7.7 Transport in xylem Flashcards
What are the cells in roots that absorb water
Root hair cells
What is transpiration
The evaporation of water from leaves is the force that pulls water molecules up through the xylem
. Energy for it is supplied by the sun, so it is passive
How does water leave the plant
. Stomata on plant leaves
. The air spaces inside the plant, next to the stomata are more humid than the atmosphere
. So there is a water potential gradient from air spaces in the stomata to air surrounding the leaves
So when stomata open, water vapour diffuses out of air spaces to the surrounding air
This lost water can be replaced by water from surrounding mesophyll cells
Describe the movement of water across the cells of a leaf through the cytoplasm
Cytoplasmic route:
- Water evaporates from mesophyll cells to the air spaces in leaf due to heat supplied by the sun
. These cells now have lower water potentials, so water enters them by osmosis from neighbouring cells
. The loss of water from these neighbouring cells reduces the water potential in them
. So they take in water from their neighbours by osmosis
This movement due to a water potential gradient pulls water from xylem, across the leaf mesophyll, and then out the stomata
Describe the transpiration pull of water up the xylem
(Cohesion tension theory)
. Water evaporates from mesophyll cells due to heat from sun. This lower volume of water leads to a lower pressure
. When this water is lost by transpiration, more water is pulled up through the xylem to replace it. Because it moves from a higher pressure to a lower pressure
. Water molecules form hydrogen bonds with each other so they stick together and are cohesive
. The molecules form a continuous column up the xylem called the transpiration pull
What is the cohesion of water
Water molecules stick together due to strong hydrogen bonds between the oxygen and hydrogen between two molecules
So this can create a column up a xylem as all the molecules stick together
What is the adhesion of water
When water sticks to other water molecules, and the xylem walls
The narrower the xylem, the more the water adheres to the walls
Why is there a pressure gradient up the xylem
At the roots there is a high pressure, as water is moving in by osmosis
This increases volume of liquid in the roots so there is a higher pressure of water in roots
Meanwhile, at the top in the leaves water is evaporating out so there is a lower volume of water, so a lower pressure
Overall, water will move up the xylem from a high to low pressure
How does tree trunk diameter support cohesion tension theory
—> Tree trunks change in diameter according to rate of transpiration
Eg when transpiration rate is at its highest, so during the day, there is more tension in the xylem and a lower pressure at the top
This pulls the walls of xylem inwards causing the trunk to shrink in diameter.
So there is a difference in width of trees in the day vs at night when no transpiration is occurring
Eg try tree hugging and notice difference in the day and at night
Evidence of cohesion tension theory
. If xylem vessel is broken and air enters it, the tree can no longer draw up water as the column of water is broken so water molecules can’t stick together
Also, when it is broken no water leaks out as there is no longer any pressure so cohesive forces break, so water can’t be pulled up the vessels to leak out
It draws air into vessels instead
What type of process is transpiration
Structure of xylem
Passive, so doesn’t require metabolic energy to take place.
However there is still energy needed to drive transpiration from the sun , in the form of heat to evaporate water from leaves
- The xylem are dead cells
- There are no end walls so they form continuous unbroken tubes from roots to leaves which is essential for cohesion tension theory of water
How do you use a potometer to measure water uptake through the xylem
. A leafy shoot is cut under water, and the potometer is filled completely with water
This is so air bubbles don’t get in the potometer or xylem vessel
Make sure to not wet leaves
. Using a rubber tube, attach the leafy shoot to the potometer under water
. Then remove the potometer from under water, and seal all joints with waterproof jelly
Put an air bubble into capillary tube, and measure the distance it travels in a certain amount of time , and repeat to get a mean value
It measures rate of transpiration
You can turn the reservoir tap on which pushes down syringe of water, to push the air bubble back to the start again if you need to repeat
What substance strengthens the xylem cell walls
Lignin forms rings and spirals around the vessel to strengthen the xylem as the cells are dead
