Cohesion-tension theory

Definition

Cohesion-tension theory explains how water moves through plants, particularly in the xylem, using the concepts of cohesion and tension. It describes the process where water molecules stick together (cohesion) and how they are pulled up from roots to leaves due to the evaporation of water from the plant's surface, creating a tension that facilitates this upward movement.

5 Must Know Facts For Your Next Test

1. Cohesion occurs because of hydrogen bonding between water molecules, allowing them to stick together as they move upwards through the xylem.
2. Tension is generated by transpiration, where water vapor leaves the leaf surfaces, causing a negative pressure that pulls more water into the xylem from the roots.
3. The combination of cohesion and tension enables plants to transport water over significant heights without relying on energy expenditure.
4. Cohesion-tension theory can help explain why taller trees are able to transport water from roots to leaves, despite gravitational forces.
5. Factors such as humidity, temperature, and wind can influence the rate of transpiration and subsequently affect the effectiveness of the cohesion-tension mechanism.

Review Questions

• How does cohesion contribute to the process of water transport in plants according to cohesion-tension theory?
  • Cohesion plays a vital role in water transport in plants by allowing water molecules to stick together due to hydrogen bonding. This cohesive property creates a continuous column of water within the xylem. As water evaporates from the leaf surface during transpiration, it generates tension that pulls more water upward from the roots. Without cohesion, this upward movement would be disrupted, making it essential for effective nutrient and water transport.
• Discuss how transpiration is linked to both cohesion and tension in the context of cohesion-tension theory.
  • Transpiration is directly linked to both cohesion and tension because it initiates the entire process of water transport in plants. When water evaporates from stomata on leaf surfaces, it creates tension within the xylem that pulls adjacent water molecules up from the roots. Cohesion ensures that these water molecules remain connected as they move upward against gravity. Thus, transpiration acts as a driving force that utilizes both properties to facilitate efficient water movement throughout the plant.
• Evaluate how environmental factors might impact cohesion-tension theory and its effectiveness in water transport within plants.
  • Environmental factors such as humidity, temperature, and wind can significantly impact cohesion-tension theory's effectiveness in transporting water within plants. For instance, high humidity may reduce transpiration rates, leading to lower tension in the xylem and potentially affecting nutrient delivery. Conversely, dry or windy conditions can increase transpiration rates, enhancing tension but also risking excessive water loss. Understanding these interactions helps explain how plants adapt their physiological processes to maintain hydration and nutrient transport in varying environmental conditions.

Related terms

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

Transpiration: The process by which water is absorbed by plant roots, moves through plants, and is released as vapor through stomata in leaves.

Adhesion: The attraction between water molecules and other substances, which helps water rise against gravity in plant xylem.