7.2.2—Water transport from soil to xylem

Water moves from soil through root hair cells, the cortex and endodermis into the xylem. Mineral-ion uptake and membrane-controlled pathways make root entry selective rather than a simple open pipe.

• Root hair entry: Root hairs increase surface area. Mineral ions may enter by diffusion or active transport, depending on concentration conditions. Ion uptake can lower the water potential of root-hair cells, so water enters by osmosis.
• Apoplast route: Water moves through cellulose cell walls and their continuous spaces. It does not cross a partially permeable membrane at each cell, so this route is relatively direct and fast.
• Symplast route: Water enters cell cytoplasm/vacuoles across membranes and moves between cells through plasmodesmata. Membrane crossings make this route slower but allow cell-level control.
• Casparian boundary: The suberin Casparian strip in the endodermis blocks the apoplast. Water and dissolved minerals must enter the symplast before reaching the xylem, helping regulate which ions enter the vascular tissue.
• Destination: The selected solution enters xylem vessels. The later transpiration and cohesion-tension objectives explain long-distance driving; this card stops at radial transport from soil to xylem.

Path cue: soil → root hair → cortex → endodermis/Casparian strip → xylem.

Apoplast movement through cell walls is not osmosis; symplast movement includes membrane crossings and plasmodesmata. Do not use root pressure or cohesion-tension as the explanation for this radial pathway. Staff-only visual brief: show the root hair-to-xylem route with apoplast and symplast branches converging at the Casparian strip; do not generate or bind an image.