Explain Cohesion-transpiration theory.
Answers
Explanation:
Transpiration: Transpiration is the technical term for the evaporation of water from plants. ... Cohesion: When water molecules stick to one another through cohesion,they fill the column in the xylem and act as a huge single molecule of water (like water in a straw).
NCERT based Answer for Class 11 Biology Students
Cohesion theory or transpiration pull:
The most accepted theory of water movement through plants was given by Henry Dixon in 1914.
According to the theory, water is mainly ‘pulled’ through the plant and driving force for this process is transpiration from the leaves. This is referred to as the Cohesion-tension- transpiration pull model of water transport.
Less than 1% of water reaching leaves is used in photosynthesis and plant growth. Most of it is lost through stomata in leaves. This water loss is known as transpiration.
Transpiration driven ascent of xylem sap depends mainly on following physical properties of water:
1. Cohesion is the mutual attraction between water molecules. This allows formation of a continuous water column throughout plant from root to stem.
2. Adhesion: Water molecules stick to lignocellulosic wall of xylem elements.
3. Surface tension:
Water molecules are attracted to each other in liquid phase more than to water in gas phase.
Evaporation of water from leaf to atmosphere results in a decrease in water potential of epidermal cells, which are in direct contact with atmosphere.
Water lost from these cells is replaced by water moving from adjacent cells along water potential gradient.
Water lost from leaf cells generates negative water potential in xylem element creating a tension or transpiration pull.
As a result water movement forms a continuous water column from roots to leaves.
Note: Measurements reveal that the forces generated by transpiration can create pressure (pull) sufficient to lift xylem sap over 130 meters high.
If you find the answer useful, plz mark it the Brainliest.
Visit our website for purchasing XI & XII Complete Syllabus Video Lectures
https://concept-tutorials.teachable.com
