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. In this way, as water is always being absorbed by the root hairs, water 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 and it is impenetrable. In order to cross the endodermis, the water that has been moving through the cell walls must now move into the cytoplasm. It then 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.