How does the cohesion-tension theory explain water transport in xylem?

The cohesion-tension theory explains water transport in xylem through the cohesive properties of water and transpirational pull.

The cohesion-tension theory is a model used to explain how water moves from the roots to the leaves of a plant through the xylem. This theory is based on the physical properties of water and the structure of the plant. The main driving force behind this movement is the process of transpiration, which is the evaporation of water from the leaves.

When water evaporates from the leaves, it creates a tension or negative pressure in the leaf air spaces. This tension is transmitted through the water in the xylem due to the cohesive properties of water molecules. Water molecules are polar, meaning they have a positive and a negative end. This allows them to form hydrogen bonds with each other, creating a continuous water column from the roots to the leaves.

The cohesive force of water is so strong that it can withstand the tension created by transpiration, allowing the water column to remain unbroken even when the plant is subjected to drought or other stresses. This is why the theory is called the cohesion-tension theory.

In addition to cohesion, the adhesion of water to the walls of the xylem vessels also plays a role in water transport. The xylem vessels are made of dead cells with lignified walls, which are hydrophilic, meaning they attract water. This adhesion helps to counteract the force of gravity and aids in the upward movement of water.

The cohesion-tension theory also takes into account the role of the endodermis in the roots, which controls the uptake of water from the soil. The endodermis has a waterproof layer called the Casparian strip, which prevents the passive flow of water and ions. This ensures that water can only enter the xylem through the symplast, a pathway that requires energy and allows the plant to control water uptake.

In summary, the cohesion-tension theory explains how the physical properties of water, combined with the structure of the plant and the process of transpiration, enable water to be transported from the roots to the leaves through the xylem.