Transport of Water from Soil to the Xylem (Cambridge International AS & A Level Biology 9700 – Topic 7)
1. Overview
Water taken up by root‑hair cells must move radially from the soil solution into the central vascular cylinder (the stele) and then ascend through the xylem to the rest of the plant. Two parallel routes are used:
- Apoplast pathway – movement through the cell‑wall matrix and intercellular spaces (non‑living route).
- Symplast pathway – movement from cell to cell through the cytoplasm linked by plasmodesmata (living route).
Both routes are driven by a water‑potential gradient ( Ψ ) from the soil (high Ψ) to the xylem (low Ψ).
2. Structure of the Transport Tissues (Syllabus 7.1)
- Root‑hair cells – increase surface area for water and mineral uptake; thin, permeable epidermal walls.
- Cortex cells – contain large vacuoles; site of both apoplastic and symplastic movement.
- Endodermis – single‑cell layer surrounding the stele; characterised by a lignin‑rich Casparian strip and a suberin‑rich outer wall.
- Pericycle – thin layer of parenchyma just inside the endodermis; gives rise to lateral roots and contributes to the radial transport of water.
- Stele – the central vascular cylinder containing the xylem and phloem.
- Xylem vessels – dead, lignified tubes with perforation plates; provide a low‑resistance conduit for bulk water flow.
- Tracheids – elongated, dead, lignified cells with tapered ends and pits; also conduct water, especially in woody roots.
- Phloem sieve‑tube elements – living cells with sieve plates; transport photosynthates.
- Companion cells – closely associated with sieve‑tube elements; supply metabolic support and drive active loading of sugars.
Diagram suggestion: transverse section of a young root showing, from outside to inside, root hairs, epidermis, cortex, endodermis (with Casparian strip), pericycle, stele (xylem vessels, tracheids, phloem), and the two routes (solid arrows = apoplast, dashed arrows = symplast).
3. Water Potential (Ψ) – The Driving Force (Syllabus 7.2)
Water moves from regions of higher water potential to lower water potential.
| Term | Symbol | Definition |
|---|
| Water potential | Ψ | Potential energy of water per unit volume; determines the direction of water movement. |
| Solute potential | Ψs | Negative contribution due to dissolved solutes (Ψs = – i C R T). |
| Pressure potential | Ψp | Positive or negative pressure exerted on the water column (turgor in cells, tension in the xylem). |
Overall: Ψ = Ψs + Ψp
Typical gradient in a root:
- Soil water – relatively high Ψ (low solute concentration, atmospheric pressure).
- Cortex & endodermis – Ψ becomes lower because solutes are taken up (more negative Ψs) and cells develop turgor (positive Ψp).
- Stele (xylem) – very low Ψ (highly negative Ψs plus tension Ψp generated by transpiration pull).
The gradient Ψsoil > Ψcortex > Ψstele drives water inward via both pathways.
4. Apoplast Pathway
Water follows a non‑living continuum of cell walls (mainly cellulose) and intercellular spaces.
- Cellulose – forms a porous matrix that allows rapid diffusion of water.
- Lignin – deposited in secondary walls of xylem vessels, tracheids and the Casparian strip; makes walls hydrophobic, prevents lateral leakage and directs water toward the lumen.
Steps in the root (apoplastic route)
- Water enters the wall of a root‑hair cell from the soil solution (no crossing of a plasma membrane).
- It moves radially inward through the apoplastic spaces of the epidermis, cortex and the cell walls of endodermal cells.
- At the Casparian strip of the endodermis the apoplastic continuum is interrupted, forcing water to cross a plasma membrane.
- After crossing, water enters the symplast of an endodermal cell and is subsequently loaded into the xylem vessels or tracheids of the stele.
5. Symplast Pathway
The symplast is the continuous cytoplasmic network formed by plasmodesmata.
- Water first enters the cytoplasm of an epidermal cell by osmosis through aquaporin channels.
- It then moves cell‑to‑cell through plasmodesmata, remaining inside the living cytoplasm.
- In the cortex the water travels inward via the cytoplasm of cortical cells.
- When it reaches the endodermis, the Casparian strip blocks any further apoplastic movement, so water must cross the plasma membrane of an endodermal cell (again via aquaporins).
