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Edexcel GCSE Biology · 1BI0
Edexcel 1BI0 · 6.4 Check the specification (PDF) (opens in a new tab)
Photosynthesis uses light energy to convert carbon dioxide and water into glucose and oxygen. Light supplies the energy, carbon dioxide supplies material for making glucose, and enzymes help the reactions take place. The rate therefore depends on the conditions acting together, not on any one condition in isolation.
A limiting factor is the condition currently holding back the rate. Imagine a plant in dim light with plenty of carbon dioxide and a suitable temperature. Giving it more carbon dioxide is unlikely to help much: it cannot use that extra supply effectively without more light. Increasing the light intensity, however, can increase the rate. Once the light is bright enough, the plant may use carbon dioxide as quickly as it is available. Carbon dioxide, rather than light, can then become limiting.
A graph of photosynthesis rate against light intensity often rises at first and then levels off. The rising part shows that more light allows a faster rate. At the plateau, extra light alone makes no difference: another condition now restricts the rate. The plateau does not by itself tell you whether that condition is carbon dioxide concentration or temperature.
Compare curves measured under different conditions to find out more. In the graph, both carbon dioxide conditions give the same rate in very dim light because light is limiting. At greater light intensities, the higher-carbon-dioxide curve reaches a higher rate. That difference shows that carbon dioxide restricted the lower curve where it levelled off. The higher curve eventually levels off too; its plateau might be caused by temperature or another condition, so the graph alone cannot identify it.
At low light intensity the curves coincide. With more light, extra carbon dioxide allows a higher rate, showing why the factor limiting photosynthesis can change.
Data for How light intensity and carbon dioxide concentration interact
| Series | Light intensity (arbitrary units) | Rate of photosynthesis (arbitrary units) |
|---|---|---|
| Lower carbon dioxide concentration | 0 | 0 |
| Lower carbon dioxide concentration | 1 | 2 |
| Lower carbon dioxide concentration | 2 | 3 |
| Lower carbon dioxide concentration | 3 | 3 |
| Lower carbon dioxide concentration | 4 | 3 |
| Higher carbon dioxide concentration | 0 | 0 |
| Higher carbon dioxide concentration | 1 | 2 |
| Higher carbon dioxide concentration | 2 | 4 |
| Higher carbon dioxide concentration | 3 | 5 |
| Higher carbon dioxide concentration | 4 | 5 |
Temperature affects the enzymes involved in photosynthesis. When temperature is low, increasing it can speed their reactions. But warming a plant in dim light may produce little increase because light still limits the rate. With sufficient light and carbon dioxide, temperature may have a much clearer effect. Above the optimum temperature, enzymes can denature and the rate falls; more light or carbon dioxide cannot simply undo that effect.
This is why the limiting factor can change as conditions change. Increasing one condition may raise the rate until a different condition becomes restrictive. To explain an interaction, identify what was changed, whether the rate responded, and what the response—or lack of one—reveals about the factor limiting the rate at that point.
Light intensity, carbon dioxide concentration and temperature interact. A factor is limiting when changing it appropriately would increase the photosynthesis rate. Once another condition takes over, increasing the first factor alone has little or no effect.
A rising light-intensity curve shows light is limiting. At a plateau, light is no longer limiting, but the curve alone does not identify the new limiting factor. A higher plateau after increasing carbon dioxide is evidence that carbon dioxide limited the lower curve.
Warming can increase the rate below the optimum if other factors allow it. Above the optimum, enzyme denaturation can reduce the rate.
On a graph, a plateau shows that increasing the factor on the horizontal axis no longer increases the rate. Do not name the new limiting factor without further evidence.
When comparing curves, use the changed condition as evidence: if raising carbon dioxide produces a higher plateau, carbon dioxide limited the lower curve at that point.
Heating helps only below the optimum temperature; above it, the rate can fall as enzymes denature.
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Last updated
Limiting factor
A condition that currently restricts the rate of a process, so changing that condition appropriately could increase the rate.
Optimum temperature
The temperature at which a process reaches its highest rate under the conditions being considered.
Plateau
A level section of a graph where increasing the factor on the horizontal axis no longer increases the measured rate.
Put your knowledge into practice — try past paper questions for Biology
Limiting factor
A condition that currently restricts the rate of a process, so changing that condition appropriately could increase the rate.
Optimum temperature
The temperature at which a process reaches its highest rate under the conditions being considered.
Plateau
A level section of a graph where increasing the factor on the horizontal axis no longer increases the measured rate.