Biology – Transport mechanisms | e-Consult

The movement of water in the xylem is largely driven by the cohesion-tension theory, a process fundamentally reliant on the unique properties of water, particularly its hydrogen bonding capabilities. Cohesion, arising from the attraction between water molecules due to hydrogen bonds, is crucial. This cohesive force creates a continuous column of water within the xylem. As water evaporates from the leaves during transpiration, it creates a negative pressure (tension) in the xylem. This tension pulls the water column upwards. The hydrogen bonds between water molecules transmit this pull down the xylem, effectively drawing water from the roots to the leaves.

Adhesion, the attraction between water molecules and other substances (specifically the cellulose cell walls of the xylem), also plays a vital role. Water molecules are attracted to the hydrophilic cellulose, which helps to counteract the force of gravity. This adhesion contributes to the upward movement of water by preventing the water column from breaking. The combination of cohesion and adhesion creates a continuous, continuous water column that can withstand the tension generated by transpiration. Without both cohesion and adhesion, the water column would likely break, and transpiration pull would be significantly reduced. The hydrogen bonds facilitate both cohesion and adhesion, making water an ideal solvent for this transport mechanism.

In summary, hydrogen bonding allows for the formation of a continuous water column (cohesion) and helps water adhere to the xylem walls (adhesion), enabling the transpiration pull to effectively transport water upwards against gravity.