Let us make in-depth study of the demonstration and types of osmosis, osmotic pressure and osmotic concentration.
In all biological systems, the solvents is water. Semipermeable membranes permit the movement of solvent molecules but prevent the movement of solute molecules.
Osmosis is a special form of diffusion in which water molecules diffuse in the presence of semipermeable membrane from hypotonic solution to hypertonic solution.
All diffusing molecules are known to possess some kinetic energy, called free energy. Free energy of pure water is higher than that of the same in a solution. The addition of solutes to the water (or solvent) lowers the free energy of the latter and thereby brings a fall in its diffusion pressure.
The lowering of free energy in the solution is due to the attraction and collision between solvent (water) and solute molecules. Thus, higher the concentration of a solution, lower is its free energy or diffusion pressure.
Demonstration of Osmosis:
The basic principles of osmosis can be illustrated most clearly by a simple physical experiment (Fig. 1.2).
A semipermeable membrane, such as parchment paper, is tied over the base of a thistle funnel. The base of the thistle funnel is then filled with a sugar solution and the apparatus placed in a jar of water. The membrane is only slightly permeable to the molecules of sugar but is readily permeable to water. Because of the presence of the sugar molecules the concentration of water within the funnel is lower than the concentration outside in the jar of water.
Water molecules pass through the membrane. The net movement of water will be into the sugar solution. Because of this inward movement, the volume of water inside increases, and hence the solution rises in the stem of the funnel.
The liquid will continue to rise until the height of the water column is such that the pressure exerted downward (i.e., hydrostatic pressure) prevents further inward diffusion. Such a hydrostatic pressure is termed osmotic pressure.
Endosmosis and Exosmosis:
Endosmosis:
‘Osmosis in which the direction of osmotic flow of water is into the cell”.
Changes in the Cell Due To Endosmosis:
(i) Increase in the volume of the cell sap causing an increase in the internal pressure, (i.e. Turgor pressure).
(ii) Dilution of the cell sap due to the absorption of water.
(iii) Enlargement of the protoplast due to endosmosis. The enlarged protoplast is pressed against the cell wall due to the Internal pressure.
(iv) Cell wall is rigid. It resists increases in the volume and exerts a counter pressure on the enlarging protoplast. (Wall Pressure).
Exosmosis:
“When the osmotic flow of water is from the inside of the cell to the outer hypertonic medium through the cell membrane.”
Changes in the Cell Due to Exosmosis:
(1) Loss of turgor and decrease in turgor pressure of the cell.
(2) Shrinking of the cell protoplast (plasmolysis).
(3) Cell wall become relaxed.
(4) Decrease in W.P. corresponding to the decrease in T.R
(5) Concentration of the cell sap increases due to the loss of water.
Osmotic Pressure:
Osmotic pressure is also defined as ‘The hydrostatic pressure developed in a solution due to the osmotic flow of solvent (such as water) when the solution is separated from pure solvent by a semipermeable membrane. Osmotic pressure term was coined by Pfeffer. Osmotic pressure is measured in atmosphere, bars or Pascal’s. Instrument used for measuring O.P. is called Osmometer.
Now a days, the term osmotic potential (i.e. solute potential = Ψs) is preferred over osmotic pressure. Numerically, osmotic potential is equal to osmotic pressure but opposite in sign. Osmotic pressure has a positive value while osmotic potential (Ψs) has equal but negative value.
Osmotic pressure can be calculated by the following relationship:
OP = CRT
C = Molar concentration of solution.
R = Gas constant which is 0.082
T = Absolute temperature 273°C
Normal Range of Osmotic Pressure Values in Different Plant Groups
Osmotic Concentration (Types of solutions):
1. Hypertonic solution (Hyper = More than):
A solution whose osmotic concentration (solute potential) is more than that of another solutions or cell sap (tonoplasm) is called hypertonic solution. If a cell is placed in hypertonic solution water comes out of the cell by the process of exosmosis and cell becomes flaccid.
2. Hypotonic solution (Hypo = Less than):
A solution whose osmotic concentration (solute potential) is less than that of another solution or cell sap (tonoplasm) is called hypotonic solution. If a cell is placed in hypotonic solution, water enters into the cell by the process of endosmosis and cell becomes of turgid.
3. Isotonic solution (Iso = Same):
A solution whose osmotic concentration (solute potential) is equal to that of another solution or cell sap (tonoplasm) is called isotonic solution. If a cell is placed in a isotonic solution, there will be no change in the cell and the solution.
