Edexcel IGCSE Biology 复习笔记 5.1.1 Crop Plants: Glasshouses & Polythene Tunnels

• If a plant is given unlimited light, carbon dioxide and water and is at a warm temperature, the only thing limiting the rate at which it can photosynthesise is its own ability to absorb these materials and make them react
• However, most often plants do not have unlimited supplies of these things, so their rate of photosynthesis is limited by whatever factor is the lowest at that time (known as the limiting factor)
• There are three main factors that limit the rate of photosynthesis:
  • Temperature 
  • Light intensity
  • Carbon dioxide concentration

   

Temperature

• As temperature increases the rate of photosynthesis increases as the reaction is controlled by enzymes
• However, as the reaction is controlled by enzymes, this trend only continues up to a certain temperature beyond which the enzymes begin to denature and the rate of reaction decreases

Light intensity

• The more light a plant receives, the faster the rate of photosynthesis
• This trend will continue until some other factor required for photosynthesis prevents the rate from increasing further because it is now in short supply

Carbon dioxide concentration

• Carbon dioxide is one of the raw materials required for photosynthesis
• This means the more carbon dioxide that is present, the faster the reaction can occur
• This trend will continue until some other factor required for photosynthesis prevents the rate from increasing further because it is now in short supply

Limiting factors and the growth of crop plants

• The knowledge about limiting factors and how they affect the rate of photosynthesis can be used by farmers to improve crop yields
• Growing crops outside does not allow farmers to control any of these factors
• Glasshouses and polythene tunnels provide an enclosed environment in which farmers can, to some extent, control the climate inside and increase their crop yields

Glasshouses

• In a glasshouse, several conditions can be manipulated to increase the rate of photosynthesis, including:
  • Artificial heating (enzymes controlling photosynthesis can work faster at slightly higher temperatures - only used in temperate countries such as the UK)
  • Artificial lighting (plants can photosynthesise for longer)
  • Increasing carbon dioxide content of the air inside (plants can photosynthesise quicker)
  • Regular watering

   

• When considering the use of glasshouses and manipulating conditions like this, farmers need to balance the extra cost of providing heating, lighting and carbon dioxide against the increased income
• In tropical countries where temperatures are much hotter, glasshouses may still be used to control other conditions however they may need to be ventilated to release hot air and avoid temperatures rising too high, which could cause the denaturation of the enzymes controlling the photosynthesis reaction

Polythene tunnels

• More commonly called polytunnels, these are large plastic tunnels that cover crops
• They can protect crops grown outside from the effects of the weather, including excessive wind, rain and extreme temperatures
• They also increase the temperature slightly inside the tunnel
• They can prevent the entry of pests that can damage plants or diseases that can kill plants

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