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Osmosis is defined as: the passage of water from a region where it has a higher water potential to a region where it has a lower water potential through a selectively
permeable membrane
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Solutions and Water Potential:

A solute is any substance that is dissolved in a solvent (water)
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Water potential is measured in units of pressure
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The highest value of water potential (pure water) is 0
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The more negative the value = the lower the water potential
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If a red blood cell is placed in pure water, it will
absorb water by osmosis because it has a lower water potential
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The membrane will therefore break, bursting
the cell and releasing its contents
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(blood plasma in this case)
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the solution on the left has a low concentration of solute molecules
while the solution on the right has a high concentration of solute
molecules
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Both the solute and water molecules are in random motion due to
their kinetic energy
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The selectively permeable plasma membrane, however, only allows
water molecules across it and not solute molecules
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The water molecules diffuse from the left hand side, which has the
higher water potential, to the right hand side which has a lower water potential = down a water potential gradient
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At the point where the water potentials on either side of the membrane are equal, a dynamic equilibrium is established and there is no
net movement of water
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When placed in
pure water, they also absorb water by osmosis because of their
lower (more negative) water potential
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Water entering a plant cell by osmosis causes the protoplast to
swell and press on the cell wall
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In this situation, the protoplast
of the cell is kept pushed against the cell wall the cell is said to be
turgid
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The volume of
the cell decreases
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The cell is said to be incipient plasmolysis
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Net movement of water

Enters cell

Neither enters or
leaves

Leaves cell

protoplast

Swells

No change

shrinks

Condition of cell

turgid

Incipient plasmolysis

plasmolysed



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