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Water enters the xylem in the roots via osmosis.
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Once in the xylem the water molecules form hydrogen bonds, forming a continuous column of water molecules up to the leaf.
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Water is constantly lost by transpiration in the leaf.
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When one water molecule is lost another is pulled along due to cohesion of water molecules.
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This creates a tension or negative pressure.
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Transpiration pull is the main cause of water movement and the upward flow of water is called transpiration steam.
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Adhesion to the wall of the xylem vessels prevents the water falling backwards (not actually needed for specification).
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The lignin in the xylem vessels prevents them from collapsing under tension.
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Root pressure is created by the osmotic pressure of xylem sap, which is, in turn, created by dissolved minerals that have been actively transported into the xylem of the root by the endodermis.
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This causes water to pass by osmosis into the xylem. The continuous inflow forces the sap up the xylem vessels creating an upward pressure.
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Although root pressure plays a role in the transport of water in the xylem in some plants and in some seasons, it does not account for most water transport.
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In extreme cases water can be pushed out of the ends of xylem vessels of the leaves. This is called guttation.
Please feel free to critique what I've wrote, as that is how it is in my head and if it needs a reshuffle before the exam then I would appreciate being told by someone.
•
Water enters the xylem in the roots via osmosis.
•
Once in the xylem the water molecules form hydrogen bonds, forming a continuous column of water molecules up to the leaf.
•
Water is constantly lost by transpiration in the leaf.
•
When one water molecule is lost another is pulled along due to cohesion of water molecules.
•
This creates a tension or negative pressure.
•
Transpiration pull is the main cause of water movement and the upward flow of water is called transpiration steam.
•
Adhesion to the wall of the xylem vessels prevents the water falling backwards (not actually needed for specification).
•
The lignin in the xylem vessels prevents them from collapsing under tension.
•
Root pressure is created by the osmotic pressure of xylem sap, which is, in turn, created by dissolved minerals that have been actively transported into the xylem of the root by the endodermis.
•
This causes water to pass by osmosis into the xylem. The continuous inflow forces the sap up the xylem vessels creating an upward pressure.
•
Although root pressure plays a role in the transport of water in the xylem in some plants and in some seasons, it does not account for most water transport.
•
In extreme cases water can be pushed out of the ends of xylem vessels of the leaves. This is called guttation.
Please feel free to critique what I've wrote, as that is how it is in my head and if it needs a reshuffle before the exam then I would appreciate being told by someone.
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