9.3 Transpiration Flashcards
Describe the process of transpiration
At the leaves, water leaves the xylem and moves into the cells, mainly by the apoplast pathway.
Water evaporates from the cell walls into the spaces between cells in the leaf.
When the stomata open to allow O2 to leave and CO2 to enter, the water (vapour)diffuses out too, down a water potential gradient, into the surrounding air.
Why are at least some stomata open all the time?
Stomata open and close to control the amount of water lost by a plant.
During the day : need to be open to take in carbon dioxide for photosynthesis
During the night : no oxygen produced by photosynthesis, so needs to takes in oxygen for cellular respiration
Describe how the transpiration stream works
Water molecules evaporate from the surface of mesophyll cells into the air spaces in the leaf and move out of the stomata into the surrounding air by diffusion down a concentration gradient.
The loss of water lowers the water potential of the cell, so water moves into the cell from an adjacent cell by osmosis, along both apoplast and symplast pathways.
This is repeated across the leaf to the xylem. Water moves out of the xylem by osmosis into the cells of the leaf.
What is the cohesion - tension theory ( transpiration stream description continued )
Water molecules form hydrogen bonds with the carbohydrates in the walls of the narrow xylem vessels ( adhesion ).
Water molecules also form hydrogen bonds with each other and tend to stick together ( cohesion ).
Adhesion and cohesion leads to capillary action. This is where water can rise up a narrow tube against the force of gravity. Water is drawn up the xylem in a continuous stream to replace the water lost by evaporation ( transpiration pull).
This transpiration pull results in a tension in the xylem , which in turn helps to move water across the roots from the soil.
What is the evidence for the cohesion - tension theory ?
Changes in the diameter of trees. When transpiration is at its height during the day, the tension in the xylem vessels is as its highest too. As a result, the tree shrinks in diameter. At night, when transpiration is as its lowest, the tension in the xylem vessels is at its lowest and the diameter of the tree increases.
When a xylem vessel is broken, in most circumstances air is drawn in to the xylem rather than the water leaking out.
If broken and air is pulled in, the plant can no longer move water up the stem as the continuous stream of water molecules held together by cohesive forces has been broken.
How can you measure transpiration ?
Using a potometer :
Seal all joints with waterproof jelly to make sure no water escapes.
Rate of water uptake = distance moved by air bubble / time taken for air bubble to move that distance.
Potomters work as most of the water that is taken up is lost by transpiration.
List the factors that affect transpiration
Light Humidity Temperature Air movement Soil - water availability
How does light affect transpiration ?
Light is required for photosynthesis and in the light the stomata open for the gas exchange needed.
In the dark, most of the stomata close.
Therefore, increased light intensity increases the amount of stomata open which increases the rate of water vapour diffusing out and therefore increasing the evaporation from the surfaces of the leaf.
How does humidity affect transpiration?
Higher relative humidity lowers the rate of transpiration because of the reduced water vapour potential gradient between the inside of the leaf and the outside air.
How does temperature affect transpiration?
Increased temperature increases kinetic energy of the water molecules and therefore increases the rate of evaporation from the spongy mesophyll cells into the air spaces of the leaf.
Increased temperature increases the concentration of water vapour that the external air can hold before it becomes saturated ( so decreases its relative humidity and its water potential )
Both these thing increase the diffusion gradient between the air inside and outside the leaf, increasing the rate of transpiration.
How does air movement affect transpiration ?
Each leaf has a layer of still air around it trapped by the shape of the leaf and features such as hair on the surface of the leaf decrease air movement close to the leaf.
The water vapour that diffuses out accumulates here and so the water vapour potential around the stomata increases, in turn reducing the diffusion gradient. This increases rate of transpiration.
How does soil - water availability affect transpiration ?
If soil is dry , the plant will be under water stress and rate of transpiration will be reduced.
How do stomata open and close?
When turgid is low the asymmetric configuration of the guard cell walls closes the pore.
When the environmental conditions are favourable guard cells pump in solutes by active transport, increasing their turgor.
Cellulose hoops prevent the cells from swelling in width, so they extend lengthways.
As inner wall of cell is less flexible than outer wall, cells become bean - shaped and open the pore.
When water is scarce, hormonal signals from root can trigger turgor from the guard cells, which close the stomata pore and conserve water.
Summarise the difference between transpiration and transpiration stream
Transpiration: evaporation of water from surface of a leaf
Transpiration stream: flow of water moved up from soil into root hair and through root cortex by osmosis, into xylem and up through stem by cohesion of water molecules, cross leaf cells by osmosis and out of leaf by evaporation and diffusion.
