Photosynthesis — Cambridge IGCSE Biology
The photosynthesis equations, uses of glucose, nitrate and magnesium, limiting factors and the practical investigations.
Revision notes
7 short notes, in the order of the specification. Each one in short:
Photosynthesis is the process by which plants synthesise carbohydrates from raw materials using energy from light. The word equation is carbon dioxide + water → glucose + oxygen, in the presence of light and chlorophyll. Chlorophyll is a green pigment in chloroplasts that absorbs light and transfers energy from light into energy in chemicals.
Plants use the glucose made in photosynthesis in five main ways. They use it in respiration to release energy, store it as insoluble starch, build cellulose cell walls from it, convert it to sucrose for transport in the phloem, and make nectar to attract insects for pollination. Plants transport sucrose, not glucose or starch.
Plants need nitrate ions to make amino acids, which they join to make proteins, and magnesium ions to make chlorophyll. Glucose contains no nitrogen, so nitrate from the soil supplies it. A lack of nitrate causes stunted growth. A lack of magnesium causes chlorosis: the leaves turn yellow because there is less chlorophyll, so less photosynthesis.
Testing a leaf for starch shows whether it has photosynthesised, because glucose made in photosynthesis is converted to starch. The plant is destarched in the dark for 24 to 48 hours first. The leaf is boiled, then heated in ethanol to remove the chlorophyll, and iodine turns from orange-brown to blue-black where starch is present.
A limiting factor is the factor in shortest supply, which stops the rate of photosynthesis from increasing. Light intensity, carbon dioxide concentration and temperature can each be limiting. Increasing a limiting factor raises the rate; increasing a factor that is not limiting makes no difference. On a graph, the factor is limiting where the line is still rising.
The rate of photosynthesis can be measured by counting the oxygen bubbles released by pondweed such as Elodea per minute, or by collecting and measuring the gas. Change light intensity by moving a lamp, carbon dioxide with sodium hydrogencarbonate, or temperature with a water bath, keeping the other factors constant. Collecting the gas is more accurate.
Hydrogencarbonate indicator is a solution that changes colour with carbon dioxide concentration: yellow when high, red at atmospheric level and purple when low. An aquatic plant in bright light turns it purple because photosynthesis uses carbon dioxide faster than respiration releases it. In the dark the plant only respires, so the indicator turns yellow.
Exam questions
7 exam-style questions (31 marks), each with its mark scheme.
Answer the questionsFlashcards
21 cards: flip them, mark what you knew, and practise the rest.
Practise the cardsMind map
The whole of plant nutrition on one page, so you can see where this subtopic fits.
Open the mind mapPractical sheets
Method, variables, risks and a results table, with questions to answer.
Print it
Free PDFs to print or save.
Photosynthesis revision notes12 pagesExam questions and mark scheme5 pagesFlashcards to print and cut out6 pagesPlant nutrition mind map1 pagePractical skill 5: What a plant needs for photosynthesis8 pagesPractical skill 6: Factors affecting the rate of photosynthesis9 pagesPractical skill 7: Light, dark and gas exchange in an aquatic plant7 pages
Quick check questions
Give the word equation for photosynthesis.
Show answer
Carbon dioxide + water → glucose + oxygen, in the presence of light and chlorophyll.
Which carbohydrate is transported in the phloem?
Show answer
Sucrose.
Why do plants need magnesium ions?
Show answer
To make chlorophyll.
What colour does iodine solution turn in the presence of starch?
Show answer
Blue-black.
Supplement Write the balanced chemical equation for photosynthesis.
Show answer
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Frequently asked questions
What are the products of photosynthesis used for?
The glucose made in photosynthesis is used for respiration, stored as starch, built into cellulose for cell walls, converted to sucrose for transport in the phloem, and used to make nectar to attract insects. Oxygen is released as a by-product. Glucose is also combined with nitrogen from nitrate ions to make amino acids.
How does light intensity affect the rate of photosynthesis?
Supplement As light intensity increases, the rate of photosynthesis increases, because light provides the energy for the process. The rate then levels off, because another factor, such as carbon dioxide concentration or temperature, becomes limiting. You can show this by moving a lamp closer to pondweed and counting the oxygen bubbles released per minute.
How does temperature affect the rate of photosynthesis?
Supplement As temperature rises, the rate of photosynthesis increases up to an optimum, because photosynthesis is controlled by enzymes and molecules move faster with more collisions. Above the optimum, the rate falls sharply because the enzymes are denatured. On a cold bright day, temperature is likely to be the limiting factor.
What are the limiting factors of photosynthesis?
Supplement The limiting factors of photosynthesis are light intensity, carbon dioxide concentration and temperature. A limiting factor is the one in shortest supply, so it stops the rate from increasing. At night, light intensity limits the rate; on a bright summer midday, carbon dioxide concentration or temperature is more likely to be limiting.
Why is a leaf heated in ethanol when testing for starch?
A leaf is heated in ethanol to remove the chlorophyll, so the leaf turns pale and the colour change with iodine can be seen. The leaf is first dropped into boiling water to kill and soften it. Ethanol is flammable, so turn off the Bunsen burner and heat the ethanol in a hot water bath.
Written and checked against the Cambridge IGCSE Biology (0610) specification · Updated October 2026