Transport of water in the xylem Flashcards
How is water absorbed in plants?
Through root hair cells.
In flowering plants, how is water transported?
Through hollow, thick-walled tube called xylem vessels.
What is transpiration?
Transpiration is the process in plants by which water travels in a stream from the roots to the leaves where it evaporates.
The energy which is needed is provided by the sun so it’s a passive process.
How is water lost through the stomata?
- As there is a higher water potential inside the air spaces next to the stomata, therefore, there’s a water potential gradient.
- When the stomata are open, water vapour molecules diffuse out of the air spaces into the atmosphere.
- As water is lost by diffusion from the air spaces into the surrounding air, it is then replaced by the water evaporating from the cell wall of mesophyll cells.
- By changing the size of the stomatal pores, plants can control their rate of transpiration.
How does water move across the cells of a leaf?
- Water is lost from the mesophyll cells by evaporation from their cell walls to the air spaces of the leaf.
- These cells now have a low water potential than the surrounding cells so water enters by osmosis.
- The loss of water from the neighbouring cells also lowers their water potential so they take water from their neighbouring cells etc. More of the water loss, higher the rate of osmosis.
- Then a water potential gradient is established therefore it pulls water from the xylem.
What is the main factor which is responsible for the movement of water up the xylem?
Cohesion-tension.
How does water move up the stem in the xylem?
- Water evaporates from mesophyll cells due to heat from the sun which causes transpiration.
- Water molecules have hydrogen bonds between them which makes them stick together known as cohesion.
- Therefore water forms a continuous, unbroken column across the mesophyll and down the xylem.
- As more water evaporates from the mesophyll cells into the air spaces, more water molecules will be drawn up as a result of cohesion.
- A column of water is pulled up the xylem due to transpiration - this is transpiration pull.
- Transpiration pull puts the xylem under tension, there is a negative pressure within the xylem - cohesion-tension theory.
What is the evidence which supports the idea or cohesion-tension theory?
1) Change in diameter of tree trunks during the day.
2) If a xylem vessel is broken and air enters, the tree will no longer be able to take up water as the continuous column of water is broken so water cannot stick together e.g. like a straw been broken.
3) When a xylem vessel is broken, water does not leak out, instead, the air is drawn in.
Is transpiration a passive or active process?
Passive, therefore, it does not require metabolic energy.
The energy that is required comes from the sun.
Why can’t the xylem vessel actively uptake water?
The xylem vessel is dead cells so it can’t actively move the water.
Why can xylem vessels form a series of continuous, unbroken tubes from root to leaves?
Xylem vessels have no end walls so they can reach all the way from root to leaves which is essential to the cohesion-tension theory of water flow up the stem.
How can you estimate transpiration rate?
By using a potometer - It measures the water uptake by a plant, assuming which water uptake is directly related to water loss by the leaves.
1) Cut the shoot underwater to prevent air entering the xylem. Cut it slant so increase SA for water uptake.
2) Assemble the potometer underwater to make sure no water can enter when you place the plant in.
3) Place the end of the capillary tube into a beaker of water.
4) Then watch how fast the bubble is travelling at and time the rate.
