Kohesionsspänningsteorin

Kohesionsspänningsteorin, or cohesion-tension theory, is a model that explains the ascent of sap in plants. It proposes that transpiration, the process of water evaporation from the leaves, creates a negative pressure or tension within the xylem. This tension pulls water upwards from the roots, overcoming gravity.

The theory relies on two key properties of water: cohesion and adhesion. Cohesion refers to the attractive forces between water molecules, which allow them to form a continuous column within the xylem vessels. Adhesion refers to the attractive forces between water molecules and the walls of the xylem vessels, which help to counteract the downward pull of gravity.

When water evaporates from the leaf surface, it reduces the water potential in the leaf cells. This creates a gradient of water potential from the roots to the leaves. Water moves from areas of higher water potential (roots) to areas of lower water potential (leaves) through the xylem. The tension generated by transpiration acts like a suction pump, drawing the water column upwards.

This theory is a widely accepted explanation for how tall trees can transport water to their highest leaves, a feat that would be impossible with simple capillary action alone. The continuous column of water is maintained by the cohesive forces between water molecules, and the entire system is driven by the evaporative pull from the leaves.