Osmotic and Water Potentials, Biology tutorial


In plants, water moves through membranes. This movement of water via membranes in plants is explained as water potential. Water potential is the fundamental term derived from thermodynamics. There are 2 significant factors affecting water potential of plant cells. They are solute concentration (osmotic concentration) and pressure produced when water enters and inflates plant cells. These 2 factors are expressed in terms solute or osmotic potential and pressure potential.

Osmotic Potential (Solute Potential) -ΨS:

Osmotic pressure is pressure that exists in solution, in which solution and water is divided by semi-permeable membrane. In thermodynamics, osmotic pressure is known as osmotic potential or solute potential. Solute potential is measure of change in water potential of system because of presence of solute molecules. It is stated as (Psi) is Greek word ΨS is generally negative (-ΨS). Effect of dissolving solute molecules in pure water is to decrease concentration of water molecules and therefore to lower water potential. All solutions thus have lower water potentials than pure water.

Turgor pressure and wall pressure:

Turgor pressure is not appropriate for free solution. This is only appropriate for living cells. Turgor pressure is also called as hydrostatic pressure. When the cell is immersed in water, water enters in cell as osmotic pressure of cell sap is higher. Cell content press on wall or develop pressure against membrane that is known as turgor pressure (T.P.) Turgor pressure is counter balanced by the equal and opposite pressure of thick cell wall on enclosed solution, called as wall pressure. It signifies amount of pressure exert by cytoplasm on cell wall is same and in opposite direction as pressure exerted by cell wall towards inner side on cytoplasm. Thus, T.P. = W.P. Animal cell burst when place in pure water as wall pressure is absent because of absence of cell wall. For instance consequence of endosmosis in animal cell can be shown by placing RBCs of human blood in distilled water contained in the dish. When examined after a little bit, RBCs are found to have burst open leaving the cell membranes as empty cases. Flaccid cell has zero turgor pressure.

Importance of turgor pressure:

  • Protoplasm of cell attached with cell wall because of turgidity of cell and cell becomes stretched. It maintains normal shape of cell in which physiological procedures are going on.
  • Spatial 3-D structure of mitochondria, chloroplast and microbodies are maintained because of turgor pressure that is essential for the physiological activities.
  • Turgor pressure is necessary for keeping definite shape of delicate organs.
  • Turgor pressure assists in cell elongation.
  • Turgor pressure is necessary for sprouting plumules and radicles from seed. Turgor pressure gives necessary support to plumule to come out of soil and assist inpenetration of radical in soil.

Diffusion pressure deficit (DPD):

Term diffusion pressure deficit [DPD] was utilized by B.S. Meyer. This is feature of water in solution. But usually, it is known as DPD of solution or cell. DPD of any solution is difference between diffusion pressure of water that is present in solution and diffusion pressure of pure water. DPD finds direction of osmosis and power of absorption of cell. In relation to plants, DPD may be stated as amount of that pressure by which water is sucked in cell or expels outside cell. Thus, it is called as suction pressure or DPD is also called as demand of water deficit. Diffusion of water or solute occurs from region of lower DPD to region of higher DPD in procedure of osmosis.

Generally in cell, osmotic pressure is greater than turgor pressure. Difference in osmotic pressure and turgor pressure is known as suction pressure or DPD. Diffusion pressure deficit is power (capacity) of water absorption. DPD of any free solution is equal to osmotic pressure of that solution as it has no turgor pressure. Thus - DPD = OP. Though, wall pressure develops because of turgor pressure through process of osmosis. This relation can be stated by the given equation:


DPD = OP - TP (Q TP = WP)

If cell is placed in pure water or less concentrated of solution than cell sap, then water enters in cell as a result turgor pressure develop in cell. Cell begins swelling because of turgor pressure. Concurrently, concentration of cell sap reduces because of continuous inflow of water. Thus OP decreases because of this. Finally, when value of TP will be equal to OP then DPD will be zero. At this phase cell becomes in completely turgid state. Thus, in fully turgid cell,


When OP = TP

So that OP - TP = 0

DPD = 0

If cell in flaccid state then its TP and WP would be zero and value of DPD would be equal to OP. If the flaccid cell placed in water then water enters in cell as DPD of cell sap is higher.

