Osmosis is the diffusion of water through a semipermeable membrane from a region of higher water concentration to lower water concentration. If a concentrated solution is separated from a dilute solution by a membrane that allows water but not other molecules to pass through, water will diffuse through the membrane from the dilute to the concentrated solution until concentrations equalize. Cell membranes are selectively permeable, allowing water and some molecules to diffuse across while blocking other molecules, regulating the movement of water and substances into and out of cells.

1. 1

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

4. Osmosis
water or
dilute solution
concentrated
solution
membrane
More water passes from
dilute to concentrated ...
...until concentrations
become equal
level riseslevel falls
4

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.
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8. Next slide
8

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
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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
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