Cohesion-tension theory

Definition

Cohesion-tension theory explains how water moves through plants, particularly in the xylem, driven by the evaporation of water from leaves. This process relies on the cohesive properties of water molecules, which stick together, and the tension created by transpiration that pulls water upward from the roots through the plant's vascular system.

5 Must Know Facts For Your Next Test

1. Cohesion-tension theory relies on both cohesion, where water molecules stick to each other, and adhesion, where they stick to xylem walls, enabling efficient water transport.
2. Transpiration is the driving force behind cohesion-tension theory; when water evaporates from leaf stomata, it creates a negative pressure that pulls water upward.
3. Xylem vessels are structured to facilitate this upward movement of water; they are hollow tubes that minimize resistance to flow.
4. The rate of transpiration can be influenced by environmental factors like humidity, temperature, and wind speed, which in turn affects the overall efficiency of water transport in plants.
5. Cohesion-tension theory helps maintain plant hydration and nutrient transport, playing a vital role in photosynthesis and overall plant health.

Review Questions

• How does transpiration contribute to the cohesion-tension theory in plants?
  • Transpiration is essential to the cohesion-tension theory as it creates a negative pressure in the leaves when water evaporates. This loss of water generates tension that pulls more water upward through the xylem. The cohesion between water molecules allows them to remain connected as they are pulled up, enabling efficient movement of water from roots to leaves.
• Discuss the structural adaptations of xylem vessels that facilitate the cohesion-tension theory.
  • Xylem vessels are specifically adapted to support cohesion-tension theory with their hollow structure and lignified cell walls. These features minimize resistance to water flow and allow for continuous columns of water to be maintained. The lignin reinforces the walls against the negative pressure created during transpiration, preventing collapse and ensuring effective transport of water throughout the plant.
• Evaluate the impact of environmental conditions on the cohesion-tension theory and its role in plant physiology.
  • Environmental conditions significantly influence cohesion-tension theory through their effects on transpiration rates. High temperatures and low humidity increase transpiration, enhancing tension within xylem vessels and improving water uptake. Conversely, high humidity or low light can reduce transpiration rates, leading to decreased water movement. Understanding these interactions is crucial for grasping how plants respond to changing environments and maintain hydration and nutrient flow.

Related terms

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

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

Water potential: A measure of the potential energy in water, determining the direction of water movement in plants; it includes factors like solute concentration and pressure.