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OSMOSIS
Osmosis is the movement of 1 molecules from a 2 concentration of water
molecules ( 3
solution) into a region of 4
concentration of water molecules
(
5
solution) through a
6
permeable membrane. Therefore
osmosis is simply another example of
7
but the term is specifically used for
the movement of
8
molecules.
In pure water the water molecules move about
9
within the volume of
the liquid. This ability to move is known as the
10
(  - psi) and
for pure water it is given a value of
11
. If any
12
molecules are dissolved in
the water then they bind to the water molecules and reduce their ability to move about.
The more solute present (ie. more
13
the solution) then the more
14
the
15
(WP) due to the interference with the movement of the water
molecules. The whole solution therefore has a lower
16
ie. more
negative value of the WP.
The contents of the cell cytoplasm are a complex mixture of dissolved solutes,
giving a certain water potential. If the cell is placed in a solution of equal concentration
then as much water will diffuse in as out and the two solutions are
17
.
If a cell is placed in a more dilute solution then the solution outside the cell is
18
the cell contents and water will pass
19
the cell by
20
since the cell
contents have a
21
WP.
Placing a cell in a more concentrated solution ie.
22
23 by osmosis since the surrounding solution now has the
) water potential.
Ref. Bailey and Hirst pg. 30 for diagrams.
to
will cause water to pass
24
(more negative
The
25
of water movement depends upon the WP of the cell contents
and the bathing solution. The
26
of movement depends upon the numerical
difference of the water potentials ie. the diffusion (osmotic)
27
.
Blood cells placed in water will
28
as water flows into them and they soon
burst. When in
29
solution animal cells
30
as water flows
31
by osmosis.
In plant cells the movement of water into cells is also influenced by the presence
of the
32
cell wall. This is fully
33
but, as the cytoplasm
swells against it , the wall presses
34
to create a constraining
pressure potential. This is always a positive value and so squeezes the cell and
influences the rate at which water can enter the cell.
As water flows into a plant cell placed in a
35
solution the cytoplasm
and cell vacuole increase in volume pushing outwards against the cell wall. The
latter stretches and applies a
36
. Eventually the cell wall
will stretch no more and the cell is now fully
37
. The water potential of
the cell is now
38
because the pressure potential of the wall is just
39
to the water potential of the solution within the cell.
If a plant cell is placed in a hypertonic solution (lower WP), then water will flow
40
by osmosis. The cytoplasm and vacuole will
41
in volume,
shrinking the cell contents away from the cell wall.
42
drops
to zero and the cytoplasm pulls away from the cell wall – this cell is now said to be
showing
43
.
Ref. Bailey and Hirst p.32