Explain the mechanism by which water moves upwards in the xylem in terms of a transpiration pull that draws up a column of water molecules, held together by forces of attraction between water molecules

Sourav PanNovember 2, 2024

Answered

The movement of water upwards in the xylem of plants is primarily facilitated by a mechanism known as transpiration pull. This process relies on the cohesive and adhesive properties of water molecules, as well as the physical structure of the xylem vessels. Here’s a detailed explanation of how this mechanism works:

Mechanism of Water Movement in the Xylem

1. Transpiration:
   • Transpiration is the process by which water evaporates from the surfaces of mesophyll cells in the leaves into the air spaces within the leaf and then diffuses out through stomata as water vapor.
   • This loss of water creates a negative pressure (tension) within the leaf’s air spaces, which extends down through the xylem vessels.
2. Cohesion and Adhesion:
   • Cohesion: Water molecules are attracted to each other due to hydrogen bonding, which creates a cohesive force that holds them together. This cohesion allows water to form a continuous column within the xylem vessels.
   • Adhesion: Water molecules also exhibit adhesion, meaning they can stick to other surfaces, such as the walls of xylem vessels. This property helps counteract gravity and supports the upward movement of water.
3. Creation of Negative Pressure:
   • As water evaporates from the stomata during transpiration, it reduces the water potential in the leaf’s intercellular spaces. This creates a negative pressure that pulls on the column of water in the xylem.
   • The negative pressure generated at the leaf level is transmitted down through the xylem vessels due to the cohesive nature of water, effectively drawing more water upward from the roots.
4. Continuous Column of Water:
   • The cohesive forces between water molecules ensure that even as some molecules are lost to evaporation, others remain connected in a continuous column. This column extends from the roots, through the stem, and into the leaves.
   • The adhesion of water molecules to the xylem walls helps maintain this column by preventing it from breaking under tension.
5. Capillary Action:
   • In addition to transpiration pull, capillary action also assists in moving water upward in small-diameter xylem vessels. Capillary action is driven by both cohesion (between water molecules) and adhesion (between water molecules and vessel walls).
   • This effect is particularly important in smaller xylem vessels where surface tension can have a significant impact.