Photosynthesis and limiting factors — Edexcel GCSE Combined Science
Photosynthesis as the source of food and biomass, its equation, the limiting factors, the light intensity core practical and the inverse square law.
Revision notes
6 short notes, in the order of the specification. Each one in short:
Photosynthetic organisms are plants and algae, which use light energy to make glucose from simple inorganic substances. This makes them the main producers of food and biomass, the mass of living material. Every food chain starts with a producer, and animals get their biomass, and the energy stored in it, by eating plants or algae or other animals.
Photosynthesis is an endothermic reaction in which light energy, absorbed by chlorophyll in the chloroplasts, is used to react carbon dioxide and water to produce glucose and oxygen. The balanced equation is 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. Light is the energy source, not a reactant, so it is written above the arrow.
A limiting factor is something in short supply that stops the rate of a process from going any faster. The rate of photosynthesis can be limited by temperature, light intensity and carbon dioxide concentration. On a graph, the rate rises while that factor is limiting, and the line goes flat when another factor becomes limiting.
Interactions between limiting factors
Higher tierTemperature, light intensity and carbon dioxide concentration all act at the same time, and the rate of photosynthesis is limited by whichever factor is in shortest supply. Where a graph levels off, the factor on the x-axis is no longer limiting. A line with a higher plateau has more of another factor, such as carbon dioxide concentration.
The light intensity core practical investigates how light intensity affects the rate of photosynthesis in pondweed. Light intensity is changed by moving a lamp to different distances, and the rate is measured by counting oxygen bubbles per minute or collecting the gas. Temperature is kept constant with a heat shield, and carbon dioxide with sodium hydrogencarbonate.
The inverse square law states that light intensity is proportional to 1 ÷ distance². When light is the limiting factor, the rate of photosynthesis is directly proportional to light intensity, so it follows the same law. Doubling the distance from the lamp makes the intensity, and the rate, one quarter. The rule only works while light is limiting.
Exam questions
7 exam-style questions (23 marks), each with its mark scheme.
Answer the questionsFlashcards
19 cards: flip them, mark what you knew, and practise the rest.
Practise the cardsMind map
The whole of plant structures and their functions 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
Why are photosynthetic organisms called the main producers of biomass?
Show answer
They make their own food (glucose) by photosynthesis, which builds biomass that other organisms feed on.
Why is photosynthesis described as endothermic?
Show answer
It takes in energy from the surroundings, in the form of light.
Name the three limiting factors of photosynthesis.
Show answer
Temperature, light intensity and carbon dioxide concentration.
In the light intensity core practical, how is the light intensity changed?
Show answer
By changing the distance between the lamp and the pondweed.
Frequently asked questions
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 line goes flat because another factor, such as carbon dioxide concentration or temperature, is now limiting.
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, and a higher temperature gives more collisions between enzymes and substrates. Above the optimum the enzymes are denatured because their active site changes shape, so the rate falls. Enzymes are denatured, not killed.
What are the limiting factors of photosynthesis?
The limiting factors of photosynthesis are temperature, light intensity and carbon dioxide concentration. A limiting factor is in short supply, so it stops the rate from going any faster. On a graph, the rate rises while the factor on the x-axis is limiting, and the line levels off when something else becomes limiting.
Why is photosynthesis endothermic?
Photosynthesis is endothermic because it takes in energy from the surroundings. The energy comes from light, which is absorbed by the green pigment chlorophyll in the chloroplasts. This light energy is used to react carbon dioxide and water to make glucose and oxygen, so light is the energy source, not a reactant.
How do you investigate the effect of light intensity on photosynthesis?
Place pondweed in water with sodium hydrogencarbonate and put a lamp at measured distances, such as 10, 20 and 30 cm. Count the oxygen bubbles released per minute at each distance, repeat and calculate a mean. Keep the temperature constant with a heat shield or LED lamp, because a lamp close to the plant warms the water.
Written and checked against the Edexcel GCSE Combined Science (1SC0) specification · Updated October 2026