The main force that pulls water up the stem of a plant.
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How does the Humidity of the Atmosphere compare to that of the air next to the stomata?
It is usually less humid in the atmosphere than in the air around the stomata.
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What happens when the Stomata are open?
Water Vapour diffuses into the surrounding air.
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What happens when water is lost from air spaces?
It is replaced by water evaporating from the cell walls of surrounding mesophyll cells.
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How do Plants control the rate of Transpiration?
By changing the size of the stomatal pores.
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What are the 2 types of water pathways?
Symplastic & Apoplastic.
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What is Cohesion-Tension Theory?
The theory that, due to hydrogen bonds between the water molecules, as water transpires, it pulls more water through the Xylem tubes.
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What is Step 1 Of The Cohesion-Tension Theory?
Water evaporates from the leaves as a result of Transpiration.
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What is Step 2 Of The Cohesion-Tension Theory?
The water molecules stick together due to pre-formed hydrogen bonds between one another. (This is cohesion)
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What is Step 3 Of The Cohesion-Tension Theory?
The water forms continuous, unbroken pathways across mesophyll cells & down the xylem.
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What is Step 4 Of The Cohesion-Tension Theory?
As water evaporates from the mesophyll cells into the air space, more molecules of water are pulled behind, as a result of cohesion.
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What is Step 5 Of The Cohesion-Tension Theory?
The pull of water up the xylem is called the TRANSPIRATION PULL.
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What is Step 6 Of The Cohesion-Tension Theory?
This 'Transpiration Pull' puts the xylem under tension, giving negative pressure within the xylem, and therefore giving way to the name of Cohesion-Tension Theory.
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Name the first piece of evidence for Cohesion-Tension Theory.
There is a change in diameter of tree trunks according to the rate of transpiration; during the day (when there is more transpiration) there is more tension (Negative Pressure) in the xylem, so the trunk shrinks in diameter and vice-versa.
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Name the second piece of evidence for Cohesion-Tension Theory.
If the xylem is broken & air enters, the tree can no longer draw up water, therefore the continuous water column is broken.
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Name the third piece of evidence for Cohesion-Tension Theory.
If the xylem is broken, water does not leak out as if the water was under pressure; air is drawn in because the air is under tension.
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Is the Transpiration Pull a Passive Process?
Yes.
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Is energy needed to drive transpiration itself?
Yes.
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Are Xylem Vessels Alive?
No.
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Why are Xylem Vessels Dead?
Because then they can form a series of continuous, unbroken tubes.
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Other cards in this set
Card 2
Front
How does the Humidity of the Atmosphere compare to that of the air next to the stomata?
Back
It is usually less humid in the atmosphere than in the air around the stomata.
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