The process of adsorption of water by solid particles of a substance without forming a solution is called 'imbibition'. It is a type of diffusion by which movement of water take place along a diffusion gradient. The solid particles which adsorb water or any other liquid are called imbibants. The liquid which is imbibed is known as imbibate. Cellulose, pectic substances, protoplasmic protein and other organic compound in plant cells show great power of imbibition.

The phenomenon of imbibition has three important characteristics :

# (a) Volume change : During the process of imbibition, imbibants increase in volume. It has been observed that there is an actual compression of water. This is due to arrangement of water molecules on surface of imbibant and occupy less volume than the same molecules do when are in free stage in the normal liquid. During the process of imbibition affinity develops between the adsorbant and liquid imbibed. A sort of water potential gradient is established between the surface of adsorbant and the liquid imbibed.
e.g. If a dry piece of wood is placed in water, it swells and increases in its volume. Similarly, if dry gum or pieces of agar agar are placed in water, they swell and their volume increases. Wooden doors and windows adsorb water in humid rainy season and increase in their volume so that they are hard to open or close, in gram and wheat the volume increase by adsorption of water, in plant systems are adsorption of water by cell wall.

# (b) Production of heat : As the water molecules are adsorbed on the surface of the imbibant, their kinetic energy is released in the form of heat which increase the temperature of the medium. It is called heat of wetting (or heat of hydration). e. g., during kneading, the flour of wheat gives a warm feeling due to imbibition of water and consequent release of heat.

# (c) Development of imbibitional pressure : If the imbibing substance (the imbibant) is confined in a limited space, during imbibition it exerts considerable pressure. The bursting of seed coats of germinating seeds is the result of imbibition pressure developed within the seeds as they soak the water. Imbibition pressure can be defined as the maximum pressure that an imbibant will develop when it is completely soaked in pure water. Imbibition pressure is also called as the matrix potential because it exists due to the presence of hydrophilic substances in the cell which include organic colloids and cell wall.
Resurrection plants of Selaginella, lichens, velamen roots and dry seeds remain air dry for considerable periods because they can absorb water from the slight downpour by the process of imbibition.

# (a) Nature of imbibant : Proteins are the strongest imbibants of water, starch less strong, cellulose being the weakest. That is why proteinaceous pea seeds swell more than the starchy wheat seeds. During seed germination seed coat rupture first because it is made up of cellulose (weak imbibant) and kernel is made up of protein, fat and starch (strong imbibant).

# (b) Surface area of imbibant : If more surface area of the imbibant is exposed and is in contact with liquid, the imbibition will be more.

# (c) Temperature : Increase in temperature causes an increase in the rate of imbibition.

# (d) Degree of dryness of imbibant : If the imbibant is dry it will imbibe more water than a relatively wet imbibant.

# (e) Concentration of solutes : Increase in the concentration of solutes in the medium decreases imbibition due to a decrease in the diffusion pressure gradient between the imbibant and the liquid being imbibed. It is due to the fact that imbibition is only a special type of diffusion accompanied by capillary action. If some solute is added into the liquid which is being imbibed, its diffusion pressure decreases and the process of imbibition slows down.

# (f) pH of imbibant : Proteins, being amphoteric in nature, imbibe least in neutral medium. Towards highly acidic or highly alkaline pH, the imbibition increases till a maximum is reached, there after it starts slowing down.

(a) The water is first imbibed by walls of root hairs and then absorbed and helps in rupturing of seed coat (made up of cellulose).
(b) Water is absorbed by germinating seeds through the process of imbibition.
(c) Germinating seeds can break the concrete pavements and roads etc.
(d) The water moves into ovules which are ripening into seeds by the process of imbibition.
(e) It is very significant property of hydrophilic surfaces.

