Transpiration Stream (Cambridge International IGCSE Biology)

Water vapour loss: Extended

Spec point — 8.3.4 · Explain how water vapour loss is related to: the large internal surface area provided by the interconnecting air spaces between mesophyll cells and the size and number of stomata

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

Spec point — 8.3.5 · Explain the mechanism by which water moves upwards in the xylem in terms of a transpiration pull that draws up a column of water molecules, held together by forces of attraction between water molecules

Extended Tier Only

• Water molecules are attracted to each other (cohesion), forming 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