9.3 - Transpiration and Factors Affecting Transpiration

Question 1

Define transpiration

Answer 1

The loss of water vapour by evaporation and diffusion from the surface of leaves and stems of a plant

Question 2

What actually is transpiration ?

Answer 2

• transpiration is a consequence of gas exchange as the stomata need to already be open to allow CO2 in and O2 out
• water moves out of the leaf from an area of high WP to low WP

Question 3

How is the rate of transpiration controlled ?

Answer 3

• waxy cuticle
• guard cells can open or close stomata
• very few stomata on upper surface of the leaf

Question 4

Define the transpiration stream

Answer 4

The flow of water in continuous columns up the xylem vessels from roots to leaves

Question 5

Describe the movement of water in transpiration

Answer 5

• water enters the leaves and passes into the mesophyll cells by osmosis
• water evaporates from the leaves to firm water vapour
• large air spaces between the mesophyll cells allow water vapour to collect and diffuse through the leaves
• as a result of this water collecting the water potential rises

Question 6

How does water travel up the stem against gravity ?

Answer 6

• movement of water from roots to leaves is called transpiration stream
• mechanisms by which water can move up the leaves by are :
• cohesion
• adhesion
• tension

Question 7

What are guard cells ?

Answer 7

Control the opening and closing of the stomata

Question 8

What happens when the stomata open ?

Answer 8

• guard cells are turgid
• water moves into the vacuoles by osmosis
• outer wall is more flexible than the inner walls , so to cell bends and opens the stoma

Question 9

What happens when the stomata close?

Answer 9

• guard cells are flaccid
• water moves out of the vacuoles by osmosis
• outer wall is more flexible than the inner wall , so to the cell bends back and closes the stoma

Question 10

What conditions do stomata open in ?

Answer 10

• low Co2 concentration inside the leaf cause stomata to open
• High CO2 causes stomata to clos
• high light intensity
• high water potential outside the stomata
• stomata open during the day and close during the night

Question 11

What are the conditions that stomata are closed in ?

Answer 11

• high CO2 concentration inside the leaf
• low light intensity ( eg. Darkness )

Question 12

What is the function of the stomata ?

Answer 12

• when stomata are open they allow gas exchange between the Leaf and the outside environment
• CO2 can enter the lead through the stomata and O2 and water Vapou can diffuse out of the stomata

Question 13

What is the role of stomata In transpiration ?

Answer 13

1 - water vapour diffuses from air spaces through a stoma by a process called transpiration = lowering the water potential
2 - water evaporates from a mesophyll cell wall into the air spaces creating a transpiration pull
3- water moves through the mesophyll cell wall ( apoplastic pathway) or out of the mesophyll cytoplasm into the cell wall ( symplastic pathway )
4- water leaves a xylem vessel through a non- lignified area it may travel by a symplastic pathway or by an apoplastic pathway
5- water moves up the xylem vessels ( transpiration stream ) to replace the water lost from the leaf

Question 14

How are guard cells adapted ?

Answer 14

• unevenly thickened cell wall - wall beside the pore is thicker
• able to change shape/ bend
• transport proteins/ ion channels in the plasma membrane - absorption of K+ ions by the guard cells
• K+ ions decrease the water potential hence water enters by osmosis and guard cells can become turgid
• presence of chloroplasts and mitochondria to provide ATP energy

Question 15

What is meant by the rate of transpiration ?

Answer 15

The rate at which transpiration occurs refers to the amount of water lost by plants over a given time period

Question 16

What are the factors that affect the rate of transpiration ?

Answer 16

1. Increase in light intensity
2. Increase in temperature
3. Increase in air movement
4. Decrease in humidity

Question 17

Explain how temperature affects the rate of transpiration ?

Answer 17

• an increase in temp = an increase in the kinetic energy of molecules
• thus an increase in temperature will increase the rate of transpiration as water molecules move out of the leaf down a concentration gradient at a faster rate
• if the temperature gets to high the stomata close to prevent excess water loss
  = this reduces the rate of transpiration

Question 18

How does light intensity affects the rate of transpiration ?

Answer 18

• stomata close in the dark = closure reduces the rate of transpiration
• when the light is sufficient for the stomata to open = rate of transpiration increases
• once stomata are open any increase in light intensity has no effect on the rate of transpiration
• stomata will remain open at relatively low light intensities
• at high light intensities the rate of photosynthesis increases
• as photosynthesis increases and he amount of stored glucose in the guard cells increases
• this lowers the water potential of the lead
• as the water potential decreases more water enters the guard cells making them more turgid
• the turgor pressure of the guard cells leads to an opening up of stomata resulting in transpiration

Question 19

How does humidity affect the rate of transpiration ?

Answer 19

• if the humidity is high = there is a large concentration of water molecules in the air surrounding the leaf surface
• this reduced the concentration gradient between inside the leaf and the outside air which cause the rate of transpiration to decrease
• at a certain level of humidity an equilibrium is reached = there is no concentration gradient and so there is no net loss of water vapour from the leaves

Question 20

What is humidity ?

Answer 20

The amount of water vapour in the air