Transpiration Stream (Cambridge (CIE) IGCSE Biology)

Water Vapour Loss: Extended

Extended Tier Only

• Evaporation takes place from the surfaces of spongy mesophyll cells

• The many interconnecting air spaces between these cells and the stomata create a large surface area

• This means evaporation can happen rapidly when stomata are open

Transpiration Stream: Extended

Extended Tier Only

• Water molecules are attracted to each other by cohesion - creating a continuous column of water up the plant

• Water moves through the xylem vessels in a continuous transpiration stream from roots to leaves via the stem

• Transpiration produces a tension or ‘pull’ on the water in the xylem vessels by the leaves

• As water molecules are held together by cohesive forces (each individual molecule ‘pulls’ on the one below it), so water is pulled up through the plant

• If the rate of transpiration from the leaves increases, water molecules are pulled up the xylem vessels quicker

The generation of the transpiration stream