How do cohesion and adhesion help maintain a water column in xylem?

Cohesion links water molecules to one another; adhesion links water to the xylem walls. Together they help maintain a continuous water pathway, while evaporation from the leaves generates the tension that drives most upward transport in a transpiring plant.

Water moves upward as the pull from the leaf is passed through the water below; this schematic is not drawn at molecular scale.

Begin at the leaf, where water evaporates from moist cell walls into internal air spaces and water vapour diffuses out through stomata. As water is lost, tension develops in the water supplying these surfaces. This means the liquid is under negative pressure relative to atmospheric pressure.

Hydrogen bonds hold water molecules together. A pull on water near the leaf can therefore be passed through the connected water farther down the plant. This difference in pressure draws water upwards from the roots through the xylem. The water flows upward, while the pull starts above and is passed through the column below. These directions describe different parts of the same process.

Adhesion is the attraction between water and material in the xylem walls. It helps water stay in contact with the vessel surface and contributes to capillary action, the movement of water in narrow spaces. But adhesion is not a separate motor pushing water up the tree. Capillary action alone cannot maintain the supply to the tops of tall trees losing water through transpiration.

Adhesion does not make the column unbreakable. Under enough tension, gas bubbles can form or expand, a process called cavitation. The resulting gas blockage is an embolism and can interrupt water flow. Vessel structure, pit membranes and other routes for water affect how far the disruption spreads. Cohesion-tension theory therefore helps explain both water transport and how it can fail.

Keep the explanation about molecules connected to the whole plant. Hydrogen bonds are attractions between molecules, and individual bonds continually break and reform. Together, these attractions allow the liquid to pass on tension without the same molecules being permanently tied together.

Three common errors can be checked against the diagram. Cohesion is not water sticking to cellulose: that is adhesion. Xylem vessel elements do not actively pump water using ATP: mature conducting elements are dead. Finally, transpiration does not mean water is pulled out of the soil directly as vapour. It enters the plant as liquid and moves through a connected liquid pathway before evaporating in the leaf.

Check your understanding with the xylem water-column question in the Biology Unity and Diversity questionbank.