Q.Write on journey of water from soil to xylem in roots.
Water enters the root hair via imbibition→diffusion→osmosis, then relays across the cortex by a falling DPD gradient (apoplast/symplast pathways) to the pericycle and finally the xylem.
Step 1. Imbibition at the root hair. Capillary soil water is adsorbed by the hydrophilic pectin/cellulose wall of the root hair, swelling it without forming a solution, and continuing until equilibrium.
Step 2. Diffusion to the membrane. The imbibed water diffuses across the freely permeable wall to reach the interface with the plasma membrane, moving down a diffusion-pressure gradient.
Step 3. Osmosis into the cytoplasm. Because the root-hair cytoplasm's solute concentration is higher than the water at this interface, water crosses the selectively permeable plasma membrane by osmosis, and the cell becomes turgid.
Step 4. Relay across the cortex. The turgid root hair has a lowered DPD, but the next cortical cell inward has a higher DPD, so it draws water from the root hair; this repeats cell to cell, setting up a continuous DPD gradient from the epiblema inward across the loosely packed cortex.
Step 5. The two parallel pathways. This inward movement follows the apoplast pathway (through cell walls and intercellular spaces, stopped at the endodermis by the Casparian strip) or the symplast pathway (from living cell to living cell via plasmodesmata), with the Casparian strip forcing apoplastic water to switch into the symplast to continue.
Step 6. Reaching the xylem. Water crosses the endodermis (via passage cells in monocots), reaches the pericycle — which lies close to the protoxylem — and finally enters the xylem vessels/tracheids, completing the journey from soil to xylem.
Water travels from soil to xylem in three steps at the root hair — imbibition (wall), diffusion (to membrane), osmosis (into cytoplasm) — then relays cell-to-cell across the cortex via a falling DPD gradient, following the apoplast pathway (blocked at the endodermis's Casparian strip) or the symplast pathway (via plasmodesmata), reaching the pericycle and finally the xylem.
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