Osmosis may be thought of a special type of diffusion, which involves the movement of water or other solvent molecules through a semipermeable (or differentially permeable) membrane from an area of high potential (pure solvent) to an area of low potential (more concentrated solution). In a plant cell, the absorption of water or solution from an outside atmosphere, from cell to cell takes place through the plasma membrane, which is semipermeable. The semi-permeable membrane permits the passage of solvent but restricts the movement of most of the solute particles.
Experiment to demonstrate Osmosis: Taken a small bag of cellophane. Cellophane is semi-permeable. Filled 2% sugar solution in it. In this solution, there is 2% sugar and the remaining 98% is water. Suspend the cellophane bag after tying its mouth with thread, in a beaker having pure water.
The concentration of water in the beaker is 100% and the concentration of water filled up in the bag is 98%. So, water molecules will diffuse from a beaker into the bag, i.e., from high concentration to low concentration. On the other hand, the concentration of sugar in the bag is 2% and it is zero percent in the beaker. Thus, sugar molecules will diffuse into the beaker, but its rate will be slow.
As a result of these, the bag will swell up because of more water coming in it. The bag will remain in the swollen position, because of the pressure generated by water filled in that bag. The diffusion of water and sugar molecules from higher concentration to lower concentration through a semi-permeable membrane is called osmosis and the pressure generated by osmosis is called osmotic pressure. Osmotic pressure helps to move water through conducting system in plants. It helps plant cells to retain water and keeping their shape and rigidity. Osmostic pressure operates the guard cells and thus, affects the opening and closing of stomata.
