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.
Photosynthesis in plants and algae is an endothermic reaction. This means it takes in energy from the surroundings. The energy comes from light, which is absorbed by the green pigment chlorophyll in the chloroplasts.
The light energy is used to react carbon dioxide and water to produce glucose and oxygen.
carbon dioxide + water → glucose + oxygen (in the presence of light)
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Where the raw materials come from and where the products go
Substance
Reactant or product
Source or fate
Carbon dioxide
Reactant
Diffuses into the leaf from the air
Water
Reactant
Absorbed from the soil by the roots and carried to the leaf in the xylem
Glucose
Product
Used for respiration, or to build new plant material (biomass)
Light is the energy source, not a reactant, so it does not go in the word equation as a substance. Write 'in the presence of light' or put 'light energy' above the arrow.
Written and checked against the Edexcel GCSE Combined Science (1SC0) specification · Updated October 2026
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.
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