2.56B Transpiration (Edexcel IGCSE Biology)

• Transpiration is defined as the loss of water vapour from the parts of the plant that are above ground (leaves, stem, flowers)
• Loss of water occurs through evaporation of water at the surfaces of the spongy mesophyll cells followed by diffusion of water vapour through the stomata
• The many interconnecting air spaces between the mesophyll cells and the stomata creates a large surface area
• This means evaporation can happen rapidly when the stomata are open

Transpiration in plants

The effect of transpiration

• Water moves through the xylem vessels in a continuous transpiration stream from the roots to the leaves via the stem to replace the water that has been lost due to transpiration
• Due to cohesion, the water in the xylem creates a continuous unbroken column (each individual molecule ‘pulls’ on the one below it)
• Transpiration produces tension or ‘pulls’ on the water in the xylem vessels
• If the rate of transpiration from the leaves increases, water molecules are pulled up the xylem vessels quicker

Water uptake, transport and transpiration

The function of transpiration

• Transpiration has several functions in plants:
  • Transporting mineral ions
  • Providing water to keep cells turgid in order to support the structure of the plant
  • Providing water to leaf cells for photosynthesis
  • Keeping the leaves cool, the conversion of water (liquid) into water vapour (gas) as it leaves the cells and enters the airspace requires heat energy. The use of heat to convert water into water vapour helps to cool the plant down