Water Potential, Osmosis, Plasmolysis, Imbibition

Plant Water Relations Water Potential, Osmosis, Plasmolysis And Imbibition

Water is an important component of plant cells. Mature plants have 5-10% of total water in the cytoplasm. The remaining 90-95% water is present within the large, centrally placed vacuole in the cell. The content of the vacuole is known as the cell sap. In actively dividing plant cells, water constitutes 80-95% of total cell mass. Water is a polar molecule.

Constituent molecules of water remain attached by forming hydrogen bonds. Due to the presence of hydrogen bonds, water molecules have high cohesive and adhesive forces. Cohesive force imparts high tensile strength to water.

plant water relations notes

All these characteristics of water molecules enable them to pass through the cell membrane easily. The factors that are related to the movement of water molecules across the cell membrane are—imbibition, water potential, plasmolysis, and osmosis. But, before discussing these factors, we must learn about the permeability of the membrane.

Permeability

Definition: The property of the cell wall and cell membrane, or any other membrane that allows different ions and molecules of elements and compounds to pass through them is called permeability.

Types of membranes on the basis of permeability

Based on the permeability, membranes are classified into the following types.

Permeable membrane: The membranes that allow movement of the molecules of both solutes and solvents through them are called permeable membranes. example, thin and cellulosic cell walls of parenchyma and meristematic tissue of plants.

Impermeable membrane: The membranes which do not allow the movement of either solute or solvent molecules through them, are called impermeable membranes. example, lignified or cutinized cell wall, a thin artificial membrane made of rubber or plastic.

Semipermeable membrane: The membranes which allow the passage of solvent molecules but do not allow the passage of solute molecules through them, are called semipermeable membranes. example parchment paper, cellophane paper, swim bladder of bony fishes, and the thin membrane under the hard shell of an egg.

Selectively permeable membrane or differentially permeable membrane: The membranes that allow only specific solute and solvent molecules to pass through them, according to the cell requirement, are called selectively permeable membranes or differentially permeable membranes. the example plasma membrane, nuclear membrane, and tonoplast.

Water Potential

The free energy per mole, i.e., per gram molecular weight of any substance is called its chemical potential. This free energy is required by all living organisms in order to maintain physiological activities. Under constant temperature and pressure, if the density of water increases in a system, then its kinetic energy also increases. This causes an increase in the chemical potential of water. On the basis of these observations, the concept of water potential was conceived by scientist Slayer (1967).

” plant water relations class 12″

Definition: Under constant temperature and pressure, RU the difference between the free energy or chemical potential of a given water sample and the chemical potential of pure water is called water potential.

Water potential is denoted by the Greek letter psi with ‘w’ (to denote water) subscript position.

Water potential (ψ_w) is measured in the following ways—

1. Water potential can be obtained by dividing the chemical potential of water, of a given sample by the volume of 1 mole of water.
2. Water potential is also expressed as free energy per unit volume of water. Its unit is Joule per cubic meter or Jm-3. The measuring unit of water potential is Pascal (Pa) and Megapascal (MPa).

Major factors controlling water potential in plants

” what is water potential in plants”

Three factors that control water potential in plants are—

Solute potential or osmotic potential (ψ)

Pressure potential (ψ_p)

gravity potential (ψ).

Water potential is expressed by these three terms as—

Ψ_w=ψ_s+Ψ_p+ψ_g

Osmotic potential (ψ_s): The potential of a solvent molecule to move from a lower concentration to a higher concentration across a semipermeable membrane, is called osmotic potential. The osmotic potential of pure water is zero. When a solute is dissolved in pure water, the osmotic potential of the solution becomes less than zero. Osmotic potential is also known as solute potential. It is denoted by ψ_s. It is measured in Bars. [1 Bar = 0.987 atmospheric pressure]. It is always negative for a solution.

Pressure potential (Ψ_p): When a plant cell is immersed in pure water, water enters the cell. This increases the turgidity. The pressure created on the cell membrane due to this turgid condition is called turgor pressure. This turgor pressure with respect to the water potential is called pressure potential. It is denoted by Ψ_p. Its magnitude is always positive. This pressure potential generates wall pressure in plant cells.Ψ_p of pure water is 0 MPa, although standard wall pressure is 0.1 MPa.

