Question

Briefly outline the Transpiration Adhesion Cohesion Tension Model of water movement in plants. Use the model to describe where and how a water molecule moves though a vascular plant -- from when it is first pulled into the root until it exits a leaf stoma.

Answer 1

Even without the presence of a circulatory system in plants, the upward flow of water through the xylem reaches fairly high rates of about 15 metres per hour.. Most researchers agree that its due to the pulling force on water due to the transpiration of water through the leaves. This is referred to as the adhesion- cohesion-tension model of water transport.

Transpiration is the evaporative loss of water by the plants and it occurs mainly through the stomata. It is this process that drives ascent of water in the form of xylem sap. The ascent of water depends on three physical properties of water:

• Cohesion: Each water molecules are attached with each other within the xylem vessel
• Adhesion: Water molecules are attached with the xylem vessels.
• Surface Tension: Water molecules are attracted to each other more in liquid phase than in gas phase.

These properties give water high tensile strength i.e. an ability to resist a pulling force and the ability to rise in thin tubes.

After the water enters the root, due to root pressure it is pushed into the stem and rises up into the xylem. Due to transpiration when water is eliminated from the plant body through stomata as water vapour a vacuum is created which in turn creates a pull on the water in the xylem(transpiration pull). The water rises in the xylem due to its properties of cohesion-adhesion-tension until it is used up for photosynthesis by the leaves or eliminated through the stomata by transpiration and the process continues in a loop.