3.2 MOVEMENT OF SUBSTANCE ACROSS THE PLASMA MEMBRANE IN EVERYDAY LIFE

THE EFFECTS OF HYPOTONIC, HYPERTONIC AND ISOTONIC SOLUTIONS ON ANIMAL AND PLANT CELLS

1. Hypertonic solution is the solution with a higher solute concentration ('hyper': more).
2. Hypotonic solution is the solution with a lower solute concentration ('hypo' : less).
3. Isotonic solution is the solution in which the concentration of solutes are equal ('iso' : equal).


A. PLANT CELLS IN AN ISOTONIC SOLUTION
1. If a plant cell is immersed in an isotonic solution such as 5% sucrose solution, there is no net movement of water across the plasma membrane.
2. Water flows across the membrane at the same rate in both directions.
3. The cell's volume and shape remain constant.





Effect of Isotonic Solution on a Plant Cell






B. PLANT CELLS IN A HYPOTONIC SOLUTION
1. When a plant cell is immersed in a hypotonic solution such as water, water molecules diffuse into the cell by osmosis.
2. The vacuole gain water, expands and exerts pressure outwards on the cell wall.
3. This pressure is called turgor pressure which cause the plant cell to become turgid.
4. The turgidity of the cells give the plant mechanical support.





Effect of Hypotonic Solution on a Plant Cell





C. PLANT CELLS IN A HYPERTONIC SOLUTION
1. When a plant cell is placed in a hypertonic solution such as 30% sucrose solution, water molecules diffuses out of the cell by osmosis.
2. Water is lost from the vacuole and cytoplasm.
3. The vacuole and the cytoplasm shrink. The plasma membrane is pulled away from the cell wall.
4. The plant cell becomes flaccid and the plant wilts.
5. This process is called plasmolysis.
6. If the plasmolysed plant cell is immersed in a hypotonic solution, there is a net movement of water into the cell. The cell will expand and become turgid again.
7. This process is called deplasmolysis.







Effect of Hypertonic Solution on a Plant Cell








A Simulation of Plasmolysis Process in Onion Cells



A Simulation of Deplasmolysis Process in Onion Cells


A Deplasmolysis Process in a Wilt Salad Leaf.



D. ANIMAL CELLS IN AN ISOTONIC SOLUTION

1. When animal cells such as red blood cells are immersed in an isotonic solution (0.85% sodium chloride solution), water molecules flow across the plasa membrane at the same rate in both directions.
2. There is no net movement of water molecules across the plasma membrane because the concentration in the red blood cells is the same as the concentration in the environment.
3. The red blood cells maintain their shape.








E. ANIMAL CELLS IN A HYPOTONIC SOLUTION
1. When red blood cells are placed in a hypotonic solution (distilled water), water molecules diffuse into the red blood cells by osmosis.
2. The red blood cells gain water and swell.Finally burst because they have no cell wall.
3. The red blood cells undergo haemolysis.






F. ANIMAL CELLS IN A HYPERTONIC SOLUTION
1. When red blood cells are placed in a hypertonic solution (4% sodium chloride solution), water molecules diffuse out of the cells by osmosis.
2. Water is rapidly lost.
3. The red blood cells will shrivel and probably die.
4. This process is called crenation.

THE EFFECTS AND APPLICATIONS OF OSMOSIS IN EVERYDAY LIFE

(a) Wilting in Plants

1. Excessive use of chemical fertilisers will cause plants to wilt.
2. Fertilisers will dissolve in the soil and cause the soil water to be hypertonic to the root cells of the plant.
3. As a result, water diffuses out of the root cells by osmosis. Plasmolysis occurs and the plant cells become flaccid, causing the plant to wilt.
4. Wilting commonly occurs in non-woody and herbaceous plants.
5. The cells in the plant will promptly recover when water is available. However, if the period of plasmolysis is prolonged, a wilted plant will eventually die.
6. Shortage of water in soil may also lead to wilting in plants.The dried soil becomes more concentrated or hypertonic. Then the plants lose water by osmosis and plasmolysis takes place.




(b) Food preservation

1. The methods of preserving food include drying, pickling, smoking, salting and sugar-curing.
2. Preservation of food is based on the concept of osmosis and plasmolysis.
3. Most food is preserved by using salt or sugar. Examples:
- A fish is preserved by covering it with salt.The moisture around the fish will be hypertonic to the cell of the fish.The water concentration in the cells of the fish exceeds the water concentration outside the fish. Water diffuses out of the fish cells by osmosis. The cells of the fish lose water and undergo crenation. Therefore, the fish can be kept longer.
- In preparing pickles of fruits like mango and papaya, the slices of fruits are immersed in a concentrated sugar solution which is hypertonic to the fruit cells. Water diffuses out of the fruit cells by osmosis into the sugar solution. The cells lose water and not conducive for the growth of microorganisms. Therefore, the fruit can last longer.

3.3 APPRECIATING THE MOVEMENT OF SUBSTANCES ACROSS THE PLASMA MEMBRANE

1. The movement of substances across the plasma membrane occur in a continuous and controlled manner for the survival of a cell.

2. The plasma membrane which is semi-permeable to different substances acts as a :
(i) gatekeeper which regulate what goes in and out of the cell.
(ii)barrier between the the contents of cells and the external environment.

3. Specific substances move across the plasma membrane at certain parts of the membrane.
(i) Oxygen and carbon dioxide move across the lipid bilayers of the plasma membrane by simple diffusion.
(ii) Water molecules move across the lipid bilayers of the plasma membrane by osmosis.
(iii) Glucose, amino acids and charged ions move across the plasma membrane through the carrier proteins and pore proteins by facilitated diffusion.
(iv) Certain molecules and ions which their concentration against the concentration gradient are pumped in or out of the cell through carrier proteins by using energy from ATP (adenosine triphosphate). Example : potassium ions inside the cell and sodium ions outside the cell.

4. The degree of fluidity of the plasma membrane depends on the temperature and the present of specific fatty acids and cholesterol.

5. To maintain the proper functioning of a plasma membrane and a cell as a whole, it is essential for us to take care of our food and water intake. Drinking sufficient water every day will help to hydrate body cells as well as regulate the osmotic pressure of the blood.

6. We should be thankful to God for the phenomenon of movement of substances across the plasma membrane which ensures the survival of the cells.