Cohesion-tension theory

Definition

Cohesion-tension theory explains how water moves through plants, particularly from roots to leaves, relying on the properties of water molecules and their interactions with each other and with plant tissues. It describes the process where water molecules stick together (cohesion) and form a continuous column, which is pulled upward due to the evaporation of water from leaves (tension), creating a negative pressure that aids in the transport of water throughout the plant.

5 Must Know Facts For Your Next Test

1. Cohesion between water molecules is primarily due to hydrogen bonding, which allows them to remain attached to each other as they move upward through the plant.
2. Tension is created when water evaporates from leaf surfaces during transpiration, leading to a decrease in pressure that helps draw water upward from the roots.
3. The structure of xylem vessels, being narrow and elongated, enhances the effectiveness of cohesion-tension theory by facilitating the upward movement of water.
4. Cohesion-tension theory explains why plants can transport water against gravity, utilizing atmospheric pressure changes and internal leaf moisture levels.
5. During periods of drought or high temperatures, transpiration rates can increase significantly, affecting how effectively water is transported through plants and potentially leading to wilting.

Review Questions

• How do cohesion and tension work together in the cohesion-tension theory to facilitate water movement in plants?
  • In cohesion-tension theory, cohesion refers to the attraction between water molecules that keeps them together as they form a continuous column within the xylem. Tension arises when water evaporates from stomata on leaves, creating a negative pressure that pulls this column of water upward. Together, these forces allow for efficient movement of water from roots to leaves, ensuring that plants receive adequate hydration for photosynthesis and other vital processes.
• Discuss how environmental conditions like humidity and temperature can impact the cohesion-tension theory and overall water transport in plants.
  • Environmental factors such as humidity and temperature play a critical role in the cohesion-tension theory. High temperatures can increase transpiration rates, enhancing tension but also potentially leading to water loss if not compensated. In contrast, high humidity may reduce evaporation rates, decreasing tension and subsequently slowing down water movement. These fluctuations can affect a plant's ability to maintain hydration, impacting growth and overall health.
• Evaluate the implications of cohesion-tension theory on plant adaptations in arid environments compared to those in more temperate regions.
  • In arid environments, plants often exhibit specific adaptations like deeper root systems and reduced leaf surface areas to minimize transpiration while maximizing water uptake. These adaptations help maintain tension within their xylem despite lower availability of moisture. In contrast, plants in temperate regions may have broader leaves that enhance photosynthesis while relying on more consistent moisture levels for effective cohesion-tension dynamics. This evaluation highlights how different environmental challenges shape plant structure and function regarding water transport.

Related terms

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

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

Capillarity: The ability of water to move through narrow spaces, such as the xylem vessels, due to adhesion and cohesion forces.