2.5.5 Transpiration

2.5.5 Transpiration: Define transpiration, explain how it creates the transpiration stream and describe how wind speed, humidity, temperature and surface area affect its rate.

Water evaporates from the surface of cells lining the air spaces inside the leaf, forming water vapour in those spaces.

The water vapour then diffuses out of the leaf through the open stomata into the drier air outside.

Water lost in this way is replaced by more water drawn up the xylem from the roots.

The result is a continuous upward flow of water called the transpiration stream.

This stream brings dissolved mineral ions from the soil and supplies water to leaf cells for photosynthesis.

Wind speed

Moving air removes water vapour from near the leaf, so the concentration gradient is steeper and transpiration speeds up.

Humidity

Humid air already holds much water vapour, so the gradient is shallower and transpiration rate falls.

Temperature

Warmer conditions give water molecules more kinetic energy, so more evaporate and diffuse out and the rate increases.

Surface area

A larger leaf surface has more stomata through which water vapour can escape, so the rate increases.

Factor	Change	Effect on rate
Wind speed	Increases	Increases
Humidity	Increases	Decreases
Temperature	Increases	Increases
Leaf surface area	Increases	Increases

Why It Matters

Water supply

Transpiration helps move water to the leaves, where it is used as a raw material in photosynthesis.

Mineral ions

Minerals dissolved in the soil water are carried upwards in the stream and distributed to cells that need them.

Cost to the plant

Open stomata lose water, so in dry conditions the plant risks wilting if loss exceeds the uptake of water.

Control

Guard cells can close the stomata in dry conditions and so reduce the rate of transpiration.

Linking Ideas

Concentration gradient

Water vapour diffuses from the high concentration inside the leaf to the lower concentration outside.

Diffusion rate

Any factor that steepens the gradient, such as wind, increases the rate of diffusion and so transpiration.

Leaf adaptations

A waxy cuticle limits evaporation from the leaf surface so that most water loss is through the stomata.

Xylem role

Xylem forms an unbroken pathway, which allows replacement water to reach the leaf as soon as water is lost.

2.5.5 – Key Points for the Exam

Transpiration: The loss of water vapour by evaporation from the leaves of a plant.
Transpiration stream: The upward movement of water in the xylem that replaces water lost from the leaves.
Concentration gradient: The difference in concentration between two regions.
Humidity: The amount of water vapour in the air.
Evaporation: The change of liquid water to water vapour.

Common Mistakes

Saying that transpiration is loss of liquid water; it is loss of water vapour.
Stating that higher humidity increases the rate of transpiration; it reduces the gradient and so lowers the rate.
Forgetting to mention the concentration gradient when explaining the effect of wind.
Confusing transpiration, the loss of water vapour, with the uptake of water by the roots.