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Movement through the Plasma Membrane and Difference Between Diffusion and
Water also follows the law of diffusion. The spontaneous movement of water molecules through a selectively permeable membrane (e.g., plasma membrane) is called osmosis. The movement of water across the plasma membrane of the cell is affected by the amount of substance dissolved in water. Thus, osmosis is the passage of water from a region of high water concentration through a semi-permeable membrane to a region of low water concentration. Osmosis is purely a mechanical diffusion process by which cells absorb water without spending any amount of energy.
Let us see what will happen if you put an animal cell (e.g., red blood cells or RBCs) or plant cells (e.g., Rheo leaves) into a solution of sugar or salt prepared in water. One of the following three things could happen:
1. If the medium surrounding the cell has a higher water concentration than the cell, i.e., if the solution is a very dilute solution, the cell will gain water by osmosis. Such a dilute solution is called
a hypotonic solution. While water molecules are free to pass across the plasma membrane in both directions, more water will enter the cell than leave. The net (overall) result is that water enters the cell. In such a situation, the cell is likely to swell up, i.e., become inflated or turgid. Such swollen RBCs may ultimately burst, i.e., haemolysed.
2. If the medium surrounding the cell is of exactly the same water concentration as the cell, there will be no net movement of water across the plasma membrane. Such a solution is called an isotonic solution (e.g., Ringer’s solution is an isotonic solution for the animal cells).
In this case, water crosses the plasma membrane in both directions, but the amount going in is the same as the amount going out, so there is no overall movement of water. In such a situation, the cell will maintain the same size.
3. If the medium has a lower concentration of water than the cell, i.e., if it is a very concentrated solution, the cell will lose water by osmosis. Such a concentrated solution is called a hypertonic solution.
In this case, too, water crosses the plasma membrane in both directions, but this time more water leaves the cell than enters it. Therefore, the cell will shrink. In this situation, the plant cell is said to be plasmolyzed and animal cells (Red blood cells) are said to crenated.
Differences between Diffusion and Osmosis
|1. Diffusion can occur in any medium.||1. It occurs only in a liquid medium.|
|2. The diffusing molecules may be solids, liquids, or gases.||2. It involves the movement of solvent molecules only.|
|3. Semipermeable membrane is not required.||3. Semipermeable membrane is required.|
|4. It is dependent upon the free energy of the molecules of diffusing substance only; the presence of other substances in the system is of no importance.||4. Though it is the diffusion of solvent molecules only, yet influenced by the presence of other substances (solutes) in the system.|
|5. An equilibrium in the free energy of diffusion molecules is achieved in the system.||5. Equilibrium in the free energy of solvent molecules is never achieved.|
Examples of Osmosis:
Unicellular freshwater organisms and most plant cells tend to gain water through osmosis. Absorption of water by plant roots (i.e., by root hairs) is an example of osmosis.