What do you understand by plasmolysis? Explain imbibition and point out the differences between imbibition and osmosis.

PLASMOLYSIS

 If a plant is placed in highly concentrated sugar or salt solution (hypertonic solution), water from cell sap flows out due to exosmosis through plasma membrane.  Loss of water from the cell sap causes contraction or shrinkage of protoplasm.  Since the cell wall is firm, it cannot keep pace with the contraction of the plasma membrane.  Ultimately, protoplasm is separated from the cell wall and assumes a spherical shape.  This condition is known as plasmolysis and the cell is said to be plasmolysed.

 

In a plasmolysed plant cell, the space between the contracted protoplasm and cell wall remains filled with the external solution.  Plasmolysis can be demonstrated in the epidermal peel of Rhoeo discolor leaf.  The cell contents are purple in color due to the presence of anthocyanin pigments.When the epidermal peels of Rhoeo discolor leaves are placed in hypertonic solution, exosmosis takes place. The purple cell contents start pulling away from the cell wall and the cell wall begins to lose turgidity.  A colorless space can be seen between the cell wall and the purple contracted protoplasm.  This initial stage of plasmolysis is called incipient plasmolysis.  If a cell showing incipient plasmolysis is immersed in water or in a solution whose concentration is less than that of the cell sap (hypotonic solution), the cell regains its turgidity due to endosmosis.   However, if the exosmosis continues, it causes permanent plasmolysis.  Such a cell cannot regain turgidity even after it is transferred to a hypotonic solution and it eventually dies.

IMPORTANCE OF PLASMOLYSIS

  1. Plasmolysis can be shown only by living cells. It can, therefore, determine whether a cell is living or dead.
  2. Plants are not allowed to grow in the cracks of the walls by the method of salting. This treatment causes plasmolysis in the root cells and the plants finally die.
  3. High concentration of sugar in jams and jelly prevents the growth of molds, bacteria etc.
  4. Bacteria do not survive in high-salted pickle because they become plasmolysed and consequently, are killed.
  5. Salting of meat and fish kill the spores of fungi and bacteria.
  6. Osmotic pressure of a cell can be measured by plasmolysis. It will be roughly equivalent to the osmotic pressure of a solution, which will be strong enough to cause only incipient plasmolysis.
  7. It shows that the cell wall is elastic as well as permeable.
  8. Excessive concentration of chemical fertilizer at one place in the soil should be avoided to prevent the plants from dying.

IMBIBITION

The absorption of water by the solid particles of an adsorbant without forming a solution is called imbibition.  The particles which absorb water are called imbibants and the pressure which is created is called imbibition pressure.  The liquid (usually water) which is imbibed is known as imbibate.  Plant imbibants are hydrophilic colloids.

If a dry piece of wood is placed in water, it swells and the volume increases.  Similarly, when seeds are soaked in water, volume of the seeds increases.  Wooden doors and windows absorb water in humid rainy season and increase in their volume.  All these are the examples of imbibition.  Increase in volume of imbiants can be seen by soaking some dry seeds of pea in water.  The dry seeds are negative to water potential, when dipped in water, a steep water potential gradient is established.  Water molecules enter through the micropore of the imbibants and get adsorbed.  It results in the swelling of the seeds.  During imbibition, temperature increases.  The water molecules possess kinetic energy.  During imbibition, the water molecules get tightly absorbed and they become immobile.  They lose most of their kinetic movement which is called heat of hydration or heat of wetting.

Imbibition capacity is maximum in phycocalloids followed by proteins, starch and cellulose.  Lignin is hydrophobic.  It, therefore, does not show imbibition of water.

 

FACTORS AFFECTING IMBIBITION

  1. Temperature.  The rate of imbibition increases with the increase in temperature.
  2. Effect of concentration of solute in the medium. The rate of imbibition decreases with increase in the concentration of solute in the medium.
  3. Effect of pH. Hydrogen ion concentration or pH of the medium largely affects the rate of imbibition.

 

IMPORTANCE OF IMBIBITION

  1. Imbibition is the initial step of germination of seeds. The water is first imbibed by the seed coat than by other tissues of embryo and endosperm.
  2. Breaking of seed coat in germinating seed is due to greater imbibitional swelling of the seed kernel (starch and protein) as compared to seed coverings (cellulose).
  3. Seedling is able to come out of the soil due to development of imbibition pressure.
  4. Imbibition pressure developed during germination of seeds and spores can even break concrete pavements and asphalt roads.
  5. Absorption of water by young cells is mostly through imbibition.
  6. Imbibition is dominant in the initial stage of water absorption by root. The water is first adsorbed by the wall of the root hairs before it is actually absorbed by the root.
  7. The movement of water into ovules that are ripening into seeds is made possible by the process of imbibition.
  8. Jamming of wooden frames during rain is caused by the swelling of wood due to imbibition.

 

DIFFERENCE BETWEEN IMBIBITION AND OSMOSIS

 

IMBIBITION

OSMOSIS

1.  Imbibition  involves the absorption of solvent or water by a solid substance. 1.  It involves the movement of water or solvent from its higher chemical potential to its lower chemical potential.
2.  Imbibition does not produce a solution. 2.  Osmosis usually takes place in solution separated by a semipermeable membrane.
3.  A semipermeable membrane is not required. 3.  A semipermeable membrane is essential for the operation of osmosis.
4.  Imbibition produces heat. 4.  Heat is not produced in osmosis.
5.  Imbibition can develop a very high pressure (up to 1000 atm) called imbibition pressure. 5.  Osmosis develops a comparatively low pressure (up to 100 amp) known as osmotic pressure.
6.  Imbibition requires the presence of colloidal particles. 6.  Osmosis usually requires the presence of solute particles.