7.7- Transport of water in the xylem

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  • Created by: Megan2413
  • Created on: 15-03-17 18:56
What is transpiration?
A means of bringing water up the stem due to the evaporation of water through the leaves
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Why is transpiration a passive process?
It doesn't use energy from ATP, rather from the sun
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What four factors affect transpiration?
- Temperature - Air humidity - Air movement - Light intensity
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What effect does raising the temperature have to the rate of trasnpiration and why?
It increases transpiration because the water particles will have more kinetic energy to diffuse and evaporate
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What effect will increasing the air humidity have to the rate of trasnpiration and why?
It will decrease trasnpiration because the water potential gradient will be reduced and it won't evaporate out of the leaves as quickly
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What effect will increasing the air movement have on the rate of transpiration and why?
It will increase the rate of transpiartion as it maintains a water potential gradient for the evaporation of water from the leaves
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What effect will increasing th elight intensity have on the rate of trasnpiration and why?
It will increase the rate of trasnpiration because when its light the stomata are open more due to photosynthesis, so more water can evaporate out
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What does the cohesion-tension theory show?
It is a theory about how water is brought up the xylem
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Where does the cohesion part of the theory come from?
Due to transpiration water is brought up from the roots and because hydrogen bonds form between water molecules, this creates a continuous, unbroken column which is known as cohesion
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Where does the tension part of the theory come from?
When the column of water created travels up the xylem due to transpiration (transpiration pull) - this puts the xylem under tension creating a negative pressure within the xylem
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Why does a tree trunks diamter decrease slightly during the day?
The change in diDuring the day transpiration rate is high so there is more tension (negative pressure) wthin the xylem and this causes the trees diameter to decrease
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How can the breaing of a xylem vessel support the cohesion-tension theory?
When it is broken and air enter it, the tree can no longer draw up water because the continuous column of water is broken so the water molecles can no longer stick together. Also water does no leak out, it is drawn in, so xylem must be under tension
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Why can't cells making up the xylem actively move the water?
Xylem is made up of dead cells
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Why doesn't xylem have end plates at intervals in its vessels?
This would break the continuous column of water hence it would not be able to be brought up by transpiration pull
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What form of energy is used to drive transpiration?
Heat energy from the sun
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Where components of the root does water need to travel through before reaching the xylem?
- Root hair cell - Cortex - Endodermis
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What two adaptations does a root have to absorb water and ions?
- Little projection coming off the root hair cell increase surface area - Many mitochondria to provide energy for active uptake of ions
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What are the two diffent pathways for water to travel across the cortex cells to the xylem?
- Cytoplasmic pathway - Cell wall pathway
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What mechanism drives the water down these pathways?
A water potential gradient
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Which pathway is quicker and why?
The cell wall pathway is faster due to there being less resistance in the cellulose cell wall as opposed to cytoplasm and cell membranes
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Why does the cell wall pathway eventually have to travel in the cytoplasm?
In the endodermis cell there is a waterproof barrier called the casparian ***** in the cell wall
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What is root pressure?
An active process by which the effcet of actve transport of ions into the endodermis cells creates a lower water potential, drawing water into these cells and this force can push water up th xylem
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Appoximately how far can root pressure push water up the xylem?
Only a few metres
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What experiment provides evidence for the existance of root pressure?
A young tree was cut at the base, and water was seen coming out of the stump. The pulling force (transpiration pull) had been removed, which suggests there must be pushing force
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What experiment provides evidence for root pressure being an active process?
The root was poisoned by respiratory inhibitors or was put in low temperatures (to stop respiration and production of ATP)- the effect was stopped, shows its an active process
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Other cards in this set

Card 2

Front

Why is transpiration a passive process?

Back

It doesn't use energy from ATP, rather from the sun

Card 3

Front

What four factors affect transpiration?

Back

Preview of the front of card 3

Card 4

Front

What effect does raising the temperature have to the rate of trasnpiration and why?

Back

Preview of the front of card 4

Card 5

Front

What effect will increasing the air humidity have to the rate of trasnpiration and why?

Back

Preview of the front of card 5
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