9.3 Transpiration Flashcards
What are stomata?
Microscopic pores in the leaf, that allow carbon dioxide, oxygen, and water vapour to enter and exit the leaf
What controls whether the stomata are open or closed?
The guard cells
What happens when the stomata are open?
-An exchange of carbon dioxide and oxygen happens
-Water vapour also moves out by diffusion and is lost
What is transpiration?
An inevitable consequence of gaseous exchange. The loss of water vapour from the leaves and stems of plants
Why are stomata open during the day?
To take in Carbon Dioxide for photosynthesis, and release oxygen as a product
Why are stomata open during the night?
To take in oxygen for cellular respiration
State the movement of water in the transpiration stream.
Xylem, through the leaf, mesophyll cell walls, air spaces, external air through the stomata
How does water move from the mesophyll cell walls to the air spaces?
By evaporation
How does water move from the air spaces to the external air through the stomata?
By diffusion
How does water travel from the xylem to the mesophyll cell walls?
Through symplast and apoplast pathways
How does the water potential of the mesophyll cells remain low so water can travel into them by osmosis?
The continuous loss of water by evaporation off the mesophyll cell walls maintains the water potential gradient
What is transpiration pull?
The method that means that water is drawn up by the xylem in a continuous stream to replace the water lost by evaporation. Capillary action
What is capillary action in transpiration caused by?
The forces of adhesion and cohesion of water molecules in the xylem, to allow the water to rise up the narrow tube against the force of gravity
Describe cohesion in transpiration.
The water molecules forming hydrogen bonds with eachother, making them stick together
Describe adhesion in transpiration.
The water molecules forming hydrogen bonds with the carbohydrates in the walls of narrow xylem vessels
What does transpiration pull result in?
Tension in the xylem, which in turn helps to move water across the roots from the soil
What is the name of the model of water moving from the soil in a continuous stream?
Cohesion-tension theory
What is the purpose of transpiration?
-Delivers water and dissolved mineral ions to the cells where they are needed
-Evaporation of water from leaf cell surfaces helps to cool the leaf down, and prevent heat damage
What is the main way in which the rate of transpiration is controlled by the plant?
The opening and closing of the stomatal pores
What drives the opening and closing of the stomatal pores?
Changes in turgor pressure in the guard cells
How does the stomatal pore close?
-When turgor in guard cells is low, the asymmetric configuration of the guard cell walls closes the pore
-When water becomes scarce, hormonal signals from the roots trigger turgor loss from the guard cells
How does the stomatal pore open?
Guard cells pump in solutes by active transport, increasing their turgor
What features of the stomata mean that when guard cell turgor increases, the stomata open, instead of simply staying closed and expanding?
-Cellulose hoops prevent the cells from swelling in width, so they extend lengthways
-Because the inner cell wall of the guard cell is less flexible than the outer wall, the cells become bean-shaped and open the pore
What factors affect the rate of transpiration?
-Light intensity
-Humidity
-Temperature
-Air movement
-Soil-water availability
Describe how light affects the rate of transpiration.
As light intensity increases, the rate of transpiration increases
EXPLAIN how light affects the rate of transpiration.
-In darkness, most of the stomata will close, because photosynthesis is not occuring
-Therefore, in more light, more stomata will be open, so more space for water vapour to evaporate out
Describe how humidity affects the rate of transpiration.
As humidity increases, the rate of transpiration decreases
EXPLAIN how humidity affects the rate of transpiration.
-Humidity is a measure of the amount of water vapour in the air
-Very high humidity lowers the rate of transpiration because of the reduced water vapour potential gradient
Describe how temperature affects the rate of transpiration.
As temperature increases, the rate of transpiration increases
EXPLAIN how temperature affects the rate of transpiration.
-Increase in temp increases the kinetic energy of water molecules, so rate of evaporation increases
-Increase in temp increases the concentration of water vapour that the external air can hold
Describe how soil-water availability affects the rate of transpiration
Low water availability decreases rate of transpiration
Describe how air movement affects the rate of transpiration.
Increased air movement increases the rate of transpiration
