cohesion-tension model of xylem transportpresthurlesscience.weebly.com/.../cohesion_tension.pdf ·...
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3.3.7
Cohesion-Tension Model
of Xylem Transport
( Not examined at ordinary level )
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Need to Know
1. How plants move water up to great heights
against the force of gravity
2. Know the contribution of Irish scientists Dixon
and Joly to plant biology
3. Understand the terms transpiration, cohesion,
adhesion, tension, osmosis and use them to
explain water movement up through xylem
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Cohesion-Tension Model
of Xylem Transport
explains how water is
transported in plants to
extreme heights against
the force of gravity
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Henry Dixon
Working in Trinity College
1894
Theory proposed by two Irish scientists
John Joly
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Similar molecules sticking together
Cohesion
Attraction between molecules
e.g. water sticking to water
H
H O
H
H O
H
H O
H
H O
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Different molecules sticking together
Adhesion
Attraction
e.g. water sticking to xylem walls
H
H O
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Causes the the water to form
into a continuous column or
stream in the xylem
The Cohesion – Tension model
In the narrow xylem tubes
Cohesion between water molecules
1.
H2O
H2O
H2O
H2O
H2O
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Replaces the water by pulling the next
water molecule up the xylem
2.
Cohesion between the water molecules
As water molecules evaporate in
transpiration at the leaf
As the column of water is hard to break this
pull is felt down the entire column of water
to the root
H2O
H2O
H2O
H2O
H2O
H O
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3.
Puts the column of water under tension
Transpiration from the leaf
Tension can pull a column of water
To great heights in plants
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Lignin in the walls prevents the xylem collapsing in
But the cohesive forces between the water molecules
Are strong enough to prevent the column breaking
Tension causes the column of water to be stretched
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Learning Check
1. Explain the terms Cohesion, Adhesion,
Transpiration, Tension, Osmosis
2. Name the Irish scientists who proposed the
tension cohesion model of water movement.
3. Explain how plants move water to great
heights against the force of gravity
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End