IGCSE Biology / Human influences on the environment

Glasshouses and polythene tunnels, and why they raise yield

Edexcel International GCSE Biology (4BI1), chapter 15: Human influences on the environment, specification point 5.1, 5.2. Free lesson.

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Glasshouses and polythene tunnels improve crop yield by controlling conditions that affect photosynthesis and protecting plants. A grower must balance the extra yield against the cost of equipment and fuel.

How a glasshouse raises yield

Photosynthesis has limiting factors: if one factor is in short supply, it limits the rate. A glasshouse lets a grower improve these conditions, but each must be kept at a useful level. Temperature: Heat can be trapped or added. Photosynthesis enzymes work faster as temperature rises, up to the optimum. Above it, the enzymes denature and the rate falls, so vents can be opened on hot days. Carbon dioxide: Its concentration can be raised above the level in ordinary air. On a bright day, carbon dioxide can be the limiting factor, so adding more increases photosynthesis. Water:…

Why growers burn fuel inside

Burning a fuel such as paraffin inside a glasshouse does two jobs at once. It releases carbon dioxide and provides heat, helping the grower address two limiting factors. The temperature still needs to stay at or below its optimum.

How glasshouses and tunnels differ

Both forms of cover can improve growing conditions, but their cost, lifespan and degree of control differ. The choice depends on the crop and the scale of growing. Glasshouses are expensive to build, but can last for decades. They allow full control of heat, carbon dioxide, light and water, and are used for high-value crops such as tomatoes and peppers. Polythene tunnels are cheap, so they can cover far more land. They mainly provide shelter and warmth, with less precise control. The plastic needs replacing after a few years. They are used for field-scale crops such as strawberries…

When adding a factor raises yield

Only the factor in shortest supply limits the rate of photosynthesis. Adding more of a factor that is already sufficient will not raise yield, and extra heat can be harmful if it pushes temperature above the optimum.

The costs of growing under cover

The equipment and fuel cost money, so the extra crop yield must be worth more than the cost of producing it. Heating a glasshouse also burns fossil fuel, releasing carbon dioxide into the atmosphere. Remember that cover can control growing conditions, exclude pests and shelter crops from weather. Polythene tunnels are cheaper and cover more land, but offer less precise control.

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