Osmosis is a physical process and is considered as a special type of diffusion. In osmosis, only the solvent molecules move from their region of higher concentration to the region of their lower concentration across a semi permeable membrane.
If there are two sugar solutions, e.g., solution A having greater concentration than solution B and they are separated by a semi permeable membrane, then water will move from B to A as there are more number of water molecules in B than that of A.
This spontaneous movement of water molecules from B to A is caused due to phenomenon of “Osmosis”. Then, Osmosis can be defined as the movement of water (or any solvent) from a solution of dilute concentration to a concentrated solution when both the solutions are separated by a semi permeable membrane.
Semi permeable membrane
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Such membranes which only allow passage of solvent molecules (water) and do not allow solute molecules to pass through it are called semi permeable membrane. Parchment paper, fish bladder, white membrane of eggs, animals bladder etc. are the examples of semi permeable membrane.
Demonstration of Osmosis
Osmosis can be demonstrated by a simple experiment in the laboratory. Take a long stemmed thistle funnel. Close the mouth of the funnel by stretching parchment paper across the mouth of the funnel. Take sucrose solution in the thistle funnel and note the level of sucrose solution in the
Stem of thistle funnel. Now place the thistle funnel containing the solution in a beaker of water in a vertical manner with the help of a stand as shown in the figure. After sometime the level of the solution in the stem of thistle funnel will rise.
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This rise of solution in the stem of thistle funnel is due to the entry of water (solvent) molecules from the beaker into the thistle funnel across parchment paper which is a semi permeable membrane.
Osmotic pressure (OP)
The pressure which develops in a solution when it is separated from its pure solvent by a semi permeable membrane under ideal conditions of osmosis is called osmotic pressure. It may be defined an excessive hydrostatic pressure which is to be applied to the solution when the solution is separated from its pure solvent (water) by means of a semi permeable membrane in order to prevent the inflow of solvent to the solution. Osmotic pressure is measured in terms o atmosphere or bars and is denoted by the symbol n It is directly related to the concentration of the) solute; more the solute particle, more is the osmotic pressure. Increase in temperature increases the osmotic pressure of the solution.
Endosmosis and Exosmosis
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If a living plant cell or tissue is placed in water or hypotonic solution (whose o.p is less than that of cell sap), water enters into the cell by osmosis.
This process is called as endosmosis. As a result of entry of the water into the cell sap, a pressure is developed which presses the protoplasm organist the cell wall and the cell becomes turgid. This pressure is called as turgor pressure.
Similarly if a living plant cell or tissue is placed in a hypertonic solution (whose osmotic pressure is higher than that of cell sap), the water comes out of the cell sap into outer solution and the cell becomes flaccid. This process is called exosmosis.
Plant cell as an osmotic system
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The physical concept of osmosis can be correlated with a plant cell. The chief components of the plant cell involved in the osmotic process are (a)the cell wall, a non living structure possessing elasticity, almost completely permeable to vacuolar solutes (b) the outer and inner cytoplasmic membranes and the tonoplast both partially semi permeable (c) the cell sap an aqueous solution of sugars and mineral salts contained in the vacuole.