Xylem and Transpiration-driven Transport of Resources

Plants take in water and nutrients from the soil but require these substances even in distant tissues. How do plants transport these substances against the force of gravity?

Water and dissolved minerals enter the plant through root hair cells in the epidermis. These substances then move to the xylem, where the fluid is called xylem sap.

The xylem carries the sap from the roots to the rest of the plant by bulk flow, a pressure-driven movement of liquid over long distances, through specialized transport cells.

These specialized cells, called tracheids and vessel elements, form a rigid, pipe-like system. These cells lack cytoplasm and organelles and have thick, lignified cell walls, which prevent the xylem from collapsing and stop sap from leaking.

The main force that pulls xylem sap through the plant is transpiration from the leaves.

A thin film of water sticks to the cell wall of each mesophyll cell. This film supports gas exchange during photosynthesis and cellular respiration. Water evaporates from this film, filling the air spaces within the leaf with water vapor.

The air outside the plant is drier than the air inside the leaves. When the stomata are open, water vapor diffuses out along its concentration gradient, moving from the moist air spaces within the leaf to the atmosphere.

As water vapor exits through the stomata, more liquid water evaporates from the mesophyll cell walls. This creates tension that pulls water from the leaf veins into the mesophyll cells to replace the lost water.

Water molecules stick to each other through cohesion and also stick to the xylem walls through adhesion, which keeps a continuous column of water intact.

Negative pressure in the leaf veins then pulls the xylem sap upward from the roots, through the stem, and into the leaves, transporting water and minerals throughout the plant.