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Movement through membranes

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					 Movement through
    membranes
OSMOSIS UNIT B2. 1.4
             OSMOSIS
• A special kind
  of diffusion
• Involving water
  moving through
  a cell
  membrane
           DEFINITION

• OSMOSIS IS: the movement of water
  molecules
• from a region of high water
  concentration (dilute solution)
• to low water concentration
 (concentrated solution)
• through a partially permeable
  membrane
                                          Solute molecule
• Explanation       Water molecule

  of osmosis
• Water
  molecules are
  small enough
  to pass freely
  through the
  membrane
• Solute
  molecules are
  too big to pass
  through the
  pores in this
  membrane              Dilute solution   Concentrated solution
     Concentration gradient
• There are more
  water molecules on
  the left of the
  membrane than on
  the right
• So there is a
  concentration
  gradient that they
  travel down
• WHY?
        OSMOSIS in a model cell (1)
 Visking tubing is partially permeable, like a cell surface membrane




                           Low water potential




                          High water potential

                                                            H2O




Water moves INTO the bag by osmosis, moving from a high to a low water
potential.
         OSMOSIS in a model cell (2)
If the bag is filled with water and is placed in a strong sugar solution, water
moves out of the bag and into the beaker.




                                                  H2O




Water moves out of the bag, moving down a water potential gradient.
     Are all solute molecules unable to pass
              through membranes?

• No!
• If there is a solution
  of sugar and starch
  in water
• Water passes
  through easily by
  osmosis
• Sugar passes through
  slowly by diffusion
• But starch is too big
  to get through at all.
EXPLANATION (1)
EXPLANATION (2)
   OSMOSIS IN ANIMAL CELLS.
• Animal cells are surrounded by
  a partially permeable
  membrane
• Which lets only certain sized
  particles through
• Cells use up water in chemical
  reactions, so the cytoplasm
  becomes more concentrated.
• Water enters the cell
• Some reactions produce water,
  and dilute the cytoplasm.
• Water leaves the cell.
 HOW DO ANIMAL CELLS RESPOND?

• E.g. red blood cells.
• If placed in pure
  water or more dilute
  solution than the cell
  contents
• Water passes into the
  cells by osmosis
• They swell up as they
  fill with water
• Until they eventually
  burst
     HOW DO ANIMAL CELLS RESPOND?


• If the cells are placed
  in a concentrated
  solution (stronger than
  the cell contents)
• Water leaves the cells
  by osmosis
• The cytoplasm becomes
  too concentrated
• The cell shrivels up
  Importance of controlling
   internal fluids in animals
• If the concentration of fluids surrounding
  cells is the same as that inside their
  cytoplasm
• Water will pass in and out of the cell in
  equal amounts
• And all cell reactions can go on as normal
• Keeping the concentration of blood and
  tissue fluids constant is a process called
  HOMEOSTASIS.
    WHAT ABOUT PLANT CELLS?
                                           Cell wall-
                                           fully
                                           permeable

• The           nucleus
  presence                                Cell surface
  of the cell   cytoplasm                 membrane -
                                          partially
  wall makes                              permeable

  a big         vacuole

  difference                           CONCENTRATED
                                       SOLUTION
                                       OUTSIDE CELL


                            WATER IN
               TURGID CELLS
• Water moves into cells by
  osmosis, if their surrounding
  solution is more dilute
• This makes them swell
• Pressure builds up so no more
  water can enter.
• The cells are hard and rigid
  (TURGID)
• If a plant’s cells are in this
  swollen state, it’s stems and
  leaves will also be firm and
  supported.
    Plant Cells in
Concentrated Solutions
• If water leaves the cells by
  osmosis
• they become limp (flaccid)
  and give no support
• Eventually the cytoplasm
  shrinks away from the wall
• Surrounded by the cell
  membrane
• The cells are plasmolysed
• And the plant will wilt.
• As long as plants do not go too long
  without water
• They will recover and become upright
  and firm again

				
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posted:7/16/2012
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