Back to: O level Biology NOTES Uganda syllabus
This is the movement of water molecules from a region of their high concentration to a region of their low concentration across a semi permeable membrane.
Or
It is the movement water molecules from a solution of low concentration to a solution of high concentration across a semi permeable membrane.
A semi/partially/selectively permeable membrane is one which can allow the passage of some materials to occur and prevent other materials from passing across it.
Diagram showing details of osmosis
When 2 solutions are separated by a semi permeable membrane having small pores, water molecules continue to move from a dilute solution to a concentrated solution through it.
Experiment to demonstrate osmosis in an artificial cell
Materials
- Cellophane /visking tube,
- Capillary tube,
- Beaker,
- Syrup or sugar solution,
- Thread or elastic band,
- Clamp
Procedure
a) Tie one end of the visking tubing using a rubber bung.
b) Make a sugar solution and pour it into the tubing
c) Tie the open end of the tubing to the capillary tube using a rubber bung or thread.
d) Pour some water in the beaker half way full
e) Insert the capillary tube with the visking tubing into water.
f) Note the level of the solution in the capillary tube and that of water in the beaker.
g) Clamp the capillary tube on a retort stand and leave the set up for 30 minutes.
Observation
In a few minutes, the level of the solution is seen to rise up the capillary tube
Interpretation
Water molecules are passed through the cellophane tubing into the sugar solution by osmosis, thus increasing its volume and forcing it up the capillary tube.
Water acts as a dilute solution
Sugar solution acts as a concentrated solution
Membrane of the visking tubing acts as the semi permeable membrane.
EXPERIMENT TO DEMONSTRATE OSMOSIS IN A LIVING TISSUE
Apparatus
- Fresh Irish potatoes,
- knife,
- Petri dishes,
- sugar or salt
- water
Procedure
a) 3 fresh Irish potatoes are peeled and their ends sliced flat. The interiors are scooped out to form a cup with walls of uniform thickness.
b) In A, some grains of sugar are placed in the cup, while the other potato B is left empty as a control.
c) The third potato is boiled to kill or destroy the tissues and also some sugar grains are put in the cup.
d) All the potato cups are placed in water in Petri dishes. The experiment is let to run for 2-6 hours.
End of experiment (2-6 hours)
The liquid in the cup potato A had risen to form a sugar solution and in the Petri dish, the level had fallen.
In potato B and in the boiled potato, the cups were still empty and the water level in the Petri dishes remained the same.
Conclusion
Osmosis takes place in living tissues and does not take place in boiled tissues. This is because, by boiling, the tissues are destroyed and loose semi permeability
Explanation
Living tissues have cell membrane or cell walls acting as semi permeable membrane and allow water to move through by osmosis while boiling a living tissue makes it impermeable.
Terms used in osmosis
Osmotic potential
This is the ability of a solution to exert osmotic pressure. This describes the concentration of the solution of the solution in terms of the ability of water molecules to move hence a solution with high osmotic potential has more water molecules able to move.
Osmotic pressure
This describes the concentration of the solution in terms of the ability of water molecules to move hence a solution with high osmotic potential has more water molecules able to move. This is the pressure exerted by a hypertonic solution to draw water in to its self.
Water potential
This is the concentration of water in a solution. Therefore a solution has a high osmotic pressure if it is highly concentrated and vice versa. This is the ability of a hypotonic solution to loose water to a more concentrated solution.
Hypotonic solutions
This is used to describe a solution containing less solute and more water molecules compared to another e.g. hypotonic solution has a lower osmotic pressure and is generally termed as less concentrated.
Isotonic solutions
These are solutions with the same concentration of salts and water i.e. Solutions with the same isotonic pressure
Hypertonic solutions
This is used to describe a solution with more solutes and less water molecules than the other. A hypertonic solution has a higher osmotic pressure and is generally termed as more concentrated solution.
Osmosis and cells
Animal cells
Unlike the plant cells, animal cells lack a cell wall and only have a cell membrane which is weak and non-resistant to high internal pressure.
Osmosis and red blood cells
When red blood cells are placed in a dilute solution (hypotonic solution) i.e. distilled water, the cells swell up and eventually burst (haemolyse). This is because water moves from the surrounding solution (distilled water) via the semi permeable cell membrane into cells.
