical energy in the glucose molecules produced.
- Limiting factorThe factor present in the shortest supply, which limits the rate of the process. For photosynthesis the three limiting factors are light intensity, carbon dioxide concentration and temperature.
- Testing a leaf for starchDestarch the plant in the dark, expose the leaf to the test conditions, then boil the leaf in water to kill it, boil it in ethanol to remove chlorophyll, rinse it in water to soften it, and add iodine solution. Blue black shows starch.
- Uses of glucoseConverted to starch for storage, to cellulose for cell walls, to sucrose for transport, used in respiration, and combined with nitrate ions to make amino acids.
- Mineral ionsNitrate ions are needed to make amino acids and therefore proteins. Magnesium ions are needed to make chlorophyll. Shortage of either causes stunted growth, and magnesium shortage causes yellow leaves.
IGCSE Biology Plant Nutrition questions and answers
4 exam-style questions written to the 0610 syllabus. Try each one on paper first, then open the worked answer to check your method against the marks.
Write the balanced chemical equation for photosynthesis and state the two conditions required for it to take place.
Show the worked answer
- The reactants are carbon dioxide and water. The products are glucose and oxygen.
- Balanced: 6CO2 + 6H2O gives C6H12O6 + 6O2.
- Check the balance: 6 carbon, 12 hydrogen and 18 oxygen atoms on each side.
- Light energy is required, and chlorophyll must be present to absorb it. These are usually written above and below the arrow.
A graph shows the rate of photosynthesis increasing steeply as light intensity rises, then levelling off to a plateau. Explain what is limiting the rate in each part of the graph.
Show the worked answer
- On the steep part of the graph, increasing light intensity increases the rate, so light intensity is the limiting factor.
- More light means more energy is absorbed by chlorophyll, so the reaction proceeds faster.
- On the plateau, increasing the light intensity no longer increases the rate, so light is no longer limiting.
- Something else has become the limiting factor, most likely carbon dioxide concentration or temperature. To test which, repeat the experiment with a higher carbon dioxide concentration and see whether the plateau rises.
Describe how to test a leaf for the presence of starch, and explain why the leaf is boiled in ethanol.
Show the worked answer
- Boil the leaf in water for about a minute to kill the tissue and break down the cell membranes.
- Boil the leaf in ethanol using a water bath, since ethanol is flammable and must not be heated with a naked flame.
- The ethanol removes the chlorophyll so the leaf becomes pale. This matters because the green colour would otherwise hide the colour change with iodine.
- Rinse the leaf in warm water to soften it, since ethanol makes it brittle.
- Spread the leaf on a white tile and add iodine solution. A blue black colour shows starch is present.
A plant grown in soil lacking magnesium ions develops yellow leaves and grows slowly. Explain these observations.
Show the worked answer
- Magnesium ions are required for the synthesis of chlorophyll.
- Without enough magnesium the plant makes less chlorophyll, so the leaves lose their green colour and appear yellow.
- Less chlorophyll means less light energy is absorbed, so the rate of photosynthesis falls.
- Less glucose is produced, so there is less material for growth and less energy released in respiration, and the plant grows slowly.
Common mistakes in this topic
- Forgetting to balance the photosynthesis equation.
- Writing that plants only respire at night. Plants respire continuously, day and night.
- Saying photosynthesis produces energy. It converts light energy into chemical energy stored in glucose.
- Describing a plateau on a limiting factor graph as the maximum rate possible. It is the maximum under those particular conditions.
- Omitting the water bath when boiling ethanol.
Exam tips
- Learn the equation both ways, word and balanced, and be able to state the conditions above the arrow.
- For every limiting factor graph, answer in two parts: what limits the steep section and what limits the plateau.
- In practical questions, mention the destarching step. Leaving it out loses the control mark.
- Link every mineral ion to the molecule it helps build: nitrate to amino acids, magnesium to chlorophyll.
- If a question gives you a variegated leaf or a leaf with part covered in foil, identify the control region first and say what it proves.
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Plant Nutrition FAQs
What is the balanced equation for photosynthesis?
6CO2 + 6H2O gives C6H12O6 + 6O2, in the presence of light energy and chlorophyll. In words, carbon dioxide plus water produces glucose plus oxygen. Light and chlorophyll are conditions written above and below the arrow, not reactants in the equation.
What are the limiting factors of photosynthesis?
Light intensity, carbon dioxide concentration and temperature. The limiting factor is whichever of these is in shortest supply at the time. On a rate graph, the factor being varied is limiting while the line rises, and a different factor is limiting once the line plateaus.
Why is a leaf boiled in ethanol when testing for starch?
Ethanol dissolves and removes the chlorophyll, leaving the leaf pale. Without this step the green colour would mask the blue black colour produced by iodine, making the result impossible to read. Ethanol is flammable, so the leaf must be boiled in ethanol using a water bath rather than a naked flame.
What do plants need nitrate and magnesium ions for?
Nitrate ions supply the nitrogen needed to make amino acids, which are used to build proteins for growth. Magnesium ions are needed to make chlorophyll. A shortage of nitrate causes stunted growth, and a shortage of magnesium causes leaves to turn yellow because less chlorophyll is made.
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Written to the published Cambridge IGCSE Biology (0610) syllabus. Check your school entry code and syllabus year, because Core and Extended candidates are assessed on different content. Last reviewed 2026-08-12.