At times value of turgor pressure is negative as in plasmolysed cell. In this state

DPD = OP - TP [TP = - ve]

DPD = OP - [- TP] = OP + TP


So DPD of plasmolysed cell is greater than osmotic pressure. It signifies:


Or demand of water in plasmolysed cell is greater. It signifies when osmotic pressure and turgor pressure will be equal, in that time DPD will be zero. Water won't enter in that kind of cell. But, when turgor pressure is lesser than osmotic pressure then some DPD will be certainly present in cell and water would enter in cell. When two different concentration of solution is divided by semi permeable membrane then entry of water relies on DPD.

Water potential (ΨW):

Water has kinetic energy that signifies that in liquid or gaseous form they move about fast and randomly from one location to another. Greater concentration of water molecules in the system, greater total kinetic energy of water molecule in that system and higher water potential. Pure water thus has highest water potential. If two systems having water are in contact (like soil and atmosphere, or cell and solution) random movements of water molecules will result in net movement of water molecules from system with higher water potential (higher energy) to system with lower water potential (lower energy) until concentration of water molecules in both systems is equal. Water potential of pure water is zero, pure water contain greater free energy. Free energy lowers down by are addition of solutes. Difference between free energy of molecules of pure water and free energy of molecules of water of solution is known as water potential of system.

Types of solutions:

Isotonic solutions:

If solution, in which cell is placed, has equal osmotic pressure to that of cell sap, outer solution is known as isotonic solution. In this kind osmotic concentration of both solutions is same. In such kind of solution is neither endosmosis nor exosmosis happens.

Hypotonic solutions:

If osmotic concentration of outer solution is less than that of cell sap, outer solution is called as hypotonic solution. If cell is placed in such solution endosmosis happens consequently, cell swells up; like raisins swell up when placed in distilled water.

Hypertonic solutions:

If osmotic concentration of solution is higher than that of other (cell sap) solution solution is called as hypertonic solution. If the cell placed in this kind of solution, exosmosis occurs. It means water of cell sap is diffused out in outer solution resulting in cell becoming flaccid; like grapes placed in higher concentration of sugar solution that becomes flaccid (contracts).


If the plant cell placed in hypertonic solution, water molecules diffuse out from cell. Consequently of exosmosis, protoplasm of cell is detached from cell membrane (cell wall) and begins shrinking in centre. This is known as plasmolysis. Different sequences of plasmolysis are as follows:

In the turgid cell, cell sap pushes away protoplasm so that it remains with contact of cell wall.

When it placed in the hypertonic solution, volume of cell decreases because of shrinking of cell as some amount of water of cell sap is diffused out by exosmosis. Turgor pressure reduces by which cell wall isn't pushed by protoplasm, so that shrinking cell wall decreased in total volume of cell. This condition is known as first stage of plasmolysis or limiting plasmolysis.

If diffusion of water to outside is continued by exosmosis then central vacuole contracts and with the protoplasm also shrinks but cell wall is not contracting. So that protoplasm is appears to detaching from corners of cell wall. This situation is called as second phase of plasmolysis or Incipient plasmolysis. Hypertonic solution is present in between cell wall and protoplasm. Shrinking of protoplasm is continuous because of continuous exosmosis, it detached from cell wall and supposed a spherical shape. This stage is called as evident plasmolysis full plasmolysis.

Kinds of plasmolysis:

Convex plasmolysis: Protoplasm is wholly contracted and becomes convex shaped in the stage.

Concave plasmolysis: Protoplasm is not contracted wholly and it is attached with cell wall at some places by protoplasmic fibres or plasmodesmata. Such fibres are known as fibres of Hetch. Due to these fibres, protoplasm appears to be concave shaped.

