Cohesion tension theory
Laura Armstrong
Teacher
Contents
Recall Questions
This topic requires prior knowledge of xylem structure. You can test your knowledge on this below.
How does the structure of xylem vessels support water transport?
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Xylem vessels are hollow tubes with no end walls, forming a continuous column for water movement.
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Their walls are thickened with lignin, which prevents the xylem vessels from collapsing when under tension or negative pressure.
What are pits in the xylem, and what is their function?
Pits are non-lignified regions in xylem walls that allow lateral movement of water between vessels, helping bypass blockages.
Why is it important that xylem cells have no organelles?
The flow of water is easier as it is not impeded by organelles.
Topic Explainer Video
Check out this @LauraDoesBiology video that explains cohesion tension theory or read the full notes below. Once you've gone through the whole note, try out the practice questions!
What is the Cohesion-Tension Theory?
The Cohesion-Tension Theory explains how water moves from roots to leaves in the xylem through the transpiration stream.
Key Processes in the Theory
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Transpiration (Water Loss from Leaves)
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Water evaporates from the mesophyll cells into air spaces and diffuses out through the stomata.
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This creates a low water potential in the leaf, drawing water from adjacent mesophyll cells.
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- Water then moves out of the xylem and into the leaves down a water potential gradient.
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Tension (Negative Pressure in the Xylem)
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Water leaving the xylem in the leaves creates tension (negative pressure), pulling more water up the stem from the roots.
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Cohesion (Water Molecules Sticking Together)
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Water molecules are polar and form hydrogen bonds between them.
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This cohesion creates a continuous column of water, preventing breakage.
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Adhesion (Water Molecules Sticking to Xylem Walls)
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Water molecules also adhere to the hydrophilic walls of the xylem, assisting in the upward pull.
Summary of Water Movement
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Water is lost from the leaves via transpiration through the stomata.
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This creates tension, pulling water up the xylem.
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Cohesion keeps the water column unbroken.
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Adhesion helps water move against gravity.
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The continuous water column moves upwards through the plant.
Water Movement into the Roots
Water enters the roots via osmosis, down a water potential gradient, before being transported up through the xylem by the cohesion-tension mechanism.
Water Uptake in the Roots
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The soil around the roots has a higher water potential than the cytoplasm of root hair cells, which contain dissolved solutes.
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Water enters the root hair cells by osmosis, moving down the water potential gradient.
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Once inside the root, water moves towards the xylem.
Key Terms
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Cohesion-Tension Theory: Model explaining how water is pulled up the xylem due to cohesion, tension, and adhesion.
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Transpiration: Evaporation of water from the leaf surface, through the stomata, driving water movement.
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Tension: Negative pressure created in the xylem as water evaporates from the leaves.
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Cohesion: Attraction between water molecules due to hydrogen bonding, forming a continuous column.
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Adhesion: Attraction between water molecules and xylem walls, assisting upward movement.
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Water potential gradient: the difference in water potential between two regions.
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Exam Tip
When explaining the Cohesion-Tension Theory, make sure to mention transpiration, tension, cohesion, and adhesion in logical order.
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Explain how water moves through a plant using the Cohesion-Tension Theory. (6 marks)
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Water evaporates from mesophyll cells and diffuses out through the stomata, during transpiration. (1)
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This reduces the water potential in the mesophyll cells. (1)
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Water is drawn out of xylem. (1)
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This creates tension (negative pressure) in the xylem. (1)
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Cohesion between water molecules (via hydrogen bonding)
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Forms a continuous water column that is pulled up. (1)
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Adhesion between water molecules and xylem walls. (1)
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Helps maintain the continuous water column (and aids movement against gravity). (1)
Practice Question
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!