Xylem and Transpiration-driven Transport of Resources

Water and dissolved minerals enter through permeable root hair cells and travel to the xylem as xylem sap. Bulk flow, a pressure-driven movement of liquid over long distances, distributes sap from roots throughout the plant via specialized transport cells called tracheids and vessel elements. These cells form a flexible pipe system that moves resources upward against gravity.

Transpiration from leaves is the primary mechanism pulling xylem sap upward. Water vapor diffuses out through open stomata along its concentration gradient. As liquid water evaporates from mesophyll cells, tension develops that pulls water from leaf veins into the cells, creating negative pressure that draws xylem sap away from roots and up through the stem.

Tracheids are elongated cells with lignified walls containing small gaps called pits that conduct sap between cells. Vessel elements are wider cells stacked vertically, connected by perforation plates with larger openings. Vessel elements transport larger volumes of sap more efficiently. Seedless vascular plants have only tracheids, while most flowering plants contain both cell types.

Cohesion between water molecules is strong because hydrogen bonding occurs between all three atoms of each water molecule. Adhesion allows water to stick to xylem cell walls, preventing sap from seeping downward through roots. These physical properties work together: cohesion links water molecules like a chain throughout the xylem, while adhesion anchors them to vessel walls.

Tracheary elements are dead upon maturity, which allows them to function as open transport conduits without cellular interference. Their lack of cytoplasm and organelles, combined with impermeable lignified cell walls, creates an unobstructed pathway for xylem sap movement. This structural simplification enables efficient bulk flow transport of water and minerals throughout the plant.

Active transport moves minerals into the xylem, creating a high solute concentration in roots. This generates a pressure potential gradient, with higher pressure in roots and lower pressure elsewhere in the plant. Although this gradient contributes to water movement toward lower-pressure areas, it is only a minor driver compared to the transpirational pull from leaves.

Mesophyll cells contain air spaces saturated with water vapor during photosynthesis and respiration. A liquid water film adheres to mesophyll cell walls, facilitating gas exchange. When stomata open, water vapor diffuses out along its concentration gradient, causing some liquid water to evaporate and generating tension that pulls xylem sap from leaf veins into the cells.