” plant water relations exercise”

Gravity or Gravitational pressure potential (ψ_g): The effect of gravitational force on water potential is called gravitational pressure potential. It is denoted by ψ_g. It depends on the height of the water column (h), the density of water (pw), and acceleration due to gravity (g), i.e., ψ_g = yowgh. Water potential for a water column of 10m height will be, 0.1 MPa. So, for a height of 5m, ψ_g is negligible. In such cases, Ψ_w=ψ_s+Ψ_p

Plasmolysis

Plasmolysis Definition: The phenomenon in which the protoplasm of the plant cell separates from the cell wall and starts shrinking at the center due to exosmosis, when the cell is placed in a hypertonic solution, is called plasmolysis.

Plasmolysis Explanation: Mature plant cells have centrally located large vacuoles. When kept in hypertonic solution (OP_e>OP_i), water moves out by the process of exosmosis, into an external solution, from cytoplasm and cell sap. This causes a reduction in turgor pressure and volume of cell vacuole.

This situation is called the first stage of plasmolysis or limiting plasmolysis. At this stage cell starts to shrink but protoplasm remains attached to the cell -wall. The cell reduces slightly from its original size.

If the diffusion of water to the external solution continues due to exosmosis, then the vacuole contracts further, and protoplasm along with the cell membrane shrinks, but the cell wall remains intact. It appears that the protoplasm separates from the corners of the cell wall. This condition is known as the second phase of plasmolysis or incipient plasmolysis.

If the shrinking of protoplasm still continues due to continuous exosmosis; it will separate completely from the cell wall and assume a spherical shape. This phase is known as evident plasmolysis or complete plasmolysis.

According to the shrunken shape of the protoplast, plasmolysis is of two types

Convex plasmolysis: The protoplast appears completely contracted and becomes convex-shaped in this stage.

Concave plasmolysis: The protoplast does not appear contracted completely and remains attached to the cell wall at some points, through the protoplasmic fibers.

Test for plasmolysis

Materials required: Leaves of Rhoeo discolor plant, sugar or sucrose solution, some Petri dishes, forceps, glass slide, coverslip, distilled water, microscope.

Plasmolysis Procedure:

1. A series of sugar solutions, of different concentrations (0.1M, 0.2 M, 0.3 M, 0.4M) are prepared.
2. These are kept in different Petri dishes and are marked accordingly.
3. A Rhoeo leaf is taken. Its lower surface is peeled off and cut into small pieces. A few of these pieces are placed in each of the sucrose solutions made and a few are immersed in distilled water as control.
4. The Petri dishes are kept undisturbed for about 30-40 minutes.

Plasmolysis Observation and inference: Anthocyanin is a water-soluble pigment present in the cell sap of Rhoeo leaves. This pigment gives a purplish color to the cytoplasm. This helps in the visual identification of the changes in the cytoplasm.

After 30-40 minutes, the peels are taken out from the Petri dishes. They are then placed on glass slides and are covered with coverslips. Now they are observed under a microscope.

In distilled water and sucrose solutions of low concentration, no changes are observed in the cell cytoplasm or the vacuole. Thus we may infer that cell sap concentration remained unchanged.

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Hence, plasmolysis has not occurred in these cells. However, leaves kept in a sucrose solution of high concentration show shrunken protoplasm which is detached from the wall. This means incipient plasmolysis has occurred.

This plasmolyzed protoplasm shows a darker purple color because of the low water content in the cell sap.

Plasmolysis Significance:

1. A living cell can be distinguished from a non-living cell through plasmolysis, as it does not occur in dead cells.
2. The osmotic pressure and osmotic concentration of any cell can be measured by limiting plasmolysis.
3. If the plasmolyzed condition prevails for a longer duration in a cell, it dies.
4. The occurrence of plasmolysis signifies the permeability and elasticity of the cell wall.

Deplasmolysis

If a plasmolyzed cell is kept in a hypotonic solution or in pure water, then the protoplast expands and restores its normal condition due to endosmosis (OP_e < OP_i). This phenomenon is called ‘deplasmolysis’.

What will happen when a filament of Spirogyra is kept in a 2% sucrose solution?

