Limiting factorsSpec 6.3
In short
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.
A limiting factor is something that is in short supply, so it 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.
| Factor | Effect on rate | Why |
|---|---|---|
| Light intensity | Rate increases as light intensity increases, then levels off | Light provides the energy for the reaction. At low light, the amount of light limits the rate. At high light, another factor is limiting. |
| Carbon dioxide concentration | Rate increases as concentration increases, then levels off | Carbon dioxide is a reactant. At low concentration, there is not enough of it. At high concentration, another factor is limiting. |
| Temperature | Rate increases up to an optimum temperature, then falls sharply | Photosynthesis is controlled by enzymes. A higher temperature gives more collisions between enzymes and substrates. Above the optimum the enzymes are denatured, so the rate falls. |
(opens full size in a new tab)On a graph, the rate rises while the factor on the x-axis is the limiting factor. Where the line goes flat, that factor is no longer limiting and something else is. Reading a graph this way is a common exam question.
Plot the rate on the y-axis and the factor you changed on the x-axis. To describe a graph, state the trend, then quote data from it, for example the values where the line levels off.
Do not write that the enzymes are 'killed' at high temperature. Enzymes are not alive. They are denatured, because their active site changes shape.
Written and checked against the Edexcel GCSE Combined Science (1SC0) specification · Updated October 2026