describe the transport of water from the soil to the xylem through the: apoplast pathway, including reference to lignin and cellulose, symplast pathway, including reference to the endodermis, Casparian strip and suberin
Cambridge A-Level Biology 9700 – Transport Mechanisms
Transport of Water from Soil to Xylem
Water moves from the soil into the plant’s vascular system by two main routes:
Apoplast pathway – movement through cell walls and intercellular spaces.
Symplast pathway – movement from cell to cell via plasmodesmata.
1. Apoplast Pathway
The apoplast consists of the continuum of cell walls (primarily composed of cellulose) and the spaces between cells. Water follows this non‑living route until it encounters a barrier that forces entry into the symplast.
Cellulose: Provides structural rigidity to the cell wall, forming a porous matrix that allows water to diffuse freely.
Lignin: A complex phenolic polymer deposited in the secondary walls of xylem vessels and tracheids. Lignification reduces wall permeability, directing water flow toward the xylem lumen.
In the root cortex, water moves radially inward through the apoplastic spaces until it reaches the endodermis. At this point, the Casparian strip blocks further apoplastic flow, compelling water to cross a cell membrane.
2. Symplast Pathway
The symplast is the continuous cytoplasmic network formed by plasmodesmata, which are microscopic channels linking adjacent plant cells.
Water enters the root epidermal cells by osmosis and then moves cell‑to‑cell through plasmodesmata.
Within the cortex, water travels inward through the cytoplasm of cortical cells, remaining in the symplast.
When the water reaches the endodermal layer, it must cross the plasma membrane of an endodermal cell because the Casparian strip blocks the apoplast.
Endodermis, Casparian Strip and Suberin
The endodermis is a single‑cell layer that surrounds the vascular cylinder. Its inner wall is impregnated with:
Casparian strip: A band of lignin that seals the space between adjacent endodermal cells, preventing passive apoplastic flow.
Suberin: A waxy, hydrophobic polymer deposited on the outer surface of endodermal cell walls, further reducing water loss and controlling solute entry.
Because of these barriers, water must cross the plasma membrane of an endodermal cell (via aquaporins) to enter the stele. Once inside the symplast of the endodermis, water can be loaded into the xylem vessels.
Suggested diagram: Cross‑section of a root showing the apoplast and symplast routes, the Casparian strip, and suberin deposition.
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
Aquaporins, plasmodesmata
Barrier encountered
Casparian strip forces entry into symplast
Must cross plasma membrane of endodermal cells
Regulation
Limited – largely passive diffusion
Highly regulated via membrane transport proteins
Speed of transport
Generally faster due to less resistance
Slower but more selective
Key Points to Remember
Water initially follows the apoplast pathway through cellulose‑rich cell walls.
Lignin in the secondary walls of xylem vessels provides structural support and limits apoplastic leakage.
The Casparian strip (lignin) and suberin in the endodermis block the apoplast, forcing water into the symplast.
Symplastic transport relies on plasmodesmata and membrane proteins, allowing selective uptake of water and dissolved minerals.
After crossing the endodermis, water enters the xylem and is driven upward by transpiration pull and root pressure.