How do the cohesive property of water and the structure of xylem vessels enable water transport under tension?

Sourav PanNovember 9, 2024

Answered

The cohesive property of water and the structure of xylem vessels work together to enable efficient water transport under tension in plants. Here’s how these two factors interact to facilitate this process:

Cohesive Property of Water

1. Hydrogen Bonding:
   • Water molecules exhibit strong cohesive properties due to hydrogen bonding. Each water molecule can form multiple hydrogen bonds with neighboring water molecules, creating a continuous column of water within the xylem vessels. This cohesion allows water molecules to “stick” together, which is crucial for maintaining an unbroken water column from the roots to the leaves.
2. Transpirational Pull:
   • When water evaporates from the stomata in leaves (a process known as transpiration), it creates a negative pressure (tension) in the leaf’s air spaces. This tension is transmitted down through the xylem due to the cohesive forces among water molecules. As one molecule evaporates, it pulls on adjacent molecules, creating a chain reaction that draws more water upward from the roots .

Structure of Xylem Vessels

1. Composition and Design:
   • Xylem vessels are composed of dead cells that form long, hollow tubes. These vessels have thick, lignified walls that provide structural support and prevent collapse under negative pressure. The lignin strengthens the walls, allowing them to withstand the tension created by transpiration without bursting .
2. Continuous Water Column:
   • The structure of xylem vessels allows for a continuous column of water to be maintained despite the high tension. The absence of end walls in vessel elements means that there are minimal interruptions in the flow path, which is essential for effective water transport over long distances.
3. Adhesion to Xylem Walls:
   • In addition to cohesion, adhesion plays a role in water transport. Water molecules are attracted to the hydrophilic surfaces of xylem vessel walls, which helps counteract gravitational forces and supports upward movement. This adhesion helps maintain contact between the water column and the vessel walls, further enhancing stability during transport