Nutrition in flowering plants — Edexcel International GCSE Biology
How plants make glucose by photosynthesis, what affects its rate, how the leaf is adapted, and the practical work.
Revision notes
6 short notes, in the order of the specification. Each one in short:
Photosynthesis is the process by which plants and algae make glucose, using light energy absorbed by chlorophyll in the chloroplasts. Carbon dioxide and water react to form glucose and oxygen: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. It converts light energy to chemical energy stored in glucose, which plants use for respiration and to build new material.
The rate of photosynthesis is affected by light intensity, carbon dioxide concentration and temperature, and any one of these can be the limiting factor. Increasing light or carbon dioxide raises the rate until it levels off, because another factor becomes limiting. Temperature raises the rate up to an optimum, then the enzymes are denatured and the rate falls sharply.
A leaf is adapted for photosynthesis and gas exchange. It is broad, flat and thin, giving a large surface area to absorb light and a short diffusion distance for gases. Most photosynthesis happens in the palisade mesophyll, whose cells contain many chloroplasts. Air spaces in the spongy mesophyll and stomata opened by guard cells let gases diffuse in and out.
Plants need mineral ions, absorbed from the soil by the roots, for healthy growth. Magnesium ions are needed to make chlorophyll, so a shortage causes yellow leaves and slower photosynthesis. Nitrate ions are needed to make amino acids, which are joined to make proteins for growth, so a shortage causes stunted growth.
Practical: oxygen from a water plant
PracticalThe rate of photosynthesis can be measured by counting the oxygen bubbles a water plant such as pondweed gives off per minute, or by collecting and measuring the volume of gas. Light intensity is changed by moving a lamp to different distances. Closer lamps give more bubbles until another factor becomes limiting. Temperature and carbon dioxide are kept constant.
Testing a leaf for starch with iodine solution shows whether it has photosynthesised, because plants store glucose as starch. The plant is destarched in the dark first. The leaf is boiled, its chlorophyll is removed in hot ethanol, and iodine is added: blue-black means starch. Covered, carbon dioxide-free or white variegated areas stay orange-brown.
Exam questions
7 exam-style questions (25 marks), each with its mark scheme.
Answer the questionsFlashcards
20 cards: flip them, mark what you knew, and practise the rest.
Practise the cardsMind map
The whole of structure and functions in living organisms on one page, so you can see where this subtopic fits.
Open the mind mapPractical sheet
Method, variables, risks and a results table, with questions to answer.
Print it
Free PDFs to print or save.
Quick check questions
Give the word equation for photosynthesis.
Show answer
Carbon dioxide + water → glucose + oxygen (in the presence of light and chlorophyll).
Which form of energy is photosynthesis converting light into?
Show answer
Chemical energy, stored in glucose.
Name three factors that can affect the rate of photosynthesis.
Show answer
Carbon dioxide concentration, light intensity and temperature.
Why do plants need magnesium ions and nitrate ions?
Show answer
Magnesium ions are needed to make chlorophyll. Nitrate ions are needed to make amino acids.
What colour does iodine solution turn when starch is present?
Show answer
Blue-black.
Frequently asked questions
What are the limiting factors of photosynthesis?
The limiting factors of photosynthesis are light intensity, carbon dioxide concentration and temperature. A limiting factor is whichever is in shortest supply, so it stops the rate from going any faster. On a graph the rate rises while that factor limits it, then the line goes flat when a different factor becomes limiting.
How does light intensity affect the rate of photosynthesis?
As light intensity increases, the rate of photosynthesis increases, then levels off. Light provides the energy for the reaction, so at low light intensity the amount of light limits the rate. At high light intensity the rate stops rising because another factor, such as carbon dioxide concentration or temperature, has become the limiting factor.
How does temperature affect the rate of photosynthesis?
The rate of photosynthesis increases with temperature up to an optimum, then falls sharply. Photosynthesis is controlled by enzymes, so a higher temperature gives more collisions between enzymes and substrates. Above the optimum the enzymes are denatured because their active sites change shape, so the rate falls. Enzymes are not killed, because they are not alive.
Why do plants need magnesium and nitrate ions?
Plants need magnesium ions to make chlorophyll and nitrate ions to make amino acids, which are joined to make proteins for growth. A plant short of magnesium has yellow leaves and absorbs less light. A plant short of nitrate shows stunted growth because it cannot make the proteins it needs for new cells.
How do you test a leaf for starch?
To test a leaf for starch, dip it in boiling water for about a minute, then put it in hot ethanol in a water bath with the Bunsen burner off to remove the chlorophyll. Rinse it in warm water, spread it on a white tile and add iodine solution. Blue-black shows starch; orange-brown shows none.
Written and checked against the Edexcel IGCSE Biology (4BI1) specification · Updated October 2026