4.1.2Use of Biological Resources
Crop Plants: Increasing Carbon Dioxide and Temperature — Edexcel International GCSE Science Double Award (Modular) - Biology Unit 2 Revision Notes & Topical Questions
5 min read 6 parts Lesson 3 of 15
Part 1 of 4The idea of a limiting factor
What limits photosynthesis?
- What limits photosynthesis?
- If a plant had unlimited light, carbon dioxide and water and a warm temperature, its rate would be limited only by how fast it could absorb and react the materials.
- In reality one factor is usually in shortest supply, and that factor limits the rate. It is called the limiting factor.
Three main factors
- Three main factors The three factors examined are temperature, light intensity and carbon dioxide concentration.
- Increasing the limiting factor raises the rate; increasing a factor that is not limiting makes no difference.
Part 2 of 4Effect of each factor
| Factor | Trend in rate | Explanation |
|---|---|---|
| Temperature | Rises to an optimum, then falls sharply | Photosynthesis is controlled by enzymes. Warmth increases their activity, but above the optimum they denature. |
| Light intensity | Rises, then levels off | Light provides the energy for the reaction. Once another factor becomes scarce, more light has no further effect. |
| Carbon dioxide concentration | Rises, then levels off | Carbon dioxide is a raw material. More of it speeds the reaction until something else becomes limiting. |
Part 3 of 4Reading a limiting-factor graph
1
Look at the rising part of the curve: here the factor on the x-axis is the limiting factor, because increasing it increases the rate.
2
Look at the flat part: some other factor is now limiting, such as light intensity or temperature.
3
To raise the plateau, identify which other factor is in short supply and increase that one instead.
4
On a temperature graph, a fall after the peak shows enzymes denaturing, not a lack of any raw material.
Part 4 of 4Using this to raise yield
1Identify the limit
- Identify the limit A farmer works out which factor is limiting at a particular time, for example carbon dioxide on a bright, warm day.
2Supply more
- Supply more In a glasshouse, carbon dioxide can be added and heat can be provided, within the optimum range.
3Faster photosynthesis
- Faster photosynthesis More glucose is made, so the plants grow faster and larger.
4Higher yield
- Higher yield More crop is harvested, but the cost of the extra heat or gas must be less than the value of the extra crop.
Key terms
| Term | Meaning |
|---|---|
| Limiting factor | The factor in shortest supply, which restricts the rate of photosynthesis. |
| Optimum temperature | The temperature at which enzymes work fastest. |
| Denaturation | Loss of an enzyme's shape at high temperature so it can no longer work. |
| Light intensity | How bright the light is that reaches the plant. |
| Plateau | The flat part of a graph where the rate no longer increases. |
Avoid these mistakes
1
Saying that a rise in temperature always speeds photosynthesis — beyond the optimum the rate drops as enzymes denature.
2
Explaining the flat part of a graph by saying 'the reaction has stopped' — it is still happening, but at a maximum set by another limiting factor.
3
Writing that enzymes are 'killed' by heat — enzymes are not alive; they are denatured.
4
Naming only one limiting factor for all situations — which one limits depends on the conditions at the time.
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