Back to: Botany 200 Level
Hello, my brilliant student! How are you today? Have you ever wondered how trees, even as tall as 100 metres, transport water all the way from their roots to the topmost leaves without a pump? Unlike humans who have a heart to circulate blood, plants rely on natural forces to move water upward. Two of the most important principles behind this movement are the Cohesion-Tension Theory and Capillary Action. Today, we’ll break these down in a simple and relatable way.
Cohesion-Tension Theory and Capillary Action in Water Transport
Plants transport water from their roots to their leaves through the xylem using a combination of physical forces. These forces allow water to move against gravity without any energy from the plant.
1. Cohesion-Tension Theory – The Water Chain Effect
The Cohesion-Tension Theory explains how water moves upward in a plant through the xylem. It is based on two key properties of water:
Cohesion – Water molecules stick to each other like tiny magnets, forming a continuous column of water.
Tension – As water evaporates from the leaves through transpiration, it creates a pulling force (like sucking through a straw), drawing more water upward from the roots.
How does it work?
Water evaporates from the leaf surface (transpiration).
This creates tension (a pulling force) in the xylem.
Cohesion between water molecules keeps the column of water intact, pulling more water from below.
Water is continuously drawn up from the roots to replace the lost water.
Example: Imagine you are drinking a bottle of soda using a straw. When you suck at the top, the liquid moves up because of the force you create. Similarly, transpiration creates a suction force that pulls water up in plants.
Why is Cohesion-Tension Theory Important?
It explains how plants transport water without using energy.
It ensures a continuous supply of water from the roots to the leaves.
It helps in cooling the plant through transpiration.
2. Capillary Action – Water Climbing Up
Capillary action is another process that helps water move through the xylem. It is based on two forces:
Cohesion – Water molecules stick to each other.
Adhesion – Water molecules stick to the walls of the xylem vessels.
Because of these forces, water climbs up the narrow xylem vessels, just like how water rises in a thin tube.
Example: Dip a paper towel in water, and you will see the water move up the towel against gravity. That’s capillary action in action!
Why is Capillary Action Important?
It helps water move even before transpiration starts.
It is most effective in narrow xylem vessels, where water molecules can cling to the walls.
Summary
Plants transport water using natural forces:
Cohesion-Tension Theory explains how transpiration creates a pulling force, moving water from the roots to the leaves.
Capillary Action helps water move up narrow xylem vessels due to cohesion and adhesion.
Together, these mechanisms allow plants to move water efficiently without using energy!
Evaluation
- What role does cohesion play in water transport in plants?
- How does transpiration create tension in the xylem?
- Why does water move up narrow tubes in capillary action?
- How do cohesion and adhesion work together to transport water in plants?
Amazing job! You’ve just learned how plants use simple physical forces to transport water all the way to their tallest branches. Keep going—every lesson brings you closer to mastering plant science. See you in the next lesson—Afrilearn is always here to make learning fun and easy for you!
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