The following points highlight the four physical processes involved in movement of substances in plant body. The physical processes are: 1. Diffusion 2. Permeability 3. Osmosis 4. Imbibition.
Plant Body: Physical Process # 1. Diffusion:
The movement of molecules of gases, liquids or solutes from the region of higher concentration to the region of lower concentration until the molecules are evenly distributed throughout the available space is known as effusion. Thus diffusion may be said to be a movement of molecules from a region of high partial pressure to the region of low partial pressure as a result of their inherent kinetic energy.
Various substances which enter into the plants from their surroundings out of which plants synthesize the numerous complex organic compounds, find their way inside by the process of diffusion. Gases, liquids or solutes can diffuse simultaneously and independently at different rates and in different directions in the same place without interfering with each other.
The rate of diffusion of gases is generally fast, while the liquids and solutes have a slower rate of diffusion. The rate of diffusion is directly proportional to the temperature and inversely proportional to the square root of density.
Plant Body: Physical Process # 2. Permeability:
A membrane which allows both water as well as solute particles to pass through is called permeable. The membrane which allows the passage of only the solvent particles and not the solute particles is called semipermeable or differentially permeable.
Impermeable membranes are those which prohibit the entry of both solvent and solute particles. A plant cell normally has a differentially permeable membrane (the plasma-membrane). The permeability of the complex cell membrane depends on the nature of the surrounding particles as well as on the changing conditions inside and outside the cell.
Among all substances, water has the most rapid rate of diffusion through cell membranes. The membrane is very much permeable to gases like CO2, O2, N2, and fat-solvents like alcohol, ether, chloroform, etc., relatively less permeable to some organic substances like mono- and di-saccharides, fatty acids, amino acids, strong electrolytes, salts, bases, etc., and relatively impermeable to polysaccharides, phospholipids, proteins, etc.
Ions also affect the permeability of cytoplasmic membrane and there exists some sort of mutual active opposition between salts and their ions having the same electric charge. This phenomenon is known as ‘antagonism of ions’.
Permeability is increased by the monovalent Na+ ions the effect of which is antagonised by the divalent Ca++ ions.
Plant Body: Physical Process # 3. Osmosis:
Osmosis is regarded as spontaneous diffusion of solvent across a semipermeable membrane from a place of its higher diffusion pressure (or concentration) to a place of its lower diffusion pressure (or concentration).
The osmotic pressure is that excess pressure which has to be applied to the solution when it is separated from the solvent by means of a semipermeable membrane in order to check the inflow of the solvent and establish equilibrium in the system.
The osmotic pressure of a solution is, therefore, a rating of the potential maximum pressure which can be developed in a solution as a result of osmosis. This pressure is directly dependent upon the concentration of the solute particles as in nonelectrolytes.
Hence molar solutions are generally used in experiments of osmosis which takes into account the total number of particles of the solutes. In the case of electrolytes, the solute dissociates into cations and anions in water, increasing the number of solute particles. Thus the osmotic pressure of a molar solution of an electrolyte will be twice as much as that of a nonelectrolyte when completely dissociated.
The pressure which develops in a cell from time to time due to osmotic diffusion of water is called the turgour pressure (or the hydrostatic pressure). The wall will exert a reverse pressure on cell solution. This is wall pressure which is always equal and opposite to turgour pressure.
The turgour pressure in a particular system may be variable whereas osmotic pressure of that system is always constant. During endosmosis, the turgour pressure increases gradually in a solution and when it has reached its maximum limit it is equal to the osmotic pressure, provided the solvent is pure.
In case of plant cells the actual value of osmotic pressure cannot be obtained because of its increasing cell volume due to endosmosis (cell wall is not perfectly inelastic).
The suction pressure or the diffusion pressure deficit of water in a solution is that pressure which is the difference of diffusion pressure of water in pure state and in solution, both under same temperature and pressure.
The following fundamental relation between the three osmotic quantities of a plant cell can be expressed in simple equations:
Suction pressure (S) = Osmotic pressure (P) — Turgour Pressure (T)
Or, S = P – T.
An ideal molar solution should have an osmotic pressure of 22.4 atmospheres at 0°C. If n gram molecules (molecular weight expressed in grams) of solutes are present in 1 litre of water, the diffusion pressure of water in the solution formed will be less than the diffusion pressure of pure water by n × 22.4 atmospheres.
At N.T.P. the diffusion pressure of pure water is about 1240 atmospheres. Solutions having equal osmotic pressure or concentration are called isotonic or isosmotic. When a solution is at a higher concentration than the other, it is called hypertonic and the latter solution having lower concentration is called hypotonic.
When a plant cell is immersed in a hypertonic solution exosmosis results and shrinkage of protoplasm takes place. This phenomenon is known as plasmolysis. When plasmolysis is just to commence (i.e., turgour is zero), the cell is said to be at incipient plasmolysis.
If a plasmolysed cell is transferred from a hypertonic solution to pure water, it slowly recovers by endosmosis of water regaining its turgour. This phenomenon is known as deplasmolysis.
In living cells many kinds of selectively permeable membranes are found such as plasma-lemma, tonoplast, nuclear membrane, etc. The cellulose cell wall is, however, permeable to most type of substances including water. These cell walls when impregnated with cutin or suberin or waxy materials become impermeable to water.
Plant Body: Physical Process # 4. Imbibition:
Many substances (lyophilic) when immersed in certain solvents or solutions imbibe the solvent into their structure forming a colloidal solution. This taking up of water and the consequent swelling of colloidal materials and other substances is called imbibition.
Substances such as proteins, gums, mucilage, starch, gelatin, pectin, agar-agar, cellulose, etc., form a viscous colloidal solution in water by the process of imbibition. Seeds contain substances which can imbibe water and the germination process is initiated. This imbibition of water by dispersed particles in a colloidal system is responsible for a number of physiological processes on the surface of protoplasm.
Different organic substances have various degrees of imbibing ability. For example, proteins are better imbibant than starch and cellulose.
The amount of liquid imbibed depends upon the amount of mutual cohesive attraction between the molecules of the imbibant as well as on the amount of adhesive force between the molecules of imbibant and the imbibed liquid.
The rate of imbibition depends on the diffusion pressure gradient of the solvents of the imbibant and the medium. The physical conditions of the medium such as pH, temperature, concentration, etc., also determine the rate of imbibition and the amount of liquid imbibed.
Increase in temperature, within certain limits, increases the rate of imbibition. During imbibition energy may be released causing change in temperature. By imbibition the imbibant swells in volume producing an outwardly directed pressure called the imbibition pressure (analogous to turgour pressure). This pressure depends upon the nature of imbibant and the imbibed liquid, concentration, temperature, pH, etc..