Cohesion-tension theory

Definition

Cohesion-tension theory explains how water moves through plants, particularly in the xylem, by utilizing the properties of cohesion and tension. This theory highlights that water molecules stick to each other (cohesion) and also adhere to the walls of xylem vessels (adhesion), which creates a continuous column of water that can be pulled upward from roots to leaves due to transpiration. The loss of water vapor from leaf surfaces generates a negative pressure (tension) that aids in drawing more water up from the roots, allowing for efficient transport of nutrients and maintaining plant hydration.

5 Must Know Facts For Your Next Test

1. Cohesion occurs because water molecules are polar and form hydrogen bonds with each other, creating a strong attraction between them.
2. Tension is generated in the xylem when water evaporates from stomata, leading to a drop in pressure that pulls more water into the leaf.
3. The combination of cohesion and adhesion helps prevent the column of water in the xylem from breaking, ensuring continuous flow.
4. Cohesion-tension theory helps explain how tall trees can transport water from their roots to leaves that are many meters above ground level.
5. This theory is vital for nutrient transport as it ensures that not only water but also dissolved minerals reach various parts of the plant.

Review Questions

• How do cohesion and adhesion contribute to the process of water transport in plants according to cohesion-tension theory?
  • Cohesion and adhesion are essential for water transport in plants as described by cohesion-tension theory. Cohesion refers to the strong attraction between water molecules due to hydrogen bonding, which allows them to form a continuous column in the xylem. Adhesion is the attraction between water molecules and the xylem vessel walls, helping to counteract gravity. Together, these forces allow water to be efficiently pulled from roots to leaves, ensuring adequate hydration and nutrient distribution.
• Describe how transpiration influences the tension created in the xylem and its role in water transport within plants.
  • Transpiration plays a crucial role in creating tension within the xylem by causing water vapor to exit through stomata on leaf surfaces. As water evaporates, it creates a negative pressure that pulls more water upward from the roots through the xylem vessels. This tension is essential for maintaining the continuous flow of water needed for plant hydration and nutrient transport. The process ensures that even tall plants can effectively draw up water against gravitational forces.
• Evaluate how cohesion-tension theory accounts for both the physical properties of water and biological processes in plants.
  • Cohesion-tension theory integrates physical properties of water, like cohesion and adhesion, with biological processes such as transpiration and nutrient transport. The theory explains that hydrogen bonding among water molecules leads to cohesive forces that maintain a continuous column in xylem vessels. This physical phenomenon works hand-in-hand with biological functions like transpiration, which creates tension necessary for pulling water upwards. Evaluating this relationship highlights how understanding both physical chemistry and plant physiology is essential for grasping how plants effectively manage their hydration and nutrient needs.

Related terms

Transpiration: The process by which water evaporates from the surface of leaves, creating a negative pressure that pulls water up from the roots through the xylem.

Xylem: The vascular tissue in plants responsible for transporting water and dissolved minerals from the roots to the rest of the plant.

Capillary action: The ability of water to move within narrow spaces, such as xylem vessels, due to cohesion and adhesion forces.