6.9 Transpiration and the Stomata

Question 1

What is transpiration?

Answer 1

The loss of water vapour from the parts
of a plant exposed to the air due to
evaporation and diffusion

Question 2

Where does the majority of transpiration take place?

Answer 2

Leaves

Question 3

Describe the process of transpiration

Answer 3

● Water evaporates from the mesophyll cell surfaces and diffuses
out of the stomata
● Water molecules (which have cohesive properties) are drawn
up the xylem vessels to replace the water that has been lost
● This causes more water molecules to be absorbed from the soil
into root hair cells

Question 4

How does the transpiration stream transport mineral

ions?

Answer 4

Mineral ions are dissolved in the water

that is carried by the transpiration stream

Question 5

What are stomata?

Answer 5

They are pores found in the lower
epidermis of a leaf which allow gas
exchange

Question 6

What are guard cells?

Answer 6

Specialised cells surrounding the stoma
that change shape to control the size of
the pore

Question 7

How do guard cells control the size of stomata?

Answer 7

To open the stomata:
Water enters guard cells. They swell and become turgid. They bend
and draw away from each other, opening the stomata.
To close the stomata:
Water leaves guard cells. They become flaccid, closing the
stomata.

Question 8

What factors affect the rate of transpiration? (3)

Answer 8

● Light intensity
● Temperature
● Air movement

Question 9

Describe how high light intensity affects the rate of transpiration

Answer 9

● High light intensity, greater number of stomata open to allow gas
exchange for photosynthesis
● Rate of photosynthesis increases so more water is taken up from the
soil, pushing water up the xylem
● More water vapour diffuses out of the stomata
∴ rate of transpiration increases

Question 10

Describe how low light intensity affects

the rate of transpiration

Answer 10

At a low light intensity, fewer stomata are
open so the rate of transpiration
decreases.

Question 11

Describe how temperature affects the rate of

transpiration

Answer 11

● Temperature increases, water molecules have more KE so rate of
diffusion increases
● Photosynthesis also increases so more water is taken up from the
soil, pushing water up the xylem
● More water vapour diffuses out of the stomata
∴ rate of transpiration increases

Question 12

Describe how air movement affects the

rate of transpiration

Answer 12

● Air movement increases, high water concentration gradient
maintained between the air spaces in the leaf and atmosphere
● Increased rate of diffusion of water molecules out of the
stomata
● Rate of transpiration increases

Question 13

What apparatus is used to measure the

rate of transpiration?

Answer 13

Potometer

Question 14

What is assumed when measuring the

rate of transpiration using a potometer?

Answer 14

rate of water uptake ≈ rate of transpiration