7.2.1—Solutes transported in water

Xylem sap is water containing dissolved mineral ions. Root hairs absorb the materials from the soil, and the solution then enters xylem vessels and is transported through the root, stem and leaf system.

• Root entry: Root hairs increase surface area. Mineral ions enter root-hair cells by diffusion or active transport, depending on the concentration conditions; ion uptake can lower cell water potential, so water enters by osmosis.
• Across the root: The absorbed solution moves through the root by two linked routes. The apoplast follows cellulose cell walls and their continuous spaces, so water and dissolved ions move without crossing a partially permeable membrane at each cell. The symplast follows cytoplasm, plasmodesmata and vacuoles, with water crossing membranes and moving between cells through plasmodesmata.
• Selective boundary: At the endodermis, the suberin Casparian strip blocks the apoplast route and forces water and dissolved ions into the symplast. This membrane-crossing step helps the root regulate which mineral ions reach the xylem.
• Xylem route: After entering xylem vessels, water and dissolved mineral ions form the xylem sap that moves from roots through stems to leaves. Xylem walls are lignified and waterproofed, supporting the conducting pathway.

The apoplast is not an osmosis pathway: water in cell walls moves through freely permeable spaces. The symplast involves cell membranes, cytoplasm, vacuoles and plasmodesmata. This card ends at root entry and the xylem pathway; the later transpiration/cohesion-tension cards explain long-distance driving mechanisms. Staff-only visual brief: a root hair→cortex→endodermis/Casparian strip→xylem route with apoplast and symplast branches; do not generate or bind an image.