Osmosis is defined as the movement of solvent from higher concentration to lower concentration region through a semi-permeable membrane until equilibrium state is reached. In simple language, it is the movement of water or solvent from its dilute solution to concentrated solution to overcome concentration difference between them. The solvent molecules are allowed to pass through a filter called semi-permeable membrane. It allows only specific solutes to pass through it.
Tonicity of solution
To know the types of osmosis, first let's know about 3-types of osmosis solution on the basis of its concentration or tonicity as compaired to normal cell sap. Cell sap is the solvent or solution present inside vacuole and cytoplasm of the cell.
[Image Caption: Demonstration of osmosis in a cell by placing in different tonicity of solution]
Isotonic solution: When the concentration of cell sap and outer solution are equal, it is called isotonic solution. When a cell is kept inside it, there is no movement of water molecules inside or outside the cell due to equal concentration or water potential in both of them. Examples: 0.9% saline water.
Hypotonic solution: When the concentration of cell sap is higher than outer solution, it is called hypotonic solution. When a cell is kept inside it, there is movement of water molecules inside the cell from outer solution. It results in endosmosis. Examples: solutions used for food packing and food dehydrating.
Hypertonic solution: When the concentration of cell sap is low and outer solution is high, it is called hypertonic solution. When a cell is kept in it, there is movement of water molecules from the cell to the outer solution. It results in exosmosis. Examples: 0.45% saline solution, 5% dextrose solution etc.
Types of osmosis
There are 2 main types of osmosis.
In this type(exo=out) water molececules comes outside the cells. When plant or animal cell is placed in hypertonic solution (or high osmotic pressure), the water molecules comes out from the cell. This happens due to movement of water from cell to outside solution in which it is kept. To demonstrate exosmosis in a cell, it should be kept in hypertonic solution. Thus, the phenomenon of outward movement of water from the cell when it is kept in hypertonic solution is called as exosmosis.
In this type(endo=inside) water molececules enter inside the cell. When plant or animal cell is placed in hypotonic solution (or low osmotic pressure), the water molecules enter inside it. This happens due to movement of water from hypotonic solution to the cell. To demonstrate endosmosis in a cell, it should be kept in hypotonic solution so that concentration inside cell sap is higher than concentration of outside solution. Thus, the phenomenon of inward movement of water inside the cell when it is kept in hypotonic solution is called as exosmosis.
Forward and reverse osmosis
If you learn osmosis in depth, you'll hear about this two types also. Forward osmosis is defined as the technique which is used to separate solute and solvent with the help of semi-permeable membrane from the solution by applying osmotic pressure. Some applications in our daily life are emergency drinks, desalination of water etc.
Reverse osmosis is just opposite in case of forward osmosis. The solvents from higher concentrated solution are forcely allowed to move towards pure solution or less concentrated solution through a semipermeable membrane. External hydraulic pressure is applied to reverse the natural process of osmosis. It has also many applications including purification of ocean's salty water into pure drinking water, waste water purification etc.
Examples of osmosis
In animals: Brusting and dying of bacterials inside prickles due to result of exosmosis, vomiting blood by leeches on sprinkling salt on its body surface, swelling of legs and fingers during rainy season due to frequent touch with water, effect of erythrocytes in different types of solutions: Inside isotoic-remains normal, Inside hypertonic-it shrinks due to exosmosis and inside hypotonic-it swells up due to endosmosis.
In plants: Opening and closing of guard cells of stomata, absorption of water from soil by roots, germination of seeds by absorbing water, maintaince of water eqilibrium inside cells and tissues.
Concept of osmosis
To learn osmosis is simple but it might frequently creates confusion in understanding and memorising. Its terms are so relative to each other that it causes confusion among learners. There are tons of definitions available on internet, books etc which defines the same terms in different ways.
But the good is to choose one definiton and understanding all single words clearly. Once you understand any one definition clearly, you will notice that all definition from any source are pointing the same things in different manner.
There is always confusion between hypotonic, hypertonic, exosmosis, endosmosis, plasmolysis and deplasmolysis terms in osmosis topic in biology. So, these 2-tips will simplify your learning.
- Hypertonic solution cause exosmosis resulting into flaccid cell. If normal cell is kept in it, it causes plasmolysis.
- Hypotonic solution cause endosmosis resulting into turgid cell. If plasmolysed cell is kept in it, it causes deplasmolysis.
1. Do dead cell show plasmolysis and deplasmolysis?
Ans: No, dead cell do not show plasmolysis ans deplasmolysis. Living cell shows both plasmolysis and deplasmolysis. Dead cell do not show both of it.
2. What are the experiments to explain osmosis?
Ans: There are 2 famous experiments to explain osmosis. Separating two different concentrated solution with egg membrane and by using potato-osmometer.