The movement of water in plants cannot be accurately explained in terms of difference in concentration or in any other linear expression. The best way to express spontaneous movement of water from one region to another is in terms of the difference of free energy of water between two regions. Free energy is the thermodynamic parameter, that determine the direction in which physical and chemical changes must occur. The potential energy of water is called water potential. e.g., water is stored behind a dam. When the water runs downhill, its potential energy can be converted to electrical energy. This conversion of energy of water is due to gravity. The other source that provides energy to water is pressure. The increasing pressure increases the free energy there by increasing water potential.
Water running downhill due to gravity can be made to run uphill by overcoming the water potential (energy) by applying pressure. This means that water moves from the point, where water potential is greater to the other, where water potential is less. The difference in water potential between two points is a measure of the amount of work or energy needed to move water from one point to the other. Thus, based on the concept of water potential, the direction of water movement can be predicted. Water potential is measured in terms of pressure.

When a cell is subjected to the movement of water, many factors begin to operate which ultimately determine the water potential of cell sap. For solutions, such as contents of cells, water potential is determined by three major sets of internal factors viz., matric potential (ψm), solute potential (ψs) and pressure potential (ψp). The water potential (ψ) in a plant cell or tissue can be written as the sum of the matric potential (ψm) due to binding of water to cell walls and cytoplasm, the solute potential (ψs) due to concentration of dissolved solutes, which by its effect on the entropy components reduces the water potential and the pressure potential (ψp) due to hydrostatic pressure, which by its effect on the energy components increases the water potential :

Suppose A and B are two adjacent plant cells where osmotic movement of water can occur. Cell A has osmotic potential ( s) of –16 bars and pressure potential of 8 bars. The cell B has osmotic potential of –12 bars and pressure potential of 2 bars. The movement of water will be as follows :

Osmosis (Gr. Osmos = a pushing or impulse) was discovered by Abbe Nollet in 1748 and also coined the term 'osmosis'. First of all Traube (1867) use copper ferrocyanide and develop semipermeable membrane to show its utility in the osmosis of plant physiology. First time Pfeffer in (1887) develop osmoscope by using semipermeable membrane.
Osmosis is special type of diffusion of a liquid, when solvent moves through a semipermeable membrane from a place of higher diffusion pressure to a place of lower diffusion pressure.
Or
It is the migration of solvent from a hypotonic solution (of lower concentration) to hypertonic solution (of higher concentration) through a semi-permeable membrane to keep the concentration equal.
In osmosis, the water (or solvent) molecules moves as following fig.

 In formalin preserved Spirogyra filament, selective permeability of plasma membrane is lost and hence no effect on placing in hypertonic solution.
 If salt presents in higher concentration in a cell than outer side, water will enter in the cell by osmosis.

Osmotic pressure (OP) : Pfeffer coined the term osmotic pressure.
Osmotic pressure of a solution is the pressure which must be applied to it in order to prevent the passage of solvent due to osmosis.
[Or]
Osmotic pressure is that equivalent of maximum hydrostatic pressure which is produced in the solution, when this solution is separated from its pure solvent by a semipermeable membrane.
It can also be defined as "the excessive hydrostatic pressure which must be applied to it in order to make its water potential equal to that of pure water". Osmotic pressure is equal to the pressure which is needed to prevent the passage of pure water into an aqueous solution through a semi-permeable membrane. In other words, it is that pressure which is needed to check the process of osmosis.
(i) 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.
 When a fish of marine water kept in fresh water than it will be die due to endosmosis.
 An animal cell placed pure water will swell up and brust.
 Pollen grains of some of plants germinate on stigma soon but they burst in water or dilute sugar solution.

# (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.

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 changes 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.
 Osmotic concentration of a solution may governed by concentration of solute, temperature of solution, ionization of solutes and hydration of the solute molecules.
 In xerophytes, the osmotic concentration of cell sap is more than normal. e.g., A molar solution of sucrose separated from pure water by such a membrane has an OP of approximately 22.4 atmospheres at 0°C. The osmotic pressure of given solution can be calculated by using the following relationship.
[ Osmotic pressure = CST ]
Where, C = Molar concentration of solution, S = Solution constant, which is 0.082 and T = Absolute temperature i.e., 273°C.

