Osmosis 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

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

Permeability The membrane must allow water molecules to diffuse through. It is permeable to water. If a concentrated solution is separated from a dilute solution by a suitable membrane, water will pass from the dilute to the concentrated solution. In fact, water passes both ways but faster from the dilute to the concentrated solution.

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

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

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.

Because there are more freely moving water molecules on Sugar molecules can pass through the membrane but, being surrounded by a cloud of water molecules, they move more slowly. 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. Molecular movement

Next slide 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 selectively permeable membrane. The cell sap and cytoplasm function as fairly concentrated solutions.

Solution Differences & Cells solvent + solute = solution Cell in a Hypotonic solution Solutes in cell more than outside Outside solvent will flow into cell In a Isotonic Solutes equal inside & out of cell In a Hypertonic Solutes greater outside cell Fluid will flow out of cell

Scale of drawings 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

Osmosis in animal cells so water diffuses into the cell by osmosis There is a greater concentration of free water molecules outside the cell than inside and the cell swells up

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 cell the same as it is inside

As a result the fish takes in water by osmosis Fresh water fish Trout 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

Perch The fish gets rid of the excess water by means of its kidneys kidneys expel excess water ... blood carries water to kidneys gills absorb water by osmosis ...in the form of dilute urine heart

Sea water is a more concentrated solution than the Sea water fish Herring 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. Sea water fish related to sharks, e.g. dogfish, avoid losing water to their surroundings by maintaining their blood at the same concentration as sea water The fish drinks sea water and the gills expel the excess salt from the blood, so keeping its concentration constant

The vacuole will expand, pushing the cytoplasm 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

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 will be firm and upright and the leaves would be expanded

Plant cells cell wall vacuole cytoplasm and cell membrane A useful analogy is the bicycle tyre. The outer casing represents the cell wall; the inner tube represents the cytoplasm and the air inside represents the vacuole. When the tyre is pumped up, the air pressure pushes the inner tube against the outer casing. The outer casing cannot expand and so the pressure in the tube builds up until the tyre is firm The cell absorbs water by osmosis .... ....but the cell wall stops the cell expanding any more

Question What did you notice about the colour of the cell sap in the two cells? Can you suggest an explanation for this?

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 Osmosis between one cell and the next plays a part in the movement of water throughout the tissues of the plant.

Limp and turgid tissue These cells are short of water; the tissue is limp and the plant is wilting The cells have taken up water by osmosis; the cells are turgid and the tissue is firm

Busy Lizzie (1) wilting (2) recovering (3) recovered (1) Wilting. The plant is short of water. The leaf cells are no longer turgid and the leaves curl up and droop. (3) Recovered. Water is once more available. The leaf cells take up water by osmosis and become turgid. The leaves are now firm and are held horizontally. Woody plants such as trees and shrubs do not collapse when they wilt but their leaves become limp.

In the growing regions of a plant, the cell walls are not inextensible but are still ‘plastic’ When the vacuole takes in water by osmosis it expands, pushing out on the ‘plastic’ cell wall and 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 causes growth

Cell extension 1 The cell has just divided 4 4 The vacuoles begin to join up and continue to absorb water and extend the cell 2 2 Vacuoles start to form in one cell 3 3 The vacuoles absorb water by osmosis and start to extend the cell 1 1 The cell has just divided Notice that the cell expands in length rather than width. This is due to the arrangement of cellulose fibres in the cell wall

Growth in a shoot tip cell division continues these cells will divide vacuoles forming cells absorb water by osmosis and expand The osmotic forces generated by the cells and tissues are very large. This why you get weeds pushing up through tarmac.

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

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

Question 3 Cell sap concentrated solution a c dilute solution b more dilute solution Water will tend to move by osmosis Plant cells (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

Question 4 Which statements are correct ? (a) A fresh water fish tends to absorb water by osmosis (b) A sea water fish tends to absorb water by osmosis (c) A fresh water fish tends to lose water to its surroundings (d) A sea water fish tends to lose water to its surroundings

Answer The colour intensity in the vacuole of the left hand cell is greater than that in the right This is because the uptake of water by osmosis has diluted the cell sap in the right hand cell

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