Notes on Transport in Plants
(1) Types of osmosis
Depending upon the movement of water into or outward of the cell, osmosis is of two types.
(a) Endosmosis: The osmotic inflow of water into a cell, when it is placed in a solution, whose solute concentration is less than the cell sap, is called endosmosis e.g., swelling of raisins, when they are placed in water.
(b) Exosmosis: The osmotic outflow of water from a cell, when it is placed in a solution, whose solute concentration is more than the cell sap, is called exosmosis. e.g., shrinkage of grapes when they are placed in strong sugar solution.
(2) Osmotic concentrations (Types of solutions)
A solution can be termed as hypotonic, hypertonic and isotonic depending upon its osmotic concentration, with respect to another solution or cell sap.
(a) Hypotonic solution (hypo = less than). A solution, whose osmotic concentration (solute potential) is less than that of another solution or cell sap is called hypotonic solution. If a cell is placed in such a solution, water start moving into the cell by the process of endosmosis, and cell become turgid.
(b) Hypertonic solution (hper = more than). A solution, whose osmotic concentration (solute potential) is more than that of another solution or cell sap is called hypertonic solution. If a cell is placed in such a solution, water comes out of the cell by the process of exosmosis and cell become flaccid. If potato tuber is placed in concentrated salt solution it would become shrink due to loss of water from its cell.
(c) Isotonic solution (iso = the same). A solution, whose osmotic concentration (solute potential) is equal to that of another solution or cell sap, is called isotonic solution. If a cell is placed in isotonic solution, there is no net change of water between the cell and the solution and the shape of cell remain unchanged. The normal saline (0.85% solution of NaCl) and 0.4 m to 0.5 m solution of sucrose are isotonic to the cell sap.
(3) Significance of osmosis in plants
(a) The phenomenon of osmosis is important in the absorption of water by plants.
(b) Cell to cell movement of water occurs throughout the plant body due to osmosis.
(c) The rigidity of plant organs (i.e., shape and form of organism) is maintained through osmosis.
(d) Leaves become turgid and expand due to their OP.
(e) Growing points of root remain turgid because of osmosis and are thus, able to penetrate the soil particles.
(f) The resistance of plants to drought and frost is brought about by osmotic pressure of their cells.
(g) 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.
(h) Opening and closing of stomata is affected by osmosis.
(4) Turgor pressure (TP)
The plant cell, when placed in pure water, swells but does not burst. Because of negative osmotic potential of the vacuolar solution (cell sap), water will move into the cell and will cause the plasmalemma be pressed against the cell wall.
(5) Wall pressure (WP)
Due to turgor pressure, the protoplast of a plant cell will press the cell wall to the outside. The cell wall being elastic, presses back the protoplast with a pressure equal in magnitude but opposite in direction. This pressure is called wall pressure. Wall pressure (WP) may, therefore, be defined as ‘the pressure exerted by the cell wall over the protoplast to counter the turgor pressure.
(6) Plasmolysis (Gr. Plasma = something formed; lysis = loosing)
If a living plant cell is placed in a highly concentrated solution (i.e. hypertonic solution), water comes out of the cell due to exosmosis, through the plasmamembrane. The loss of water from the cell sap causes shrinkage of the protoplast away from the cell wall in the form of a round mass in the centre. “The shrinkage of the protoplast of a living cell from its cell wall due to exosmosis under the influence of a hypertonic solution is called plasmolysis”.