- After crossing, water joins the symplast of the stele and is loaded into the xylem lumen.
5.1 Endodermis, Casparian Strip and Suberin (Syllabus 7.2)
- Casparian strip – a band of lignin impregnating the radial and transverse walls of endodermal cells; creates an impermeable barrier to apoplastic flow.
- Suberin – a waxy, hydrophobic polymer deposited on the outer surface of the endodermal cell wall; further reduces uncontrolled water loss and solute entry.
- Consequences
- All water and dissolved minerals must cross a plasma membrane, allowing the plant to regulate uptake via transport proteins.
- Once inside the endodermal cytoplasm, water joins the stele symplast and is loaded into the xylem vessels or tracheids.
6. Comparison of the Two Pathways
| Feature | Apoplast Pathway | Symplast Pathway |
|---|
| Medium of movement | Cell walls & intercellular spaces (non‑living) | Cytoplasm linked by plasmodesmata (living) |
| Key structural components | Cellulose, lignin (especially in xylem vessels, tracheids, Casparian strip) | Aquaporins, plasmodesmata, plasma membrane |
| Barrier encountered | Casparian strip forces entry into the symplast | Must cross plasma membrane of endodermal cells (regulated) |
| Regulation | Mostly passive diffusion; limited control | Highly regulated by membrane transport proteins (aquaporins, ion channels) |
| Speed of transport | Generally faster (lower resistance) | Slower but selective |
7. Cohesion–Tension and Root‑Pressure (Syllabus 7.2)
- Cohesion–tension theory – water molecules form a continuous column in the xylem; transpiration from leaves creates a negative pressure (tension) that pulls water upward.
- Root pressure – when soil water potential is high, osmotic uptake of solutes into the xylem generates a positive pressure that can push water upward, especially at night.
- Both mechanisms rely on the water‑potential gradient established by the apoplastic and symplastic pathways.
8. Phloem Transport – Pressure‑Flow (Mass‑Flow) Hypothesis (Syllabus 7.2)
- Loading – sucrose is actively transported into sieve‑tube elements (often via companion cells), lowering Ψs in the phloem.
- Osmotic water entry – water follows the solute gradient, raising turgor pressure (Ψp) in the source region.
- Pressure gradient – high turgor at the source and lower turgor at the sink drives bulk flow of sap from source to sink.
- Unloading – sucrose is removed at sink tissues, raising Ψ and allowing water to exit the phloem.
Although the focus of these notes is water uptake, the same water‑potential principles underpin phloem transport.
9. Practical Investigation: Role of the Casparian Strip
Objective: Demonstrate that the Casparian strip blocks apoplastic movement of solutes.
Method:
- Grow two sets of seedlings on agar containing a water‑soluble dye (e.g., eosin).
- In the experimental set, gently scrape away the outer endodermal layer of a few roots to disrupt the Casparian strip.
- After 24 h, cut transverse sections of control and treated roots and observe dye distribution under a light microscope.
Expected outcome: In intact roots the dye remains confined to the cortex; in roots with a damaged Casparian strip the dye penetrates the stele via the apoplast, confirming the barrier function.
10. Key Points to Remember
- Water enters the root through hair cells and can travel either apoplastically (via cellulose‑rich walls) or symplastically (via plasmodesmata).
- Lignin in the secondary walls of xylem vessels, tracheids and the Casparian strip provides rigidity and creates an impermeable barrier to apoplastic flow.
- The Casparian strip (lignin) and suberin in the endodermis force water to cross a plasma membrane, allowing selective uptake.
- The water‑potential gradient (Ψsoil > Ψcortex > Ψstele) drives net movement toward the xylem.
- After crossing the endodermis, water enters the xylem lumen and is pulled upward by transpiration‑induced tension; root pressure can assist, especially at night.
- Phloem transport of sugars follows the pressure‑flow hypothesis, also based on water‑potential differences.
11. Suggested Diagram
Cross‑section of a young root showing:
- Root‑hair cells, epidermis, cortex.
- Apoplastic route (solid arrows) and symplastic route (dashed arrows) from soil to stele.
- Endodermal layer with highlighted Casparian strip (lignin band) and outer suberin layer.
- Pericycle, stele (xylem vessels, tracheids, phloem sieve‑tube elements, companion cells).