Biology – Transport mechanisms | e-Consult

The concentration gradient of mineral ions in the xylem is maintained through a combination of root pressure and transpiration pull. Root pressure is the positive pressure exerted by the water column within the xylem due to the continuous uptake of water by the roots. This pressure pushes water and dissolved mineral ions upwards in the xylem. However, root pressure is generally a minor force in tall plants.

Transpiration pull is the primary driving force for water movement in 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 from the roots.

The interplay between root pressure and transpiration pull is crucial. During the day, transpiration pull is dominant, driving the bulk flow of water and mineral ions upwards. At night, when transpiration is reduced, root pressure can become more significant, ensuring a continuous supply of mineral ions to the xylem. The continuous uptake of mineral ions by the roots also helps to maintain the concentration gradient, ensuring that mineral ions are constantly being drawn into the xylem. The efficiency of this process is dependent on factors such as humidity, wind speed, and temperature, which affect the rate of transpiration.