Biology: Unit 3 Osmosis

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  • Created by: Nat Meier
  • Created on: 05-05-13 11:35
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  • Biology-Osmosis
    • Osmosis is like diffusion
      • Osmosis is a type of diffusion.
        • The passive movement of water particles from high water concentration to low water concentration
    • It is the movement of water molecules across a partially permeable membrane
      • It is from a region of high water concentration to a region of low water concentration
    • Partially permeable membranes are membranes with very small holes in
      • The holes are so small only tiny molecules like water can pass through but bigger molecules like sucrose can't
    • Water molecules can pass both ways through the membrane during osmosis
      • This is because water molecules move about randomly all the time
    • Because there are more water molecules on one side compared to the other, there is a steady net flow of water into the side with less water molecules, i.e. into the stronger sugar solution.
      • This means the stronger sugar solution gets more dilute.
        • The water acts like it is trying to 'even up' the concentration either side of the membrane.
    • Water moves in and out of cells by osmosis
      • Tissue fluid surrounds the cells in the body
        • Tissue fluid is water with oxygen, glucose and other things dissolved in it
          • The tissue fluid usually has a different concentration to the fluid inside a cell. This means that water will move into the cell from the tissue fluid, or, out of the cell. This happens because of osmosis
            • If a cell doesn't have enough water, the solution inside will become rather concentrated. This means the solution outside the cell is more dilute, and so water will move into the cell by osmosis
              • If a cell has a lot of water inside it will be more dilute, so water will be drawn out of the cell and into the fluid outside by osmosis
    • An experiment you can do to show osmosis is as follows...
      • First cut up a potato into identical cylinders and get some beakers with different sugar solutions in them. One beaker should be full of pure water and the other should be full of a very concentrated sugar solution. Then you can have some more beakers full of the different concentrations in between these two extremes.
        • Measure the lengths of the potato cylinders and then leave a few pieces of potato in the different beakers for about half an hour
          • Then take the potato out and measure the lengths of them again. If the potato has drawn in water by osmosis they will be a bit longer
            • If water has been drawn out they will have gotten a bit smaller. After you have colllected this data you can then plot the graphs
        • The dependent variable is the potato length and the independent variable is the concentration of the sugar solution.
        • The variables such as the volume of solution, temperature, time, type of sugar used etc.. must be kept the same in each case or the experimentwould not be a fair test.
    • This is why it is bad to drink sea water
      • The high salt content means you end up with a much lower water concentration in your blood and tissue fluid than in your cells
        • All the water is sucked from your cells by osmosis and then they shrivel and die
  • Osmosis is like diffusion
    • Osmosis is a type of diffusion.
      • The passive movement of water particles from high water concentration to low water concentration
  • First cut up a potato into identical cylinders and get some beakers with different sugar solutions in them. One beaker should be full of pure water and the other should be full of a very concentrated sugar solution. Then you can have some more beakers full of the different concentrations in between these two extremes.
    • Measure the lengths of the potato cylinders and then leave a few pieces of potato in the different beakers for about half an hour
      • Then take the potato out and measure the lengths of them again. If the potato has drawn in water by osmosis they will be a bit longer
        • If water has been drawn out they will have gotten a bit smaller. After you have colllected this data you can then plot the graphs
    • The dependent variable is the potato length and the independent variable is the concentration of the sugar solution.
    • The variables such as the volume of solution, temperature, time, type of sugar used etc.. must be kept the same in each case or the experimentwould not be a fair test.

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