·
In AS/A-level Biology, do you know how does water enter root and xylem?
Movement in the Root
- Water enters through the root hair cells and then moves across into the xylem tissue in the centre of the root. Water moves in this direction because the soil water has higher water potential, than the solution inside the root hair cells.
- Cell sap has organic and inorganic molecules dissolved in it. The root hairs provide a large surface area over which water can be absorbed.
- Minerals are also absorbed but their absorption requires energy in the form of ATP because they are absorbed by active transport. They have to be pumped against the concentration gradient.
- Water taken up by the root hairs moves across the cortex of the root either via the cytoplasm of the cells in between the root hair cell and the xylem (the symplast pathway) or through the cell walls of these cells (the apoplast pathway).
- The root hair cell will have higher water potential than the cell next to it.
- Water moves by osmosis to where the water potential is lower.
- As water is always being absorbed by the root hairs, it will always move towards the centre of the root.
- When the water reaches a part of the root called the endodermis, it encounters a thick, waxy band of suberin in the cell walls. This is the casparian strip which is impenetrable.
- In order to cross the endodermis, the water that has been moving through the cell walls must now move into the cytoplasm.
- Water moves towards the xylem vessel in the symplast pathway.
- Once it has moved across the endodermis, it continues down the water potential gradient until it reaches a pit in the xylem vessel.
- It enters the vessel and then moves up towards the leaves.
Movement in the Xylem
- Water evaporates from the mesophyll cells into air spaces in the leaf.
- If the air surrounding the leaf has less water vapour than the air in the intercellular spaces, water vapour will leave the leaf through stomata.
- This process is called transpiration and will continue as long as the stomata are open and the air outside is not too humid.
- On dry, windy days when water vapour is continually diffusing out and being removed, transpiration will increase in rate.
- Although this loss of water can cool the plant, it is essential that the plant does not lose too much water.
- The xylem ensures that water is continuously supplied to the leaves.
- Water is removed from the top of xylem vessels into the mesophyll cells down the water potential gradient.
- This removal of water from the xylem reduces the hydrostatic pressure exerted by the liquid so the pressure at the top is less than at the bottom. This pushes the water up the tube.
- The surface tension of the water molecules, the thin lumen of the xylem vessels and the attraction of the water molecules for the xylem vessel wall (adhesion), helps to keep the water flowing all the time and to keep the water column intact.
- Pressure to push water up can also be increased from the bottom. By actively pumping minerals from cells surrounding the xylem into the xylem itself, more water is drawn into the xylem by osmosis.
- This is called root pressure and it also helps in the process but is less important than the simple movement of water down the water potential gradient. This is because moving water this way does not require energy (it is passive).
- Root pressure helps to push the water up the xylem vessel.
That's the end of the topic!
Drafted by Bonnie (Biology)