Main Notes:
Two Routes for Water Transport in Roots:
The apoplast is the non-living continuum of cell walls and intercellular spaces. The symplast is the living continuum of cytoplasm connected by plasmodesmata.
| Feature | Apoplast Pathway | Symplast Pathway |
|---|---|---|
| Route | Cell walls + intercellular spaces | Cytoplasm via plasmodesmata |
| Speed | Faster | Slower |
| Selectivity | Non-selective (passive) | Selective (membrane-regulated) |
| Living cells required? | No | Yes |
| Membranes crossed? | None | Plasma membranes at entry |
| Barrier | Casparian strip at endodermis | None (continuous) |
The Casparian Strip — Critical Barrier:
- Location: Radial and transverse walls of endodermal cells
- Composition: Suberin (waxy, hydrophobic substance)
- Function: Blocks apoplast, forces all water into symplast
- Significance: Ensures selective mineral uptake (only ions with membrane transporters can enter xylem)