Glasshouses, polythene tunnels and fertilisers
Why grow crops under cover?
Glasshouses (glass, permanent) and polythene tunnels (plastic sheeting over hoops, cheap and moveable) both create a controlled environment around a crop. Both increase yield because the farmer controls the factors that normally limit growth.
What they do:
- Trap heat — light energy enters, warms the air and soil inside; the structure stops warm air escaping, so the temperature is higher than outside.
- Extend the growing season — crops can be grown earlier, later, or out of season.
- Protect from frost, wind and heavy rain, which would damage or kill seedlings.
- Keep out pests (insects, birds) so less crop is eaten; any pesticide used stays inside.
- Control watering — irrigation systems give the exact amount of water needed.
- Allow carbon dioxide enrichment — CO₂ can be added to the enclosed air.
5.2 Increased carbon dioxide
Carbon dioxide is normally a limiting factor for photosynthesis (only about 0.04% of air).
Burning a paraffin heater or piping in CO₂ raises the concentration → rate of photosynthesis increases → more glucose made → more glucose converted to new plant tissue (starch, cellulose, protein) → greater biomass and higher yield.
Eventually CO₂ stops being limiting and something else (light, temperature) limits the rate, so the yield stops rising.
5.2 Increased temperature
Photosynthesis is controlled by enzymes.
| Temperature | Effect |
|---|---|
| Low | Molecules have little kinetic energy; few enzyme–substrate collisions; slow photosynthesis, slow growth |
| Rising to optimum | Rate of photosynthesis increases; faster growth; higher yield |
| Above optimum | Enzymes are denatured — the active site changes shape, substrate no longer fits — rate falls sharply; plants may also wilt as transpiration increases |
So glasshouses are heated in cold weather, but ventilated or shaded when too hot. The extra fuel and equipment cost money, so the increase in yield has to be worth more than the running cost.
5.3 Fertilisers
Harvesting removes plants — and the mineral ions in them — from the field, so the soil becomes depleted. Fertiliser (natural manure or artificial NPK) puts those mineral ions back.
Plants absorb mineral ions dissolved in water, through the root hair cells, and use them to make the molecules they need:
| Mineral ion | Used by the plant for |
|---|---|
| Nitrate | Making amino acids, which are joined to make proteins for growth |
| Phosphate | DNA and cell membranes; root growth |
| Potassium | Helping enzymes of photosynthesis and respiration work |
| Magnesium | Making chlorophyll |
More mineral ions → more protein and chlorophyll → more growth, greener leaves, more photosynthesis → higher crop yield. Without nitrate, plants are stunted with yellow older leaves; without magnesium, leaves are yellow (chlorosis) so photosynthesis is slow.
Common mistake: plants do not absorb protein, and they do not "need nitrate for protein" the way you might describe a human diet. Say: nitrate ions are absorbed and used to make amino acids, which are built into proteins. Confusing plant mineral requirements with human dietary requirements loses marks.
Examiner tip. Watch the command word. Describe how a glasshouse increases yield = say what it does (traps heat, adds CO₂). Explain = give the reason (higher CO₂ concentration means CO₂ is no longer the limiting factor, so the rate of photosynthesis increases, so more glucose is made for growth). Also, more photosynthesis means more oxygen released, never less — examiners report candidates writing the opposite.