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Class 9, Science, Chapter-5, Lecture-3, Plasmolysis (Notes)

MECHANISM OF OSMOSIS IN CELLS:

If the medium (external environment) has higher water concentration than the cell, water enters the cell through endosmosis.
If the medium has lower water concentration than the cell, water moves out of the cell to the medium through Exosmosis.
Thus, movement of water takes place between the cell and its external environment.

OSMOSIS:

The spontaneous movement of solvent through a semi permeable membrane from a region of low solute concentration to a region of high solute concentration is called osmosis.

  1. Endosmosis: The osmotic inward movement of water into a system is called endosmosis.
    * occurs in HYPOTONIC solutions.
  2. Exosmosis: The osmotic outward movement of water from a system is called Exosmosis.
    * occurs in HYPERTONIC solutions.

HYPOTONIC SOLUTION:

A solution medium which has lower solute concentration than the substance kept in it is termed as a hypotonic solution.

HYPERTONIC SOLUTION:

A solution medium which has higher solute concentration than the substance kept in it is termed as a hypertonic solution.

ISOTONIC SOLUTION:

A solution medium which has equal solute concentration as the substance kept in it is termed as a isotonic solution.

A cell swells up when placed in a hypotonic solution.

Reason: When a cell is placed in a hypotonic solution, the medium has lower solute concentration and higher water concentration than the cell. Endosmosis occurs and water molecules enter the cell. Movement of water molecules take place in both directions but amount of water entering the cell is more than the amount of water going out of the cell.
The cell swells due to the increase in its matter content.

A cell shrinks when placed in a hypertonic solution.

Reason: When a cell is placed in a hypertonic solution, the medium has higher solute concentration and lower water concentration than the cell. Exosmosis occurs and water molecules exit from the cell. Movement of water molecules take place in both directions but amount of water entering the cell is less than the amount of water going out of the cell. The cell shrinks due to the decrease in its matter content.

A cell remains of same size when placed in an isotonic solution.

Reason: When a cell is placed in an isotonic solution, the medium has same solute concentration and water concentration as the cell.
Osmosis does not occur. Movement of water molecules take place in both directions but amount of water entering the cell is equal to the amount of water going out of the cell.
The cell remains of same size as its matter content remain same.

ENDOCYTOSIS:

The engulfing of food and other particles by a cell is called endocytosis.

  • Phagocytosis: The engulfing of solid particles by a cell is called phagocytosis
  • Pinocytosis: The engulfing of liquid particles by a cell is called pinocytosis

PLASMOLYSIS:

The contraction of the contents of a living cell away from the cell wall due to loss of water to the surrounding is termed as plasmolysis.

Cell wall prevents bursting of plant cells.

When a cell is placed in a hypotonic solution, the medium has lower solute concentration and higher water concentration than the cell. Endosmosis occurs and water molecules enter the cell. Movement of water molecules take place in both directions but amount of water entering the cell is more than the amount of water going out of the cell.
The cell swells due to the increase in its matter content.
If the cell swells beyond a certain limit, it may burst. The cell wall exerts pressure against the swollen cell and stops further swelling.
Thus, the cell wall prevents the cell from bursting.

Activity to demonstrate endosmosis and exosmosis:

Step I : The shell of two eggs are removed by dissolving them in dilute hydrochloric acid.
Step II: One of the de-shelled egg is placed in pure water (hypotonic solution) for 5 minutes.
Another de-shelled egg is placed in concentrated salt solution (hypertonic solution) for 5 minutes.
Observation: The egg in the hypotonic solution is observed to be swollen.
The egg in the hypertonic solution is observed to be shrunken.
Conclusion: Swelling of egg in hypotonic solution indicates endosmosis of water to the egg.
Shrinking of egg in hypertonic solution indicates exosmosis of water from the egg.

Activity to demonstrate that dead plasma membrane is not selectively permeable:

Step I : A slide of living Rheo leaf and another slide of dead Rheo leaf (boiled in water for few minutes) are prepared.
Step II: Few drops of concentrated sugar solution are added to each of the slides.
Step III: The slides are observed after 1 minute.
Observation: In the LIVING cell, the content of the cell contracts and moves away from the cell wall. Plasmolysis takes place. In the DEAD cell, the content of the cell does not contract. Plasmolysis does not take place.
Conclusion: Plasmolysis in the living cell demonstrates that osmosis takes place in living cells.
Absence of Plasmolysis in the dead cell demonstrates that osmosis does not take place in dead cell.