Osmosis is the most important physical process that brings about the movement of water in living systems as well as in their environments.
Definitions:
A few definitions of osmosis are given below:
(1) Osmosis is the net diffusion of water molecules from a dilute solution to the concentrated solution when the two are separated by means of a semipermeable membrane. If the solution is separated from its pure solvent, the molecules of solvent move from pure solvent to the solution.
2) When the two solutions having different osmotic concentrations are separated by means of a semipermeable membrane, the molecules of water (or solvent) move from the region of its higher chemical potential to the region of its lower chemical potential. This movement of water (or solvent) is called osmosis.
(3) When the two solutions having different osmotic concentrations are separated by means of a semipermeable membrane, the molecules of sol vent or water move from the region of their higher diffusion pressure (or free energy) to the region of their lower diffusion pressure (or free energy). This movement of water or solvent is called osmosis.
Explanation
The three definitions given above are fundamentally the same. Basically, the osmosis involves diffusion of solvent (or water) across a semipermeable membrane (selectively on differentially permeable membrane in biological systems A pure water (or solvent) has maximum free energy, diffusion pressure and chemical potential (in case of water: water potential). When some soluble substance is dissolved into it, the free energy of its water molecules decreases. Likewise, the diffusion pressure and chemical potential (or water potential) also decrease. Therefore, the free energy, diffusion pressure and chemical potential of solvent (or water) molecules in a solution is always less than those of pure solvent (or water).
Similarly, the free energy, chemical potential and diffusion pressure of solvent molecules in a dilute solution are always more as compared to the free energy, diffusion pressure and chemical potential of solvent in a concentrated solution.It is the property of ions or molecules that they move from the region of higher energy level to the region of lower energy level. The water (or solvent) molecules, therefore, move from the region of more diffusion pressure, chemical potential (water potential) and free energy to the region of less diffusion pressure, chemical potential (water potential) or free energy across a semipermeable membrane in osmosis.
There is a tendency for solutions separated by such membranes to become equal in molecular concentration. In plants the only solvent involved in living cells is water. The water can move from the environment into the cell as from the soil into a root cell or from one cell to another, or from the cell into the environment by the simple process of osmosis.
When a plant cell is placed in pure distilled water, the water enters into the cell. The plant cell wall is permeable both to solvent and to solute molecules. The plasma membrane, on the other hand, is differentially permeable. The water molecules enter into the cell sap as a result of simple process of osmosis. This is known as endosmosis. If the direction of movement is reverse i.e., from cell sap to outside, the osmosis is termed exosmosis.
Differences between diffusion and osmosis:
| Diffusion | osmosis |
|---|---|
| 1. Diffusion is the net downward ment of a given sub- stance (ions, atoms or molecules, etc.) along the free energy gradient from the place of its higher concentration to an area of its lesser con- centration. | 1. Diffusion is the net downward ment of a given sub- stance (ions, atoms or molecules, etc.) along the free energy gradient from the place of its higher concentration to an area of its lesser concentration. |
| 2. The diffusion may occur in any medium and the diffusing particles may be solid, liquid or gas. | 2. The osmosis occurs in liquid medium and only the solvent molecules move from one place to another. |
| 1. Osmosis is a special move- type of diffusion of sol-vent molecules from low concentration of solution to higher concentration of solution, when the two are separated by a semipermeable membrane. | 3. Presence of semi-permeable membrane in between the two solutions is required. |
Osmotic Concentrations
(Hypertonic, hypotonic and isotonic solutions)
The hypertonic, hypotonic and isotonic solutions are relative terms which are defined as follows:
Hypertonic solution.
A solution having a concentration such that it gains water or solvent by osmosis across a semipermeable membrane from some other specified solution
Hypotonic solution.
A solution having a concentration such that it loses water or solvent by osmosis across a semipermeable membrane to some other specific solution (Fig. 1.2).
Isotonic solution.
A solution having a concentration such that it neither gains nor loses water by osmosis when separated by a semipermeable membrane from a specified solution (Fig. 1.2).
Demonstration of Osmosis
Expt. To demonstrate the phenomenon of osmosis by thistle funnel method (Fig. 1.3).
Requirements.
Thistle funnel, goat’s bladder membrane, water, thread, salt solution or sugar solution, beaker, stand.
Method.
Take a thistle funnel. Cover its wide mouth with a goat’s bladder membrane and tie it with thread. Now pour the salt or sugar solution in the inverted funnel till it reaches upto the middle of vertical tube. Dip the inverted funnel in a beaker containing pure water. Fix the vertical tube of the funnel with a stand. Mark the level of solution in the tube. Observe the change in the level after some time.
Observation and Conclusions.
The level of solution in the vertical tube starts rising. This shows that the water molecules enter into the thistle funnel through goat’s bladder. This experiment demonstrates the osmotic movement of water (or osmosis).
Explanation
The goat’s bladder membrane acts as a semipermeable membrane (this membrane perch of paper or cellophane). Osmosis is the phenomenon where the water molecules move from pure water (or lower concentration of solution) to a higher concentration of solution when the two are separated by means of a semipermeable membrane.
This movement Occurs because the water. molecules in pure water have high free energy (higher potential) as compared to those in a sugar or salt solution. The movement of water occurs from higher free energy level to lower free energy level. Therefore, in the present experiment the water molecules move from pure water in the beaker towards the solution in the thistle funnel by the process of osmosis.
Expt. To demonstrate the phenomenon of osmosis through plant membrane with the help of potato osmo- scope
Requirements.
