2. Osmosis is a special case of diffusion
Osmosis involves the diffusion of water
through a membrane
The membrane may be artificial and
non-living e.g. Cellophane
In biology, the important membrane is the
cell membrane
2
3. If a concentrated solution is separated from
a dilute solution by a suitable membrane,
water will pass from the dilute to the
concentrated solution.
The membrane must allow water molecules
to diffuse through. It is permeable to water.
In fact, water passes both ways but faster
from the dilute to the concentrated solution.
Permeability
3
5. There are microscopic pores in the membrane.
Molecules below a certain size can diffuse
through the pores.
Water molecules can easily diffuse through
the pores.
In the next slides represents a water molecule
and represents a sugar molecule
5
6. membrane
water sugar solution
There are as many water molecules on the right as
there are on the left but many of them are attached to
sugar molecules and are not free to move.
6
7. Because there are more freely moving water molecules on
the left, more diffuse through the pores of the membrane from
left to right than from right to left.
7
9. Because the membrane allows only molecules
of a certain size to diffuse through it, it is
called selectively permeable.
The cell membrane functions as a selectively
permeable membrane.
The cell sap and cytoplasm function as
fairly concentrated solutions.
9
10. Question 1
In osmosis, water will diffuse through a membrane
(a) From water to a concentrated solution
(b) From a concentrated solution to a dilute
solution
(c) From a dilute solution to a concentrated
solution
(d) From a dilute solution to a more
dilute solution
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11. Question 2
Which statements are correct ?
(a) The plant cell wall is permeable to water
(b) The plant cell wall is selectively permeable
(c) The cell membrane is selectively permeable
(d) The vacuole contains a solution of salts and
sugars
27
12. Question 3
concentrated solution
dilute solution
Cell sap
more dilute solution
b
a
c
Water will tend to move by osmosis
(a) from b to a and c, and from c to a
(b) from c to a and b, and from a to b
(c) From a to b and c, and from c to b
(d) From c to b and a, and from b to c
Plant cells
28
Editor's Notes
In the left hand picture the plant is wilting from lack of water. On the right it has recovered largely due to the process of osmosis. The presentation explains how this happens
Sugar molecules can pass through the membrane but, being surrounded by a cloud of water molecules, they move more slowly.
