Why can't plants absorb water over their general body surface?
Because they are terrestrial organisms that need to conserve water and so are covered by a waterproof layer
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Where is the exchange surface located?
In the soil
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What is the exchange surface for a plant?
The root hairs
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What are root hair cells responsible for?
The absorption of water and mineral ions
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How do plants lose water?
Transpiration
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What is a root hair?
A long, thin extension of a root epidermal cell
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Why are root hairs efficient surfaces for gas exchange?
They provide a large surface area as they are long extensions and occur in thousands and they have a thin surface layer across which materials can move easily
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What is the water potential of the soil surrounding the root hairs?
High
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What is the water potential of the root hairs?
Low
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How does water move?
By osmosis from the soil (area of high water potential) into the root hairs (area of low water potential)
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Name the 2 ways in which water can move across the root
The apoplastic pathway and the symplastic pathway
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Where does water travel if it is using the apoplastic pathway?
Along the cell walls and through the spaces between cells
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What happens as more water is drawn into endodermal cells?
It pulls more water along behind it due to the cohesive properties of the water molecules
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What does this movement of water create?
A tension that draws water along the cells walls of the cells of the root cortex
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What are the cell walls made of?
Cellulose
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What is the resistance like?
Little or no resistance
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What prevents the water from moving along the apoplastic pathway?
The waterproof casparian ***** located at the endodermis
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What happens to the water once it reaches the casparian *****?
Water has to take the symplastic pathway
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Where does the symplastic pathway take place?
Across the cytoplasm of the cells
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How does water pass through the cell walls?
Along tiny opening called plasmodesmata
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What is each plasmodesmata filled with?
A thin strand of cytoplasm
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By which process does water move by?
Osmosis
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What effect does water have on the water potential of the root hair cell?
Increases the water potential of the root hair cell so it has a higher water potential than the first cell in the cortex
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Where does water move to next?
Into the first cell in the cortex by osmosis as it has a lower water potential
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What effect does this have on previous cells?
Lowers the water potential so that water enters by osmosis from the root hair cell
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How does water get into the xylem?
Active transport of salts
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What cells actively transport salts into the xylem?
Endodermal cells
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What does this process require?
Energy
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Where can this process take place?
In living cells
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Where does it take place in cells?
Carrier proteins in the cell-surface membrane
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If water is to enter the xylem what must it first enter?
The cytoplasm of endodermal cells
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How is the water potential in the xylem lowered?
By active transport of mineral ions into the xylem by the endodermal cells
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How does water now move?
By osmosis into the xylem along the water potential gradient
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What force is created as a result of this?
Root pressure
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Explain one thing that is evidence of root pressure due to active transport of salts into the xylem
The pressure increases with a rise in temperature and decreases at lower temperatures
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Explain another point that is evidence of root pressure due to active transport of salts into the xylem
A decrease in the availability of oxygen or respiratory substances
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