Osmosis

Osmosis

• Osmosis is a special case of diffusion• Osmosis involves the diffusion of

water through a membrane• Water moves from a high

concentration of water to a low concentration of water

Osmosis Example

dilute solutionconcentratedsolution

membrane

More water passes fromdilute to concentrated ...

...until concentrationsbecome equal

level riseslevel falls

• There are microscopic pores in the membrane.

• Molecules below a certain size can diffuse through the pores

• Water molecules can easily diffuse though the pores

In the next slides represents a water moleculeand

represents a sugar molecule

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.

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

• If there is a greater concentration of free water molecules outside the cell than inside

• Then water will diffuse into the cell by osmosis

• The cell will swell up

Osmosis and animal cells

• If osmosis continued the animal cell would burst

• This would be bad news for animals

Osmosis and animal cells

• In this picture a red blood cell is put in a glass of distilled water. Because there is a higher concentration of water outside the cell, water enters the cell by OSMOSIS.  In this case too much water enters and the cell swells to the point of bursting open. 

Osmosis and plant cells• In a plant cell, the cell membrane

acts as a selectively permeable membrane

• The cell wall is freely permeable to water

• The vacuole contains a solution of salts and sugars

• If there is water outside the cell, it will diffuse by osmosis into the vacuole

• The vacuole will expand, pushing the cytoplasm outwards against the cell wall

Osmosis and plant cells• The cellulose cell wall cannot stretch,

so the vacuole cannot continue to expand

• But the pressure of the vacuole against the cell makes the cell very firm

• A plant cell in this condition is said to be turgid

cell wall cytoplasm and cell membrane

vacuole

The cell absorbs waterby osmosis ....

....but the cell wall stops the cell expanding any more

Plant cells 18

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

concentrated solution

dilute solution

Cell sap

more dilute solutionb

ac

Water will tend to move by osmosis

(a) from b to a and c

(b) from c to a and b

(c) From a to b and c

(d) From c to b and a

Plant cells

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Tonicity• The measurement of osmotic

pressure preventing the inward flow of water across the semipermeable membrane

• Solute: A solute is a substance dissolved in another substances

e.g. Salt• Solvent: A substance that dissolves

a solutee.g. Water

Tonicity is a relative term

• Hypotonic Solution - One solution has a lower concentration of solute (e.g salt) than the other

• Hypertonic Solution - One solution has a higher concentration of solute (e.g. salt) than the other

• Isotonic Solution - Both solutions have same concentrations of solute

Plant and Animal Cells put into various solutions