Transpiration Flashcards
What is cohesion?
• Water molecules have dipoles which cause an attraction between them
What is adhesion?
There are forces of attraction between the polar water molecules and polar groups in cell walls so water molecules
can also ‘stick’ to the wall of the xylem vessels
What is cohesion tension theory?
• Water is drawn up the xylem vessels by the tension generated from transpiration, this is aided by the cohesion and adhesion of the water molecules.
• For this to work, the xylem vessel must be a continuous column of water i.e. contain no bubbles.
Why are stomata closed?
To prevent water loss
What happens to guard cells when turgid and flaccid?
They curve apart (turgid)
Edges of cells lie close together (flaccid)
What happens to transpiration when stomata are closed?
drastically slows transpiration and
means that no carbon dioxide can enter the leaf (=no photosynthesis).
Explain the process of transpiration (6)
• Water evaporates from the leaves/transpiration
• Due to heat/kinetic energy from sunlight
• Water diffuses out of the stomata from a high water potential to low
• The diffusion of water causes a tension in the xylem
• This is due to water potential gradient
• Cohesion tension forms a continuous column of water that is pulled through the xylem in a transpiration stream
• Water’s adhesive properties aid the movement through the xylem
• Transpiration stream lowers water potential in the root cells
• Water is absorbed through the root hair cells by osmosis from a higher water
potential to low.
Evidence to support transpiration
• Change in diameter of tree trunks (smaller during the day)
• If an air bubble enters a xylem vessel
• Transpiration stops
• If a xylem vessel breaks air enters instead of water leaking out
How does temperature affect rate of transpiration?
Increases the rate of evaporation
Due to molecules having more
kinetic energy
Increases the rate of diffusion of
water vapour from the leaf
How does humidity affect rate of transpiration?
High humidity
more water vapour in the air outside the leaf
This reduces the diffusion gradient
Therefore reduces rate of transpiration.
Low humidity
less water vapour
– steeper diffusion gradient
- increases transpiration rate
How does stomatal opening affect rate of transpiration?
Open
more water vapour can diffuse
out of the leaf
– transpiration rate increases
Closed
less water vapour can diffuse
out
– transpiration rate decreases
Guard cells are light sensitive –
this means that when it gets
darker, they close the stomata.
How does wind speed affect rate of transpiration?
Wind blows/carries away any water vapour which has diffused out of the leaf
– increases the diffusion gradient between the inside and outside of the leaf
– so as wind speed increases transpiration rate also increases