- Sucrose is non-ionizing substance while NaCl is ionizing substance. Osmotic pressure of a solution of ionizing substance is greater than that of equimolar concentration of non-ionizing substance. e.g., 0.1M sucrose solution has an OP of 2.3 bars while 0.1M sodium chloride solution has value of 4.5 bars.

(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.

In a plant cell, however, two membranes are present between the cell sap and the surroundings the cell-wall is a permeable membrane that does not interfere with the movement of water and solutes into or out of the cell. The plasma membrane and vacuolar membrane (tonoplast) with the thin layer of cytoplasm between them behave as differentially permeable membrane. Cell sap of a cell is a mixture of water and soluble substances. Water absorption in root hair from soil is depends on the concentration of cell sap. So a cell behave as a osmotic system in which endosmosis generate following pressures –

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. The actual pressure that develops that is the pressure responsible for pushing the membrane against cell wall is termed turgor pressure.
In other words, we can say that when water enters the living cell, a pressure is developed within the cell due to turgidity. The hydrostatic pressure developed inside the cell on the cell wall due to endosmosis is called turgor pressure. It is responsible for growth of young cells.
Significance of turgidity in plants
 It provides stability to a cell.
 Turgidity keeps the cell and their organelles (mitochondria, plastids and microbodies) fully distended. This is essential for plants to live and grow normally.
 Turgor pressure helps in cell enlargement, consequently in stretching of the stems and in keeping leaves erect and fully expanded.
 The turgid cells provide mechanical support necessary for the non woody tissues (maize, sugarcane, banana etc.).
 Loss of turgidity leads to wilting of leaves and drooping of shoots.
 The opening and closing of stomata are regulated by the turgidity of the guard cells.
 Leaf movements (seismonastic movement) of many plants (such as bean, sensitive plant Mimosa pudica) are controlled by loss and gain of cell turgor.
 Due to turgid pressure plumule and radicles force out from seeds at the time of seed germination.

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. Normally wall pressure is equal and opposite to turgor pressure (WP =TP) except when the cell become flaccid. The value of the two forces continue to rise with the continued entry of water, till the cell becomes fully turgid.

DPD indicates the sucking power of suction pressure. As water enters into the cell the TP of the cell is increased. Cell wall exerts equal and opposite WP against TP. The actual force responsible for entry of water will be therefore OP–TP
i.e., [ DPD = OP – WP (As WP = TP) ]
[ DPD = OP – TP ]

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". The stage of plasmolysis, when the protoplast just begins to contract away from the cell wall is called incipient plasmolysis. The stage when the cell wall has reached its limit of contraction and the protoplast has detached from cell wall attaining spherical shape is called evident plasmolysis. In a plasmolysed cell, the space between the contracted protoplast and the cell wall remains filled with external solution. If a cell with incipient plasmolysis is placed in a hypertonic solution it will show more plasmolysis.
If a plasmolysed cell is placed in pure water or hypotonic solution, endosmosis takes place. The protoplast attains its original shape and the cell regains its original size. "The swelling up of a plasmolysed protoplast due to endosmosis under the influence of a hypotonic solution or water is called deplasmolysis'. Deplasmolysis is possible only immediately after plasmolysis otherwise the cell protoplast becomes permanently damaged. Leaf of Tradescantia is used for demonstration of plasmolysis in laboratory. The value of TP becomes zero at the time of limiting plasmolysis and below zero during incipient and evident plasmolysis.

It proves the permeability of the cell wall and the semipermeable nature of the protoplasm.
 The OP of a cell can be measured by plasmolysis. The OP of a cell is roughly equal to the OP of a solution that causes incipient plasmolysis in the cell.
 Salting of pickles, meat, fishes etc. and addition of sugar to jams, jellies, cut fruits etc., prevent their decay by microbes, as the latter get killed due to plasmolysis or due to high concentration of salt or sugar.
 By salting, the weeds can be killed from tennis courts and the growth of plants can be prevented in the cracks of walls.
 Plasmolysis is helpful in determining whether a particlular cell is living or dead as plasmolysis does not occur in a dead or non living cell.