Importance of plasmolysis:

  • A living cell is distinguished from non living [dead] cell via plasmolysis. This is because of fact that plasmolysis occurs in dead cell only.
  • Osmotic pressure of any one cell can be estimated by limiting plasmolysis.
  • If plasmolysis remains for long duration in the cell then it dies. For instance, to demolish weeds, salts are placed in the roots. Fish and meat are prevented from spoilage by salting that inhibits development of bacteria and fungus. Also, higher concentration of sugar in jams and jellies stops development of bacteria and fungus. High amount of manure near root causes death of plant because of plasmolysis.


Imbibition is described as adsorption of undissolved liquid by any solid materials or adsorption of water by hydrophilic colloids. It refers to the physical process by which the dry solid colloid material swells up by absorption of water. Plant's cell wall is composed of colloidal substances like cellulose, pectin, hemicellulose, etc. All these are hydrophillic in nature. Therefore they tend to imbibe water simply. Adsorption of liquid occurs on materials in process of imbibition. Agar - agar, Proteins, starch etc. all substances are imbibants. Agar - agar can absorbs 99 times more water than that of the weight. Some of proteins absorb 15 times more water. Affinity must be present between imbibants and liquid material. Cellulose relatively has very less power of imbibition. Seeds contain good amount of colloidal materials so that they are good imbibant materials. Imbibition primarily is diffusion of water. DPD of dry imbibant material is zero. When they come in contact of water or solution water begins diffusing in imbibant materials. When diffusion pressure of imbibant material is equal to diffusion pressure of outer liquid in saturation state then kinetic equilibrium is established. Excluding of liquid form, imbibition is also found in vapor forms. Wooden doors absorb water in form of vapors and swells up in rainy season.

Effects of Imbibition:

Swelling: Volume of materials increased in process of imbibition. But total volume is found less by sum of both volumes. Imbibant material + water = swelling. 10 + 90 = less than 100 (always)

Liberation of heat: Heat is released during imbibition. Water molecule becomes motionless because of imbibition by which they lose kinetic energy. This energy again seems in form of heat. This is called as heat of wetting.

Imbibition pressure: The huge pressure is developed in the material limited in space because of imbibition. Value of this pressure is reaches up to several thousands atmospheric pressure. This method is utilized in ancient period for breaking of rocks. Dry wood is filled in natural grooves of rocks and water is poured over them. Rocks are broken because of their swelling.

Factors affecting Imbibition:

Temperature: Imbibition is directly proportional to temperature.

Texture of imbibant: Imbibition is found less in efficiently arranged material such as wood, and more in lighter or soft material such as gelatin.

pH: Negative charged colloids like cellulose illustrates greater imbibition in alkaline (high pH) environment (medium). Positive charged colloids illustrate more imbibition in acidic medium. Protein due to being amphoteric illustrates very less imbibition in neutral medium and it illustrates more imbibition in acidic and alkaline medium.

Electrolytes: Electrolytes neutralize charges of imbibants or decrease imbibition process by influencing osmotic pressure.

Pressure: Imbibition decreases with increasing pressure.

Importance of Imbibition:

  • Absorption of water in seed germination is only through imbibition.
  • Breakage of seed coat during seedling is because of imbibition process proteins, fats and starch is present in kernel. This kernel swells up more as compared to seed coat that breaks seed coat. Seed coat is composed of cellulose.
  • Several hydrophillic colloids are present in cell wall and protoplasm of plant. Absorption of water occurs in young cells through imbibation process.
  • Resurrection in several plants such as Selaginella, Lichen occurs because procedure of imbibition.
  • Hydrophilic materials decrease amount of free water that protects cell during dryness and it protects from lowest temperature during winter.
  • DPD of some fruits is higher than their osmotic pressure. Higher DPD is because of imbibation, so that they can absorb water in extreme xeric conditions (dry).
  • Several dry fruits (cotton balls, pods of Moong, Urad) dehiscence through absorption of water in dry conditions by imbibition. Water enters in aerial roots and dry fruit through imbibition.

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