Water uptake
Section: Biology | Syllabus: Cambridge IGCSE Combined Science (0653)
Root Hair Cells
Root hair cells are specialised cells on the surface of roots that have long, thin extensions (root hairs), which greatly increase the surface area for the absorption of water and mineral ions from the soil.
- Root hair cells absorb water from the soil by osmosis - soil water has a higher water potential than the contents of the root hair cell
- Root hair cells absorb mineral ions (e.g. nitrate, magnesium) from the soil by active transport
- The large surface area provided by the root hair extensions increases the rate of uptake of both water and mineral ions
The long extension of a root hair cell greatly increases the surface area in contact with soil water
Pathway of Water Through the Plant
Water follows this route from the soil to the leaves:
- Root hair cells - water enters by osmosis from the soil water (higher water potential) into the root hair cell (lower water potential, due to dissolved solutes)
- Root cortex cells - water passes by osmosis through the cells of the cortex, moving from cell to cell towards the centre of the root
- Xylem - water enters the xylem vessels in the centre of the root and is carried upward through the stem to the leaves
- Mesophyll cells - water leaves the xylem and passes into the mesophyll cells of the leaf; some evaporates into the air spaces inside the leaf and exits through the stomata as water vapour
Investigating the Pathway of Water
The pathway of water through the above-ground parts of a plant can be investigated using a coloured dye (stain):
- Place a cut stem (e.g. a celery stalk or a white carnation) into a beaker of coloured dye
- Leave it for several hours in bright light, so transpiration drives water uptake
- Cut a cross-section through the stem and observe the coloured areas
- Result: the dye is visible only in the xylem vessels, appearing as coloured dots in the stem cross-section - confirming that xylem is the pathway for water transport. In leaves and petals, the coloured veins become visible
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