Transpiration

What is transpiration?

The loss of water vapour through the stomata by evaporation.

What is transpiration stream?

The pull of water and mineral ions up the stem of plants due to cohesion.

Explain how transpiration causes transpiration pull / stream?

• As water molecules leave xylem cells in the leaf, they pull up other water molecules.

• This pulling effect is known as the transpiration pull.

Why is transpiration pull/stream possible?

Due to:

• Large cohesive forces between water molecules.

• Adhesive forces between water molecules and the hydrophilic lining of vessels.

How is water cohesive?

Water molecules form hydrogen bonds with each other, causing them to 'stick' together.

“Not all the water absorbed through the roots is lost through the leaves.” Explain how water may be used by a plant.

• Water is used during photosynthesis

• Some water is retained in cells for support – it maintains turgor pressure.

• Some water is produced through respiration.

This means that the volume of water taken up in the roots is not exactly equal to the volume of water lost from the leaves through transpiration.

What happens to a plant if water loss exceeds water uptake?

• For a plant to absorb water into its roots, water must continue to be lost from its leaves.

• If water loss exceeds water uptake, plants will wilt as the leaves and stems lose turgor pressure.

• The conditions that favour wilting are hot, sunny and windy days.

State the 4 factors that affect the rate of transpiration.

• Temperature

• Humidity

• Air movement

• Light intensity

How does temperature affect the rate of transpiration?

• As temperature increases the kinetic energy of water vapour increases.

• As a result, molecules will diffuse and evaporate faster and osmosis will also take place at a higher rate.

How does humidity affect the rate of transpiration?

• Air which is more humid contains more water vapour.

• This decreases the water potential gradient and fewer water molecules can diffuse out of the leaves.

How does air movement affect the rate of transpiration?

• As air movement increases, water vapour on the surface of the leaf is blown away.

• The water potential gradient between the inside and outside of the leaf increases so there is a higher rate of diffusion of water vapour.

How does light intensity affect the rate of transpiration?

• If the light intensity is higher, the rate of photosynthesis in the guard cells of stomata increases and the stomata are more likely to open.

• The more stomata that open the higher the rate of diffusion of water vapour from the leaf.

How can the rate of transpiration be measured?

Using a potometer.

A potometer actually measures rate of water absorption, but if the cells are fully turgid water absorption and rate of transpiration should be the same.

How do you set up a potometer correctly?

• Cut the shoot under water (to prevent air bubbles forming in the xylem).

• Keep the leaves dry.

• Set up apparatus under water.

• Ensure all joints are airtight.