Transport of water Flashcards
What are the two main functions of the xylem?
The transport of water and mineral ions
Support for the plant
What is the direction of flow in the xylem?
From roots up to shoots and leaves
What is the xylem made up of?
Dead, hollow cells with no end cell walls. This forms one continuous tube when the xylem cells are stacked on top of each other and fused together
What are the two other types of tissues associated with the xylem?
Xylem parenchyma
Lignin
What is xylem parenchyma?
Xylem parenchyma packs around xylem vessels, and stores food and contains tannin deposits
What is tannin?
A bitter tasting chemical that protects plant tissues from attack by herbivores
What is the function of lignin in the xylem?
Lignin lines the walls of the xylem.
It is important in:
- Supporting and strengthening the xylem
- Impermeable so prevents incorrect water loss
Where does water leave the xylem?
Un-lignified bordered pits
How is lignin arranged in the xylem?
It can form rings, spirals or solid tubes
How does the structure of xylem cells allow for the fastest flow of water?
They have no cytoplasm, cell organelles, or end walls, so there is nothing to slow the flow of water.
What occurs at the unlignified bordered pits in the xylem?
Water and minerals can leave the xylem tube it’s currently in, either entering a new xylem tube, or entering cells
4 reasons why water is important in plants
- Turgor pressure provides a hydrostatic skeleton
- Turgor pressure drives root expansion
- Mineral ions and products of photosynthesis are transported in water
- Water is a raw material for photosynthesis
What cell is responsible for the uptake of water into the plant?
Root hair cells are the exchange surface in plants where water is taken into the plant from the soil
What do root hair cells have attached to them?
Root hairs
Why are root hairs well adapted exchange surfaces?
- Small size can easily penetrate soil
- Large SA:V ratio
- Thin layer
- High water potential gradient between the soil water and the cell
Why is there a high water potential gradient between soil water and root cells?
Soil water has a very low concentration of dissolved minerals so a high water potential.
The cytoplasm and cell sap of the root hair cells has many different solutes (e.g sugars, minerals), and so has a low water potential.
Therefore water moves into the root hair cells via osmosis
Once water has entered the root hair cell, how does it move across the root to the xylem?
The water can travel either through the symplast or apoplast pathway to reach the xylem
How does water move in the symplast pathway?
It moves through the continuous cytoplasm of living plant cells by diffusion and osmosis
What joins cytoplasms together in the symplast pathway?
Plasmodesmata is a fine strand of cytoplasm that act as a bridge linking adjacent cells
How does water move in the apoplast pathway?
It moves through the cell walls and the intercellular spaces. It does this through water tension
What is the endodermis?
The layer of cells that surround the vascular tissue
What is more common for water to move through, symplast or apoplast pathway?
Most water goes from the root cell to the xylem through the apoplast pathway
What is the Casparian strip?
A band of waxy material called suberin, which runs around the endodermal cell walls, forming a waterproof barrier
What happens in the apoplast pathway when the water reaches the Casparian strip?
It cannot pass through the Casparian strip, so must join the symplast pathway
What is the significance of apoplast water joining the symplast pathway?
The water must pass through the selectively permeable cell surface membranes; this excludes any potentially-toxic solutes in the soil water
What happens once the water enters the endodermal cells, just outside the xylem?
Endodermal cells actively transport mineral ions into the xylem
What is the impact of the active pumping of minerals into the xylem?
It lowers the water potential in the xylem, causing water to enter the xylem via osmosis
What is root pressure?
The movement of water up the xylem due to the active pumping of mineral ions. Root pressure is independent of transpiration
What is evidence to suggest that active transport has a role in root pressure?
- When ATP inhibiting poisons like cyanide are added to root cells, there is no energy so root pressure disappears
- Root pressure increases with a rise in temperature and falls with a fall in temperature
What is the definition of transpiration?
The loss of water vapour by evaporation through stomata
What are the pros of transpiration?
- Cools plants down (prevents heat damage)
- Causes the transpiration pull, which allows minerals and water to be delivered to cells
What is the transpiration pull?
Water is continuously drawn up the xylem to replace the water lost by evaporation/transpiration
What are the cons of transpiration?
Loss of water = loss of turgor pressure = plant wilting
What is the cohesion of water molecules in plants?
Water molecules form H bonds with each other, so are able to stick together and pull each other
What is the adhesion of water molecules in plants?
Water molecules form H bonds with the carbohydrates in the walls of the narrow xylem vessels
What is the cohesion-tension theory?
The model of water moving from the soil in a continuous stream up the xylem and across the leaf due to the cohesive property of water
What is capillary action, and what properties of water allow it to occur?
Capillary action is the process by which water can move up a narrow tubes against the force of gravity. This is made possible by the combined effects of the adhesion and cohesion of water moving up the xylem
Why is it important that the xylem tubes are narrow enough?
So the water can flow in a continuous stream
How does water leave the plant?
Water molecules evaporate from mesophyll cells into the air spaces of the leaf and move out of the stomata into the outside air by DIFFUSION
What happens once the water evaporates from the mesophyll cell out of the plant?
The water potential of the mesophyll cell is lowered, causing water to move from an adjacent cell to replace the lost water by osmosis. This has a knock on effect, and this process repeats throughout the plant continuously (this is the transpiration pull).
Evidence of the cohesion-tension theory
If a xylem vessel is broken, water can no longer move water up as the continuous stream of water held together by cohesive forces is broken
Why must transpiration occur?
Transpiration is a consequence of photosynthesis, as stomata must open to allow CO2 to enter and oxygen to leave, which allows transpiration to occur
Why does increasing the temperature increase transpiration rate?
Water molecules have more kinetic energy, so evaporate more quickly
Why does a lower humidity increase transpiration rate?
There is a higher water vapour concentration gradient, so water vapour will diffuse out more quickly
Why does a higher light intensity increase transpiration rate?
More stomata will open to allow more CO2 so the plant can photosynthesise more
Why does higher air movement increase transpiration rate?
Stronger winds will more quickly get rid the moist water vapour air from around the lead, increasing the water vapour concentration gradient
Why does a lower soil-water availability decrease transpiration rate?
The plant will be under water stress, so stomata will close and rate of transpiration will decrease
What instrument can be used to estimate the rate of transpiration?
Potometer
Why is using a potometer an estimation not a measure of transpiration?
Potometers measure the rate of water uptake by a plant, and not all water taken up by the plant will transpire; some will be used in photosynthesis or in maintaining turgor pressure. Therefore not all water taken up will transpire
How do potometers work?
They measure the distance travelled by the air bubble, and use this to find the volume of the water taken up
How do you convert the distance travelled by the bubble into volume of water taken up?
Distance travelled by the bubble x (pi x radius^2) = volume of water
To undergo an accurate potometer practical, what steps must you take?
- Cut leafy shoots underwater
- Place the shoot into a tube with a bung for a tight seal; seal joints with waterproof jelly so any water loss is from the plant
- Do not get water on the leaves, or if you do leave it to dry
Issues with using a potometer
Rate of water uptake does not equal water transpired
It is a complex set up, in which leaks are common and hard to detect
