4.1.1Use of Biological Resources
Crop Plants: Glasshouses and Polythene Tunnels — Edexcel International GCSE Science Double Award (Modular) - Biology Unit 2 Revision Notes & Topical Questions
5 min read 6 parts Lesson 2 of 15
Part 1 of 4Why enclose crops?
Limiting factors
- Limiting factors The rate of photosynthesis is held back by whichever factor is in shortest supply: light intensity, carbon dioxide concentration or temperature.
- Crops grown in open fields are at the mercy of the weather, so farmers cannot control these factors.
The aim
- The aim An enclosed structure lets the farmer adjust the environment so that photosynthesis, and therefore growth and yield, is as high as possible.
Part 2 of 4Controlling conditions in a glasshouse
| Condition altered | Method | Effect on the crop |
|---|---|---|
| Temperature | Artificial heating, mainly in cooler countries such as the UK | Enzymes in photosynthesis work faster at warmer temperatures, so growth is quicker |
| Light | Artificial lighting at night or in winter | Plants can photosynthesise for more hours each day |
| Carbon dioxide | Burning fuel or releasing the gas into the air inside | A raw material is no longer scarce, so photosynthesis speeds up |
| Water | Regular watering or irrigation systems | Water is a reactant, and it keeps the plants from wilting |
Part 3 of 4Polythene tunnels (polytunnels)
Polytunnel
Weather protection
- Weather protection The plastic cover shelters crops from strong wind, heavy rain, hail and extreme temperatures.
Slight warming
- Slight warming The air inside is a little warmer than outside, which speeds growth and can lengthen the growing season.
Pests and disease
- Pests and disease The barrier makes it harder for insects and other pests to reach plants, and reduces the spread of disease.
Lower cost
- Lower cost Polytunnels are cheaper to build and run than glasshouses but give less control.
Part 4 of 4Balancing cost and benefit
Costs
- Costs Heating, lighting and carbon dioxide supplies all cost money, and the structure must be built and maintained.
Benefit
- Benefit Higher yields and crops available out of season bring more income.
- The farmer must check that the extra income is greater than the extra cost.
Hot climates
- Hot climates In tropical countries glasshouses may still protect crops, but they need ventilation to let hot air escape.
- If the temperature rises too high, the enzymes controlling photosynthesis denature and the rate falls.
Key terms
| Term | Meaning |
|---|---|
| Glasshouse | A glass building in which temperature, light, carbon dioxide and water can be controlled to grow crops. |
| Polytunnel | A large plastic-covered tunnel that shelters crops and warms them slightly. |
| Limiting factor | The factor in shortest supply, which sets the maximum rate of a process. |
| Yield | The mass of crop produced from a given area of land. |
| Denaturation | The permanent change in shape of an enzyme, which stops it working. |
Avoid these mistakes
1
Saying a glasshouse keeps heat in 'because glass attracts heat' — it reduces heat loss and lets farmers add heating.
2
Forgetting the economic balancemore control also means more cost, so higher yield only pays if income rises by more.
3
Suggesting heating is always helpful — too high a temperature denatures enzymes and lowers the rate.
4
Mixing up the two structurespolytunnels give protection and slight warming, whereas glasshouses allow much greater control.
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