In the cells of a filament of Spirogyra cytoplasm is present as primordial utricle. Its central vacuole contains cell sap. But cell sap is less concentrated than the external 2% sucrose solution. So, water will move out of the cell to external sucrose solution, through the cell membrane.

This will cause a reduction in turgor pressure in the cell. Cytoplasm, with cell membrane, will shrink and become, almost spherical. Hence, plasmolysis takes place.

However, if these plasmolysed filaments of Spirogyra are kept in pure water, then OP_e< OP_i. Thus, water will enter the cell or endosmosis will occur. The cell will become turgid again. Hence, we can say deplasmolysis has occurred.

Imbibition

Imbibition Definition: The physical process by which water or any other liquid is absorbed by hydrophilic colloids like protein, polysaccharides, etc., is called imbibition.

Imbibition Explanation: Dry seeds swell up when they are soaked in water for some time. Wooden doors absorb water in the form of water vapor present in the atmosphere and swell during the rainy seasons. As a result, we face difficulty in opening doors. These are all consequences of imbibition.

This is a physical process by which hydrophilic colloid materials swell due to the absorption of water or other fluids. Components of cell walls such as cellulose, pectin, etc., are hydrophilic in nature. Therefore, they absorb water and swell up.

Different types of organic materials have imbibing properties. Those substances which have this property are called imbibing. The liquids which can be imbibed, are called imbibate. Protein, agar agar, starch, gelatin, etc., are some imbibing. Imbibants in the living organisms are mainly hydrophilic colloids.

Imbibition is absent in hydrophobic substances like lignin, cutin, etc. Imbibition is mainly the diffusion of an imbibe (for example water) within an imbibing. When the diffusion pressure of the imbibing is equal to the diffusion pressure of the imbibing, then the equilibrium is established. This leads to the cessation of the imbibition process. Gaseous substances also can be imbibed.

Effects of imbibition: Imbibition has several effects on the imbibing, such as—

Imbibition Swelling: The volume of the imbibant increases during the process of imbibition. This is commonly known as swelling.

Liberation of heat: Heat is released during imbibition

Imbibition pressure: Due to imbibition, huge pressure develops within the imbibant. This may cause the implant to burst or crack. This pressure is called imbibition pressure.

Imbibition Characteristic features:

1. Imbibition is a physical process.
2. The more hydrophilic the colloidal substances are, the faster will be the rate of imbibition.
3. During imbibition, imbibate molecules are attracted by adhesive forces, present on the surface of the imbibant. These molecules move fast towards the imbibant and their movement generates kinetic energy. As soon as they bind to the surface of the imbibing, the kinetic energy gets converted to heat energy.
4. The affinity between imbibing and imbibing facilitates the process of imbibition.
5. Imbibition pressure is generated in the implants due to an increase in volume property. Those substances which have thiafter the absorption of the imbibate.
6. Temperature, nature of solution, pH, presence of ions, etc., regulate the rate of imbibition.
7. This process does not require any semipermeable membrane.
8. Transport of imbibate molecules does not occur during imbibition. In fact, they remain bound to the imbibant molecules but do not get transported.
9. It is a passive process, that takes place along a diffusion gradient.
10. Imbibition requires direct contact between imbibant and imbibate molecules.

Factors affecting the rate of imbibition: The rate of imbibition is influenced by the following factors.

Imbibition Temperature: With the increase of temperature, I rate of imbibition increases.

The texture of Imbibing material: Imbibition occurs slowly in compact materials, like wood, and fast in lighter or soft materials, like gelatin.

Nature of imbibition: The nature of organic substances influences the rate of imbibition. For example, proteins have more imbibition capacity than starch. Cellulose has even less imbibition capacity than starch.

Imbibition Pressure: The rate of imbibition decreases with increasing pressure on imbibing.

Imbibition pressure or matric potential: Any colloidal substance that has imbibition property, is called matric. These substances are hydrophilic in nature and so, absorb water upon contact. The pressure that develops in the imbibant by imbibing water or other fluids, is called imbibition pressure or matric potential.

Imbibition Significance:

1. Absorption of water during germination of seeds takes place only through imbibition. Breaking of seed coat during germination occurs due to imbibition of water.
2. Many dry fruits (cotton balls, pods of mung bean, etc.) undergo dehiscence after absorbing water.
3. In roots, the cellulose molecules of the cell wall, absorb water by this process.