Water uptake and movement up the stem
- Created by: Nicola Carter
- Created on: 04-01-13 11:47
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- Water uptake and movement up the stem
- Water uptake from the soil
- The outermost layer of cells (the epidermis) contains root hair cells that increase the surface area of the root.
- These cells absorb minerals from the soil by active transport using ATP for energy.
- The minerals reduce the water potential of the cell cytoplasm. Makes the water potential lower in the soil.
- Water is taken up across the plasma membrane by osmosis as the molecules move down the water potential graiden
- The outermost layer of cells (the epidermis) contains root hair cells that increase the surface area of the root.
- Movement across the root
- The movement of water across the root is driven by an active process that occurs at the endodermis.
- The endodermis is a layer of cells surrounding the xylem.
- Also known as the starch sheath as it contains granules of starch (a sign that energy is being used)
- The endodermis consists of special cells that have a waterproof ***** in some of their walls, called the Casparian *****
- The 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
- This decreases the water potential in the xylem. As a result, water moves from the cortex through the endodermal cells to the xylem by osmosis
- Reduces the water potential in the cells just outside the endodermis.
- This combined with water entering the root hair cells, creates a water potential gradient across the whole cortex.
- Therefore water is moved along the symplast pathway from the root hair cells, across the cortex and into the xylem
- This combined with water entering the root hair cells, creates a water potential gradient across the whole cortex.
- Reduces the water potential in the cells just outside the endodermis.
- This decreases the water potential in the xylem. As a result, water moves from the cortex through the endodermal cells to the xylem by osmosis
- At the same time, water can move through the apoplast pathway across the cortex.
- This water moves into the cells to join the symplast pathway just before passing through the endodermis.
- The movement of water across the root is driven by an active process that occurs at the endodermis.
- What is the role of the Casparian *****?
- The Casparian ***** blocks the apoplast pathway between the cortex and the xylem
- This ensures that water and nitrate ions have to pass into the cell cytoplasm through all cell membranes
- There are transporter proteins in the cell membranes.
- Nitrate ions are actively transported from the cytoplasm or the cortex cells into the xylem
- This lowers the water potential in the xylem so water from cortex cells follows into the xylem by osmosis
- Once the water has entered the xylem it cannot pass back into the cortex as the apoplast pathway of the endodermal cells is blocked
- How does water move up the stem?
- Root Pressure
- The action of the endodermis moving minerals into the xylem by active transport drives water into the xylem by osmosis
- This forces water into the xylem and pushes the water up the xylem.
- Root pressure can push water a few metres up a stem, but can't account for water getting to the top of tall trees
- Transpiration pull
- The loss of water by evaporation from the leaves must be replaced by water coming up from the xylem.
- Water molecules are attracted to each other by forces of cohesion.
- These cohesion forces are strong enough to hold the molecules together in a long chain or column
- As molecules are lost at the top of the column, the whole column is pulled up as one chain. Creates the transpiration stream
- These cohesion forces are strong enough to hold the molecules together in a long chain or column
- The pull from above can create tension in the column of water.
- This is why the xylem vessels must be strengthened by lignin.
- The lignin prevents the vessel from collapsing under tension
- Called the cohesion-tension theory
- Relies on the plant maintaining an unbroken column of water all the way up the xylem.
- This is why the xylem vessels must be strengthened by lignin.
- Capillary action
- The same forces that hold water molecules together also attract the water molecules to the sides of the xylem vessel. Called adhesion
- Because the xylem vessels are very narrow, these forces of attraction can pull water up the sides of the vessel
- Root Pressure
- How water leaves the leaf
- Most of the water that leaves the leaf exits through the stomata
- Only a tiny amounts leaves through the waxy cuticle.
- Water evaporates from the cells lining the cavity immediately below the guard cells
- This lowers the water potential in these cells, causing water to enter them by osmosis from neighbouring cells.
- Water then enters these neighbouring cells from deeper in the leaf, and so on until eventually 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 neighbouring cells.
- Water uptake from the soil
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