Edexcel IGCSE Biology · Spec 5.1-5.4
Crop Production
How glasshouses, fertilisers and pest control are used to increase crop yields.
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Crop Production
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Explained
Controlling the limiting factors
A crop grows as fast as its most limiting factor allows. Commercial growing is therefore an exercise in finding which factor is holding the crop back and removing it, which is why glasshouses exist.
What a glasshouse controls
Light. Lamps extend the day length and allow photosynthesis outside daylight hours, so more glucose is made.
Temperature. Heaters keep the enzymes controlling photosynthesis and growth near their optimum. Too cold and the reactions run slowly; too hot and the enzymes denature, so a glasshouse also needs ventilation.
Carbon dioxide. Burning a fuel such as paraffin raises the concentration, which supplies more of the raw material for photosynthesis, and conveniently produces heat at the same time.
Water and mineral ions. Irrigation and fertiliser make sure neither runs short.
The glasshouse also keeps pests out and protects the crop from wind and frost, which is worth mentioning when a question asks for advantages.
Fertilisers
Plants need nitrates for amino acids and therefore proteins, magnesium for chlorophyll, and phosphates for DNA and cell membranes. Harvesting a crop removes those minerals from the soil, so fertilisers replace them.
The cost is downstream. Excess fertiliser washed into rivers causes eutrophication, and the chain from nitrates to algal bloom to oxygen depletion to dead fish is the one examiners want written out in full.
Pest control
Chemical pesticides act quickly and kill a high proportion of the pest. Against that, they can leave residues on the crop, kill useful species including pollinators and the pests' natural predators, accumulate along a food chain, need reapplying, and select for resistant pests.
Biological control uses a predator, parasite or pathogen. It is specific, leaves no residues and continues working once established, but it acts slowly, never removes the pest entirely, and carries the risk that the introduced organism attacks something else or spreads.
An evaluation question wants both sides and a judgement that refers to the situation described.
What the mark scheme accepts and rejects
An Edexcel International GCSE Biology mark scheme asks what happens to a crop deprived of a mineral, for three marks, and lists the chain: less respiration or less energy, less protein, less growth or fewer new cells, and less fruit production such as fewer or smaller oranges.
Then come two instructions that catch people out. There is no credit for less sugar, less amino acid or less glucose. And it says to ignore less crops and less yield.
Ignoring less yield on a question about crops seems perverse until you see the logic. Yield is what the question is asking about, so saying the yield falls restates the question rather than explaining it. The marks are for the biological steps in between, and fewer or smaller fruits is the specific version of the same idea that does earn credit.
This is the same principle these mark schemes apply everywhere: name the mechanism, not the outcome you were asked to explain.
Answering a glasshouse question
Name the factor, say what it does to photosynthesis, and say what that does to the yield.
Adding carbon dioxide increases the rate of photosynthesis, so more glucose is made, so the plants grow larger and produce more fruit.
And remember that raising one factor only helps if it was the limiting one. Adding more light to a crop already short of carbon dioxide changes nothing, and saying so is often the final mark on a graph question where the line has flattened out.
Spec 5.1-5.4
What you need to know
- Explain how glasshouses increase yield
- Describe the effects of carbon dioxide, temperature and fertiliser
- Weigh up the pros and cons of pest control
Active recall
Quick check
Answer each question before opening the answer.
Why are crops grown in glasshouses or polytunnels?
To control temperature, light and carbon dioxide levels, increasing the rate of photosynthesis and the yield.
Why are fertilisers added to crops?
To supply mineral ions such as nitrate and phosphate needed for healthy growth.
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