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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

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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.

Permeability3

Osmosis

water ordilute solution

concentratedsolution

membrane

More water passes fromdilute to concentrated ...

...until concentrationsbecome equal

level riseslevel falls

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There are microscopic pores in the membrane.

Molecules below a certain size can diffusethrough the pores.

Water molecules can easily diffuse throughthe pores.

In the next slides represents a water molecule

and represents a sugar molecule

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membranewater 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.

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Because there are more freely moving water molecules onthe left, more diffuse through the pores of the membrane from left to right than from right to left.

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Molecular movement

Next slide

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Animation

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 selectivelypermeable membrane.

The cell sap and cytoplasm function as fairly concentrated solutions.

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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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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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Question 3

concentrated solution

dilute solution

Cell sap

more dilute solutionb

ac

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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