Plant cells and water

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  • Plant cells and water
    • Water potential
      • Water  potential is a measure of the tendency of water molecules to diffuse from one place to another.
      • Water always moves from a region of higher water potential to a region of lower water potential.
      • The water potential of pure water is zero.
      • In a plant cell, the cytoplasm contains salts and sugars that will reduce water potential. This is because there are fewer free water molecules available than in pure water.
        • As a result, the water potential in plant cells is always negative
      • If you place a plant cell in pure water, it will take up water molecules by osmosis. This is because the water potential in the cell is lower than the water potential of the water.
        • But the cell will not continue to absorb water until it bursts. This is because of the strong cellulose cell wall.
          • Once the cell is full of water it is described as being turgid. The water inside the cell starts to exert pressure on the cell wall, called the pressure potential. As the pressure potential builds up it reduces the influx of water
      • If plant cell is placed in a solution with a very low water potential, it will lose water by osmosis.
        • This is because the water potential of the cell is higher than the water potential of the solution. So water diffuses down its potential gradient out of the cell
          • As water loss continues, the cytoplasm and vacuole shrink. Eventually the cytoplasm no longer pushes against the cell wall. Called incipient plasmolysis
            • If water continues to leave the cell, the plasma membrane will lose contact with the wall, known as plasmolysis.
    • How does water move between cells?
      • When plant cells are touching each other, water molecules can pass from one cell to another.
      • The water molecules will move from the cell with the higher water potential (less negative) to the cell with the lower water potential (more negative)
    • What route can water take between cells?
      • Apoplast pathway
        • The cellulose cell walls have many water-filled spaces between the cellulose molecules.
        • Water can move through these spaces and between the cells.
        • The water doesn't pass through any plasma membranes.
          • This means that dissolved mineral ions and salts can be carried with the water
      • Symplast pathway
        • Water enters the cell cytoplasm through the plasma membrane.
        • It can then pass through the plasmodesmata from one cell to the next.
          • The plasmodesmata are gaps in the cell wall that contain a thin strand of cytoplasm.
          • This means that cytoplasm of adjacent cell is linked. Once inside the cytoplasm, water can move through the continuous cytoplasm from cell to cell
      • Vacuolar pathway
        • Similar to the symplast pathway
        • The water is not confined to the cytoplasm of the cells. It is able to enter and pass through the vacuoles as well


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