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Unit 3 - Osmosis

Osmosis refers specifically to the movement of water molecules through partially permeable membranes, and in this GCSE Biology quiz we take a close look at how the process works.

Partially permeable membranes allow water molecules to pass through them in both directions, and this is how osmosis occurs.. If there are solutions of different concentrations on opposite sides of the membrane, it is important that you realise that water molecules are passing through in both directions. It is a special case of diffusion as it involves only water molecules and a partially permeable membrane. To understand it properly involves your knowledge of the particle theory of matter (kinetic theory - atoms and molecules are constantly in motion).

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Because there are more water molecules in a less concentrated solution than a more concentrated solution, more of them will be crossing through the membrane at any given time. The result of this is that the more concentrated solution will gain water molecules from the less concentrated one. You have probably seen this happening in lessons at school where a concentrated sugar solution in placed into a tube made from a partially permeable membrane or by placing a piece of potato into a glucose solution. The level of the concentrated sugar solution rises as it gains water molecules whereas the piece of potato shrinks since it loses water to the glucose solution surrounding it.

Eventually the concentrations will become equal. At this point, there will be the same number of water molecules passing through the partially permeable membrane in both directions. Equilibrium has been reached and so in practical terms, osmosis stops as there is no further change in concentration, but you must remember that the water molecules are still passing through the cell membranes.

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  1. Osmosis takes place in which of the following?
    Water is taken up by root hair cells from the soil as there is a concentration gradient from soil to the root hair cell
  2. Osmosis takes place in this important structure.
    Water is reabsorbed by osmosis in the nephrons of the kidneys, keeping the blood plasma at the right concentration to avoid the lysis (bursting) of the red blood cells in particular
  3. Potato cylinders are put into a strong sugar solution. After an hour they are...
    Water leaves the potato cells by osmosis, so the potato cylinders shrink in size due to water loss
  4. Osmosis is the movement of which molecules?
    Osmosis is only about water molecules
  5. Water molecules move...
    Along the gradient is always marked as incorrect in exams
  6. Water molecules move from...
    It is a special case of diffusion as it involves a movement of only water particles from a higher concentration to a lower concentration across a partially permeable membrane
  7. Dilute means...
    Dilute means weak and watery. A small amount of solute, e.g. salt, dissolved in a lot of water
  8. In living organisms, the partially permeable membrane is the...
    This contains the cytoplasm and when there is a concentration gradient between the outside of the cell and the cytoplasm, osmosis occurs, restoring the equilibrium
  9. Why is osmosis not the same thing as diffusion?
    True diffusion occurs when two substances are in direct contact with one another, here they are physically separated by the partially permeable membrane
  10. Plants use osmosis to prevent...
    Plant cells become flaccid when they are short of water. Osmosis makes them turgid once more

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