5.1 describe how glasshouses and polythene tunnels can be used to increase the yield of certain crops

Greenhouses, sometimes called glasshouses, provide the right conditions for plants to grow for several reasons:

The transparent material allows sufficient natural light for photosynthesis during the summer months, while additional lighting gives a 'longer day' during the winter.

The greenhouse effect also happens in greenhouses. Short wave radiations entering the greenhouse  becomes longer wave radiation as it reflects off surfaces. This longer wave radiation cannot leave as  easily so the greenhouse heats up.

Burning fuels to raise the temperature when the external heat is too low also produces carbon dioxide and water vapour. the water vapor maintains a moist atmosphere and therefore reduces the loss of water by transpiration. The carbon dioxide is a raw material of photosynthesis.

Glasshouses also protect plants from things such as strong winds and heavy rain.

 Also, growing plants in a hydroponic culture provides exactly the right balance of mineral ions for the specific crop being grown.