8.2) Water uptake
Root hair cells:
The root hairs are where most water absorption happens. They are long and thin so they can penetrate between soil particles, and they have a large surface area for absorption of water.
Water passes from the soil water to the root hair cell’s cytoplasm by osmosis. This happens because the soil water has a higher water potential than the root hair cell cytoplasm:
Osmosis causes water to pass into the root hair cells, through the root cortex and into the xylem vessels
The large surface area of root hairs increases the rate of the absorption of water by osmosis and ions by active transport
The elongated section of the root hair, basically provides a large surface area for the absorption of water and inorganic ions.
Additionally, the membrane of the root hair cell is semi-permeable. What that means is basically only minerals and water can go through the membrane, but not necessarily go back out.
Investigate, using a suitable stain, the pathway of water through the aboveground parts of a plant.
- Cut the base, non-leafy end, of a fresh stalk of celery underwater.
- Place the cut end into a beaker of water stained with red food dye.
- Leave the celery stalk in bright light at room temperature in a breeze.
- Note the red lines moving up the stalk and then along the veins of the leafy parts.
- Cut across the stalk and note the curve of red dots close to the outer edge.
- If a thin section is examined under the microscope it is the xylem which has been stained red in colour.