A large potato tuber, 10% sugar solution, beaker, water, scalpel, pin.
Method.
Take a large potato tuber and peel off its outer skin with the help of scalpel. Cut its one end to make the base flat. Now make a hollow cavity on the opposite side nearly upto bottom. Pour some sugar solution to the half of the cavity and mark the level by inserting a pin in the wall of tuber Put the potato into the beaker containing a small amount of water and allow the apparatus to stand for sometime. Make sure that the level of water should be below the level of potato.
Observation and Conclusions.
The level of sugar solution in the cavity rises. It is because of the movement of water molecules into the cavity from pure water in the beaker. This experiment shows the phenomenon of osmosis.
Explanation.
Same as in experiment number 1. The living cells of potato tuber collectively act as differentially permeable membrane. The two solutions, i.e., pure water in the beaker and sugar solution in the cavity, are separated by the living cells of potato.
Thus, the osmotic movement of water occurs from pure water to the solution passing through a differentially permeable membrane. If sugar solu- tion is taken in the beaker and pure water in the cavity, the result will be reversed. The movement of water will not occur if the skin of the potato is not removed because the skin acts as an impermeable membrane
Expt. To demonstrate the phenomenon of exosmosis and endosmosis.
Requirements.
Petri dishes (or watch glasses), water, concentrated salt (or sugar) solution, fresh grapes, dry raisins.
Method.
Pour some concentrated salt (or sugar) solution in a Petri dish (or watch glass) and dip some fresh grapes in it. Similarly, dip some dry raisins in water in another Petri dish (or watch glass). Observe after some time.
Observation and Conclusions.
The grapes show shrinkage due to exosmosis and dry raisins swell up due to endosmosis.
Explanation.
Fresh grapes are filled with dilute solution of sugars. When they are dipped in concentrated salt (or sugar) solution, they lose water to outside due to osmosis. Since water comes out from grapes, it is called exosmosis.
The dry raisins, on the other hand, have concentrated solution. When they are dipped in water, the water enters into the raisins due to osmosis. Since water enters into the raisins, it is called endosmosis. In this experiment the grapes and raisins should be taken with their stalks intact.
Osmotic Pressure (OP)
The osmotic pressure of a solution is equivalent to the pressure which must be exerted upon it to prevent the flow of solvent into it across a semipermeable membrane. In other words, the maximum amount of pressure that can be developed in a solution separated from pure water by a semipermeable membrane is termed osmotic pressure.
It is usually measured in pascals, Pa (1 Pa = 1 New- ton/m²).The osmotic pressure of a solution largely depends upon the ratio between the number of solute and solvent particles present in a given solution. For example, a molar solution of sucrose has an osmotic pressure of approximately 22.4 atmos- pheres at 0°C.
However, a molar solution of sodium chloride has almost twice the osmotic pressure of a molar solution of sucrose. This is because sodium chloride is an electrolyte and dissociates almost completely into Nat and Cl- ions (giving twice the particle number) whereas sucrose or glucose molecules are non-electrolytes and do not dissociate in water.
The osmotic pressure of given solution can be calculated by using the following relationship.Osmotic pressure CST [where C is the molar concentration of solution, Sis the solution constant, which is 0-082 and T is the absolute temperature i.e., °C + 273].
The Importance of Osmosis in Plants
(1) The phenomenon of osmosis is importantin the absorption of water by plants.
(2) Cell-to-cell movement of water occurs throughout the plant body due to osmosis.
(3) The rigidity of plant organs (i.e., shape and form of organism) is maintained through osmosis.
(4) Leaves become turgid and expand due to their OP.
(5) Growing points of root remain turgid because of osmosis and are thus, able to penetrate the soil particles.
(6) The resistance of plants to drought and frost is brought about by osmotic pressure of their cells.
(7) Movement of plants and plant parts, for example, movement of leaflets of Indian telegraph plant, bursting of many fruits and sporangia, etc. occur due to osmosis.
(8) Opening and closing of stomata is affected by osmosis.
The differences between osmosis and imbibition
Turgor Pressure (T.P.)
It is a unique property of a plant cell that it is surrounded by a rigid cell wall. If a living plant cell is immersed in water, it swells due to the absorption of water. As a result of entry of the water into the cell sap pressure is developed in the protoplasm which presses against the cell wall.
The actual pressure exerted by the protoplasm against the cell wall is the turgor pressure (TP), which is always less than osmotic pressure unless the cell is in distilled water. Cell wall is rigid and elastic, therefore, it exerts an equal and opposite pressure against the expanding protoplasm. This pressure is called wall pressure (WP). At a given time TP equals to WP: TP = WP
Diffusion Pressure Deficit (DPD)Or Suction Pressure (SP)
The term diffusion pressure (DP) and diffusion pressure deficit (DPD) were put forth by B.S. Meyer in 1938. Originally, the DPD was described by the term suction force (Saugkraft) or suction pressure (SP) by Renner (1915). Nowadays, the term water potential (v) is used which is equal to DPD, but negative in value.The pressure exerted by the diffusing particles is known as diffusion pressure (DP).
This pressure is proportional to the concentration of diffusing particles. Pure water has maximum diffusion pressure. If some solute is dissolved, the resulting solution comes to attain less diffusion pressure than that of pure water. In other words, diffusion pressure of a solvent in a solution is always lower than that in the pure solvent. The amount by which the diffusion pressure of a solution is lower than that of its pure solvent is known as diffusion pressure deficit (DPD). The DPD of a cell denotes the net pressure that causes water to enter the cell. read more
