Diffusion and OsmosisUnit 3

Diffusion

• Let’s look at the movement of itsy bitsy particles that move in liquids or gasses. Sometimes even moving through the membrane of cells.

• This is called DIFFUSION!

Real life scenario!

• Your aunt May comes to visit. • A few minutes after she enters the house can

smell her perfume as you hear her talking to your mother in the kitchen.

• The gasses from your aunts perfume were in a high concentration in one area. However, after the concentrated source of the perfume particles entered they began to spread out to be evenly dispersed.

When water molecules do this is it called Osmosis.

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. a Coffee filter

In biology, the important membrane is the cell membrane

2

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.

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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Next slide

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Because the membrane allows only molecules of a certain size to diffuse through it, it is called semi or selectively permeable.

The cell membrane functions as a selectivelypermeable membrane.

The cell sap and cytoplasm function as fairly concentrated solutions.

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The scale of the following drawings is greatly distorted.

Even if the cells were as large as they appear on the screen, the molecules would still be invisible particles

10Scale of drawings

There is a greater concentration of free water molecules outside the cell than inside

so water diffuses into the cellby osmosis

and the cell swells up

Osmosis in animal cells 11

If osmosis continued the animal cell would burst

This would be bad news for animals

Consequently there are processes in the animal’s body which control osmosis

Mainly, this is done by keeping the concentration of body fluids outside the cellthe same as it is inside

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In fresh water fish, the blood is more concentrated than the surrounding water

As a result the fish takes in water by osmosis

Not so much through the skin, which is thick,but through the gills which have a very thin membrane

Trout

Fresh water fish 14

The fish gets rid of the excess water by means of its kidneys

Perch

gills absorb water by osmosis

blood carries waterto kidneys

kidneys expel excess water ...

...in the form of dilute urineheart

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Sea water is a more concentrated solution than the fish’s blood. What will be the osmotic effect ?

A sea water fish will lose water by osmosis through its gills.

The fish drinks sea water and the gills expel the excess salt from the blood, so keeping itsconcentration constant

Herring

Sea water fish 15

In a plant cell, the cell membrane acts as a selectively permeable membrane

The vacuole contains a solution of salts and sugars

If there is water outside the cell, it will diffuseby osmosis into the vacuole

The cell wall is freely permeable to water

The vacuole will expand, pushing the cytoplasmoutwards against the cell wall

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The cellulose cell wall cannot stretch, so the vacuole cannot continue to expand

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

A plant cell in this condition is said to be turgid

If all the cells in the plant are turgid, the plant willbe firm and upright and the leaves would beexpanded

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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 19

What happens in osmosis?a. movement of solute molecules against their concentration gradientb. movement of solute molecules down their concentration gradientc. movement of water molecules against their concentration gradientd. movement of water molecules down their concentration gradient

Osmosis between cells

If the concentration of the cell sap is greater in one cell than in its neighbour, water will pass by osmosis from the less concentrated to the more concentrated.

cell sap more concentrated

cell sap less concentrated

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These cells are short of water; the tissue is limp and the plant is wilting

The cells have taken upwater by osmosis; thecells are turgid and thetissue is firm

Limp and turgid tissue 21

22(1) wilting

(2) recovering (3) recovered

In the growing regions of a plant, the cell walls arenot inextensible but are still ‘plastic’

When the vacuole takes in water by osmosis it expands, pushing out on the ‘plastic’ cell walland increasing the size of the cell.

Cell division at a growing point increases the number of cells but cell expansion causes the increase in size.

Cell division followed by cell expansion causesgrowth

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1

1 The cell has just divided

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2 Vacuoles start to form in one cell

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3 The vacuoles absorb water by osmosis and start to extend the cell

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4 The vacuoles begin to join up and continue to absorb water and extend the cell

Cell extension 24

these cells will divide

vacuolesforming

cells absorb water by osmosis and expand

cell divisioncontinues

Growth in a shoot tip 25

Osmosis or not?

• My stomach absorbing the water I just drank?

• A paper towel absorbing the mess I just made?

• Our little Hibiscus absorbing water through its roots?

• Excess water leaving your blood and passing to your kidney?

• Water spreading it self evenly on my patio as I hose it down?