Download Osmotic, or Water Potential is simply a measure of the tendency for

Osmotic, or Water Potential is simply a
measure of the tendency for water to cross
a selectively permeable membrane from low
concentration to high concentration of
solute. (or from high to low concentration
of water)
Distilled water has the highest potential (zero).
When water has substances dissolved in it,
the water molecules have less potential to
move, and the osmotic potential is
NEGATIVE.
Water molecules always move from less
negative to more negative water potential.
(or higher potential, to lower potential)
Less Negative
More Negative
-200 MPa
-400 MPa
Remember…the “less” negative the number is, the greater
the osmotic/water potential is.
What is the highest possible osmotic
A
potential?
B
What is the only fluid to exhibit this
potential?
Which of these two fluids has the greatest
osmotic potential?
-14 MPa
-12 MPa
Which of these two fluids has the greatest
osmotic potential?
The osmotic potential of a cell is known as its
WATER POTENTIAL. For animal cells, the water
potential is the osmotic potential of the cytoplasm.
An animal blood cell with water
potential of –50 MPa is placed in a
solution…
Osmotic potential of the solution
is -20 MPa.
If the osmotic potential of the
solution is less negative than the
water potential of the cytoplasm
(the solution is hypotonic),
osmosis will occur, and water will
move into the cell from the
solution. This is known as
endosmosis.
-50 MPa
-20 MPa
The result will be haemolysis
(the blood cell will burst, or
“lyse”)
If the osmotic potential of
the solution is more
negative than the water
potential of the cytoplasm
in the animal blood cell,
(ie…the solution is
hypertonic), osmosis will
occur and water will move
OUT of the cell. This is
called exosmosis.
The result will be
crenation (the cell will
shrivel up)
-50 KPa
-80 KPa
Water potential of cytoplasm= -50
Osmotic potential of solution = -80
Water potential of cytoplasm= -50
Osmotic potential of solution= -50
If the osmotic
potential of the
solution is the same
as the water
potential of the
cytoplasm (the
solution is isotonic),
there will be no net
osmosis
The cell is in dynamic equilibrium with its environment, and
osmosis WILL occur, but no measureable difference on
either side of the membrane will be noticed.
What is haemolysis, and when is it likely to occur?
Describe the conditions under which exosmosis would
occur.
When water from a hypotonic environment moves into a
cell, what type of osmosis has occurred?
What happens when the cell’s cytoplasm has an osmotic
potential of -25, and the environment does as well?
What “state” is that cell and isotonic environment in?
In animal cells, the water potential is equal to the osmotic
potential of the cytoplasm, but this is different in plant
cells…
Plant cells have a cell wall, which exerts an inward
pressure when the cell is turgid. This is known as the
pressure potential. This must be considered when
calculating osmotic potential.
A plant cell with water potential
-50 KPa
–50 is placed in a solution which
has an osmotic potential of -20…
The solution is hypotonic, so net
endosmosis occurs and the cell
becomes fully turgid.
-20 KPa
Animal cells, without a cell wall,
would lyse, without that protection.
If the solution is
hypertonic, net
exosmosis occurs and
causes plasmolysis (the
cell membrane pulls
away from the cell wall.
The cell wall stays
intact).
-50 KPa
-80 KPa
Remember, animal cells will “crenate”, or
shrivel, when placed in a hypertonic
solution. This cell wall is very protective of
plant cells, and they remain safe in most
environments as a result!
If the solution in
which the plant cell is
placed is isotonic, no
net osmosis occurs.
The cell is not
plasmolysed, but it is
not fully turgid
either.
-50 KPa
-50 KPa
Why is osmotic potential different in plant cells
than it is in animal cells?
Which solution has higher osmotic potential?
When plant cells are placed in a solution with
higher osmotic potential, what would you expect
to occur?
-50 KPa
-40 KPa
What is crenation, and what happens to plant cells
instead of this?