9.3 - Transpiration and Factors Affecting Transpiration Flashcards
Define transpiration
The loss of water vapour by evaporation and diffusion from the surface of leaves and stems of a plant
What actually is transpiration ?
- 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
How is the rate of transpiration controlled ?
- waxy cuticle
- guard cells can open or close stomata
- very few stomata on upper surface of the leaf
Define the transpiration stream
The flow of water in continuous columns up the xylem vessels from roots to leaves
Describe the movement of water in transpiration
- 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
How does water travel up the stem against gravity ?
- 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
What are guard cells ?
Control the opening and closing of the stomata
What happens when the stomata open ?
- 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
What happens when the stomata close?
- 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
What conditions do stomata open in ?
- 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
What are the conditions that stomata are closed in ?
- high CO2 concentration inside the leaf
- low light intensity ( eg. Darkness )
What is the function of the stomata ?
- 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
What is the role of stomata In transpiration ?
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
How are guard cells adapted ?
- 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
What is meant by the rate of transpiration ?
The rate at which transpiration occurs refers to the amount of water lost by plants over a given time period
What are the factors that affect the rate of transpiration ?
- Increase in light intensity
- Increase in temperature
- Increase in air movement
- Decrease in humidity
Explain how temperature affects the rate of transpiration ?
- 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
How does light intensity affects the rate of transpiration ?
- 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
How does humidity affect the rate of transpiration ?
- 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
What is humidity ?
The amount of water vapour in the air
