Water uptake and movement to the xylem
Water from soil to xylem
- Water gets in at the root hair cells.
- It then crosses the root to reach the xylem.
- A clever waterproof barrier lets the plant control what gets through.
Practice
Water enters a root hair cell by osmosis because the root hair has:
Water moves down the water-potential gradient, into the root hair which has the lower water potential.
Into the root hair
- A root hair cell has a lower water potential than the soil water around it.
- So water enters by osmosis, down the water-potential gradient.
- Root hairs give a huge surface area for this uptake.
Practice
The apoplast and symplast pathways differ because:
Apoplast = through the cell walls and spaces (fast); symplast = through the cytoplasm via plasmodesmata.
Two pathways across the root
- Apoplast pathway: water moves through the cell walls and spaces between cells — without entering the cytoplasm. This is fast.
- Symplast pathway: water moves through the cytoplasm, passing cell to cell through the plasmodesmata.
Practice
The Casparian strip at the endodermis:
The waterproof suberin band blocks the apoplast, so all water must cross a membrane — giving the plant control.
Practice
By forcing water through cell membranes, the Casparian strip lets the plant control what enters the xylem.
Crossing a membrane means selective transport proteins decide what passes — so the plant controls xylem contents.
The endodermis and Casparian strip
- At a ring of cells called the endodermis, the apoplast route is blocked by the Casparian strip — a waterproof band of suberin.
- This forces all the water through the cell membranes, so the plant can control what enters the xylem.
You've got it
Key idea
- water enters the root hair by osmosis (root hair has lower water potential than soil)
- apoplast = through cell walls (fast); symplast = through cytoplasm (via plasmodesmata)
- the Casparian strip (suberin) at the endodermis blocks the apoplast
- this forces water through the membranes → the plant controls what enters the xylem