Haemolysis in red blood cells
When the red blood cells are placed in a more concentrated solution (hypotonic solution) e.g. a strong sugar solution, water moves out of the cells to the surrounding solution by osmosis. As a result, the cells shrink the process called crenation or laking.
However, when red blood cells are placed in isotonic solution they neither gain nor lose water.
Turgor
This is the attainment of enough water in the cell to make it expand to its maximum volume.
Turgor pressure
This is the force exerted on the cell wall of the plant cell due to pushing of the cytoplasm as a result of water entering the cell vacuole and expanding.
Turgidity
Is a destination of a cell which has attained enough water and expanded to maximum size.
When a plant cell is placed in a dilute solution (water) than the cell sap, water enters by osmosis through the semi permeable cell wall and cell membrane into the cell sap. The volume of cell sap increases and it makes the sap vacuole expand. This causes the cytoplasm move towards the cell wall and gaining turgidity.
Time comes when all the cytoplasm is pressing against the cell wall and no more water can be absorbed. At this state, the cell is said to have gained full turgidity and the force on the cell wall is called turgor pressure.
Diagram showing a cell gaining turgidity
Plasmolysis
This is the loss of water from the cell to the surrounding causing the vacuole to shrink and cause the cytoplasm to lose contact with the cell wall.
When the cell is in this condition, it is said to be flaccid or plasmolysed. Therefore a flaccid cell is one whose cytoplasm has lost contact with the cell wall due to loss of water from the cell sap of the vacuole.
When the cell is in a more concentrated solution than the cell sap, water moves from the cell sap through a cytoplasm than the cell wall to the surrounding solution. This causes the vacuole to shrink and the cytoplasm to lose contact with the cell wall and the cell is said to be flaccid or plasmolysed.
Diagram showing a plasmolysed cell
Experiment to demonstrate turgor and plasmolysis
Materials
Cock borer
Four beakers
Water
Irish potato
Razor blade
Sugar crystal
Procedure
i) Get four beakers and pour ¾ of water in 3 of them and leave one empty.
ii) Mix the sugar in one beaker to make 5% solution
iii) Mix sugar in another beaker to make 50% solution
iv) Leave one with pure water
v) Use a cock borer to make 4 potato cylinders the same length e.g. 3 cm.
vi) Name this initial length
vii) Deep the potato in each cylinder
viii) Leave the setup for one hour and observe.
Setup
Remove the cylinder from each beaker and measure each length. Also feel the texture. Tabulate your results in the table below.
Observation
The cylinder in water had increased in length and became tougher.
The cylinder in 5% sucrose solution didn’t have any change in length and the texture remained the same
The cylinder in 50% sucrose solution had decreased in length and become soft, flaccid and curved.
The potato in the empty beaker decreased in length.
Conclusion
Turgor and plasmolysis occur in plant cells.
Explanation
The cylinder in water increased in length because water molecules moved into it from the surrounding water by osmosis because the cell sap had a higher concentration than the surrounding water
There was no change in length for the cylinder in 5% sucrose solution because the solution had the same concentration as the cell sap of a potato cylinder hence no net osmosis.
There was a decrease in length for the cylinder in 50% sucrose solution because water molecules moved out of the cylinder which had a lower concentration by osmosis.
There was a decrease in length for the cylinder in the empty beaker because water was lost to the surrounding through evaporation.
Significance of osmosis in plants
i) Absorption of water by root hairs from soil
ii) It enhances movement of water from root hairs via the cortex to the xylem.
iii) For support in non- woody plants
iv) It facilitates opening and closing of stomata
v) In germination, the initial absorption of water is by osmosis
Significance of osmosis in animals
i) It enables movement of water to capillaries in villi
ii) Movement of water in to unicellular animals
iii) Movement of water from tissue fluid to the cell
iv) It enables reabsorption of water into the blood stream via the kidney tubules.
Note: many semi- permeable membranes allow the passage of solute and solvents though not to the same extent. All that is required for osmosis to occur is that the solvent molecules move more rapidly than the solute molecules.