Unit 1 Section 6.2 Water Transport

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  • Water Transport
    • how does water enter a plant
      • water enters through root hair cells and then passes through the root cortex including the endodermis, to reach the xylem
      • water always moves from areas of higher water potential to areas of lower water potential
        • it goes down a water potential gradient
          • the soil around roots generally has high water potential and leaves have a lower water potential
            • this creates a water potential gradient that keeps water moving through the plant in the right direction from roots to leaves
    • water transport through the root
      • water travels through the roots via the root cortex into the xylem by two different pathways
        • the Symplast Pathway
          • the Symplast Pathway goes through the living parts of cells - the cytoplasm. the cytoplasm of neighbouring cells connect through plasmodesmata
        • The Apoplast Pathway
          • goes through the non-living parts of the cells - the cell walls.
            • the walls are very absorbent and water can simply diffuse through them, as well as passing through the spaces between them
          • when water in the apoplast pathway gets to the endodermis cells in the root, its path is blocked by a waxy strip in the cell walls, called the casparian strip
            • now the water has to take the symplast pathway
              • this is useul, because it means the water has to go through a cell membrane
                • cell membranes are able to control whether or not substances in the waster get through
                  • once past this barrier, the water moves into the xylem
    • water transport through the leaves
      • xylem vessels transport the water all around the plant
        • at the leaves, water leaves the xylem and moves into the cells mainly by the apoplast pathway
          • water evaporates from the cell walls into the spaces between cells in the leaf
            • when the stomata open, water moves out of the leaf as vapour into the surrounding air
              • the loss of water vapour from a plants surface is called transpiration
    • water movement up a plant
      • the movement of water from roots to leaves is called the transpiration stream
        • The mechanisms that move the water include cohesion, tension and adhesion
          • Cohesion and Tension
            • cohesion and tension help water move up plants, from roots to leaves, against the force of gravity
              • 1. water evaporates from the leaves at the 'top' of the xylem
                • 2. this creates a tension which pulls more water into the leaf
                  • 3. water molecules are cohesive so when some are pulled into the leaf others follow. this means the whole column of water in the xylem, from the leaves down to the roots, move upwards
                    • 4. water enters the stem through the root cortex cells
          • adhesion
            • is also partly responsible for the movement of water. as well as being attracted to each other, water molecules are attracted to the walls of the xylem vessels. this helps water to rise up through the xylem vessles


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