Transpiration & Translocation Flashcards
What is the xylem? What does it do
A tube of dead cells with their ends removed
Transports water and dissolved mineral ions from the root hair cells to the leaves in the shoot
Define transpiration stream
Flow/movement of water
Define transpiration
A loss of water - evaporation through the stomata
What is the transpiration stream
A high tension water column
How are xylem vessels adapted to cope with the transpiration stream
The xylem cells are dead so no cell contents are needed which creates a hollow tube
Cell walls break down at the ends of cells, this forms a continuous tube so the water column is unbroken
Cell walls are thick and reinforced with spiral and rings of lignin which provides strength
What does lignin do? Why is this good
Thickens vessels in a spiral arrangement rather than a continuous thickening allowing the xylem vessels to withstand tension
This is good because it means less lignin is needed so it is more efficient and also it is good because it can grow longer tubes more easily as the plant grows
What is the cohesion tension theory
The idea that water molecules form hydrogen bonds between one another and between the walls of the plant to stick together and be pulled up the transpiration stream
What is the process of cohesion tension
H2O molecules evaporate out of the stomata
The cohesivity which forms an electrostatic attraction between the positive hydrogen and negative oxygen of 2 water molecules (due to the polarity of the water) pulls a stream up and out of the xylem
This allows the movement of water through the xylem
Is transpiration active or passive? What does this mean
Passive so doesn’t require metabolic energy
What happens in the cohesion tension theory if a column is broken
Water can flow into adjacent xylem vessels via pits (gaps in the xylem tube)
Why wouldn’t the xylem be able to provide energy if transpiration was an active process
Because it is made of dead cells
What is the process of transpiration driven by
Driven by heat energy from the sun (evaporation of water)
Define capillary action
The ability of a liquid to flow in a narrow space without the assistance of, and in opposition to external forces, like gravity
What does capillary action propose
Proposes that the smaller the tube diameter, the further the liquid moves
What is the evidence for cohesion tension theory
Tree trunk diameter
The fact that water cannot move up the xylem vessel if it is broken
The fact that water does not leak out when the xylem vessel break
How is tree trunk diameter evidence for cohesion tension theory
During the day - High transpiration rate, therefore we can see there is more tension and therefore a negative pressure in the xylem. This causes a shrink in diameter
At night - Low transpiration rate, so there is less tension in the xylem, so the trunk increases in diameter
This can be observed using a data logger
How does the fact that water cannot move up the xylem vessel if it is broken act as evidence for cohesion tension theory
If the xylem vessel is broken and air enters, the tree can no longer draw up water. So the continuous column is broken, because of this water molecules can no longer stick together
This is also why we shouldn’t get air bubbles in a potometer
How does the fact that water does not leak out when the xylem vessel breaks act as evidence for cohesion tension theory
Air is drawn in. This is consistent with being under tension
What factors affect the rate of transpiration
Water potential gradient
Light
Temperature
Air movement
How does water potential gradient affect the rate of transpiration
The greater the H2O potential gradient between the inside and the outside of the leaf, the faster the water will move out and therefore the greater the rate of transpiration
How does light affect the rate of transpiration
Photosynthesis occurs during the day, so the stomata will be open which means there will be water loss
How does temperature affect the rate of transpiration
A rise in temperature increases the kinetic energy of water molecules. This increases evaporation and therefore transpiration
Also a rise in temperature increases the amount of water air can hold (humidity) which increases rate of transpiration
How does air movement affect the rate of transpiration
Water diffuses through the stomata and accumulates as water vapor around the stomata. Wind then moves this vapor so there is less on the stomata which makes the concentration gradient higher. The higher conc grad means there is more transpiration
What do potometers do? What are they used for
Measure how fast a plant cutting takes up water
They are used to work out the rate of transpiration of a plant
It can be used to investigate the uptake under different conditions
