Water Uptake and movement up the Stem
- Created by: Eleanor Izzard
- Created on: 31-12-12 10:26
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- Water Uptake & Movement up the Stem
- From the Soil
- the outermost layer of cells in a plant root contain root hair cells that increase the surface area of the root
- root hair cells absorb minerals from the soil by active transport using ATP for energy
- they reduce the water potential of the cell cytoplasm, making it lower than the water potential of the soil, so water is taken up across the plasma membrane by osmosis
- root hair cells absorb minerals from the soil by active transport using ATP for energy
- the outermost layer of cells in a plant root contain root hair cells that increase the surface area of the root
- Movement across the root
- the endodermis cells move minerals by active transport from the cortex into the Xylem
- this decrease the water potential of the xylem, causing water to move into it
- this reduces the water potential in the cells just outside the endodermis, creating a water potential gradient across the whole cortex (due to the water entering the root hair cells)
- water is moved along the symplast pathway from the root hair cells, across the cortex and into the Xylem
- this reduces the water potential in the cells just outside the endodermis, creating a water potential gradient across the whole cortex (due to the water entering the root hair cells)
- this decrease the water potential of the xylem, causing water to move into it
- driven by an active process occurring in the endodermis (a layer of cells around the Xylem)
- known as the starch sheath as it contains granules of starch - a sign that energy is being used
- consists of special cells that have a waterproof ***** in some of their walls (Casparian *****)
- ***** blocks the apoplast pathway, forcing water into the symplast pathway
- the endodermis cells move minerals by active transport from the cortex into the Xylem
- Casparian *****
- blocks the apoplast pathway between the cortex and the Xylem, ensuring that the water and nitrate ions have to pass into the cytoplasm through cell membranes
- there are transporter proteins in the cell membranes, allowing nitrite ions to be actively transported from the cytoplasm of the cortex into the Xylem
- lowering the water potential in the Xylem so water moves from the cortex in to the Xylem via osmosis
- the water cannot pass back into the cortex because the apoplast pathway of the endodermal cells is blocked
- lowering the water potential in the Xylem so water moves from the cortex in to the Xylem via osmosis
- there are transporter proteins in the cell membranes, allowing nitrite ions to be actively transported from the cytoplasm of the cortex into the Xylem
- blocks the apoplast pathway between the cortex and the Xylem, ensuring that the water and nitrate ions have to pass into the cytoplasm through cell membranes
- Up the Stem
- Transpiration Pull
- Water molecules are attracted to eachother by forces of cohesion - forces strong enough to hold molecules together in a long column
- as molecules are lost at the top of the column, the whole thing is pulled up 1 chain, creating a Transpiration Stream
- the pull from above can create tension in the column of water, lignin must be used to strengthen the Xylem Vessels and stop them from collapsing under the tension
- as molecules are lost at the top of the column, the whole thing is pulled up 1 chain, creating a Transpiration Stream
- Cohesion-Tension theory relies on the plant maintaining and unbroken column of water all the way up the Xylem
- if the column is broken in one vessel, it can be maintained through another vessel via pits
- Water molecules are attracted to eachother by forces of cohesion - forces strong enough to hold molecules together in a long column
- Root Pressure
- the action of the endodermis moving minerals into the Xylem by active transport drives water into the Xylem by osmosis
- forcing the water into the Xylem pushes it up the Xylem, up to a few metres upwards (not far enough to get to the top of a tree)
- the action of the endodermis moving minerals into the Xylem by active transport drives water into the Xylem by osmosis
- Capillary Action
- the forces that hold water molecules together also attract the water molecules to the sides of the Xylem vessel (ADHESION)
- due to the narrow vessels, these forces can pull the water up the sides of the vessel
- the forces that hold water molecules together also attract the water molecules to the sides of the Xylem vessel (ADHESION)
- Transpiration Pull
- Leaving the Leaf
- most water leaves through the Stomata, tiny pores in the epidermis
- water evaporates from the cells lining the cavity immediately below the gaurd cells
- this lowers the water potential in these cells, causing water to enter them by osmosis from neighboring cells
- water enters the neighbouring cells from deeper in the leaf and so on until water leaves the Xylem and enters the innermost leaf cells
- this lowers the water potential in these cells, causing water to enter them by osmosis from neighboring cells
- water evaporates from the cells lining the cavity immediately below the gaurd cells
- a tiny amount leaves through the waxy cuticle
- most water leaves through the Stomata, tiny pores in the epidermis
- From the Soil
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