Study the phenomenon of osmosis

Objective

To study the phenomenon of osmosis. 

 

Theory

Osmosis 

Osmosis is a process of movement of solvent molecules, usually water, across a selectively permeable membrane from an area of lower solute concentration to an area of higher solute concentration. This movement continues until equilibrium is reached, with an equal concentration of water on both sides of the membrane. 

  

Characteristics of osmosis: 

Selective Permeability: The membrane involved in osmosis is selectively permeable, meaning it allows the passage of certain molecules while restricting others. In the context of osmosis, water molecules can pass through the membrane, but solute molecules (such as ions or larger particles) are generally restricted. 

Concentration Gradient: Osmosis occurs along a concentration gradient. The water movement's direction is from an area of lower solute concentration to a higher solute concentration. This is driven by the natural tendency of solvent molecules to move from regions of higher water concentration to lower water concentration. 

Equilibrium: The process of osmosis continues until equilibrium is achieved. At equilibrium, the concentrations of water on both sides of the membrane are equal. It's important to note that equilibrium does not mean equal concentrations of solute on both sides; rather, it refers to equal concentrations of water. 

Biological Significance: Osmosis plays a critical role in biological systems, particularly in maintaining the balance of water and solutes in cells. Cells have semi-permeable membranes that allow water to move in and out, influencing cell volume and turgor pressure. 

Cellular Effects: In a hypertonic solution (higher solute concentration outside the cell), water moves out of the cell, leading to cell shrinkage or plasmolysis. In a hypotonic solution (lower solute concentration outside the cell), water moves into the cell, causing it to swell or even burst. 

                                                                    

                                                                                                                              Fig. 1 Osmosis 

 

Osmosis in Different Types of solution 

Hypertonic, hypotonic, and isotonic are terms used to describe the relative concentrations of solutes in solutions on either side of a selectively permeable membrane, such as a cell membrane.  

Hypertonic Solution: 

  • Definition: A hypertonic solution has a higher concentration of solutes (such as ions or other particles) compared to another solution. 
  • Effect on Cells: When a cell is placed in a hypertonic solution, water tends to move out of the cell, causing the cell to shrink or undergo plasmolysis. This is because water moves from an area of lower solute concentration (inside the cell) to an area of higher solute concentration (outside the cell). 

Hypotonic Solution: 

  • Definition: A hypotonic solution has a lower concentration of solutes compared to another solution. 
  • Effect on Cells: When a cell is placed in a hypotonic solution, water tends to move into the cell, causing it to swell or even burst (cytolysis). This is because water moves from an area of higher solute concentration (outside the cell) to an area of lower solute concentration (inside the cell). 

Isotonic Solution: 

  • Definition: An isotonic solution has the same concentration of solutes as another solution. 
  • Effect on Cells: When a cell is placed in an isotonic solution, there is no net movement of water. The concentration of solutes is equal inside and outside the cell, resulting in the cell maintaining its normal shape and size. Isotonic solutions are often used in medical settings for intravenous fluids to prevent cells from shrinking or swelling. 

 

                                                                     

                                                                                                    Fig. 2 Osmosis in different types of solution 

 

Semi-Permeable (or Selectively Permeable) Membrane: 

A semi-permeable membrane is a type of biological or synthetic membrane that allows certain molecules or ions to pass through it while restricting the movement of others. It selectively permits the passage of substances based on factors such as size, charge, and solubility.   

Example: Cell membrane   

De-plasmolysis: 

  • De-plasmolysis is the process by which a plasmolyzed cell (a cell that has lost water and undergone plasmolysis) is restored to its original turgid state by placing it in a hypotonic solution. 
  • Plasmolysis is the contraction of the protoplasm away from the cell wall due to the loss of water from the cell. It occurs when a plant cell is placed in a hypertonic solution (higher solute concentration outside the cell), causing water to move out of the cell. 
  • When the plasmolyzed cell is placed in a hypotonic solution (lower solute concentration outside the cell), water moves into the cell, and the protoplasm expands, pushing the cell membrane against the cell wall. This restoration of the cell to its original turgid state is called de-plasmolysis.  

Turgidity: 

Turgidity refers to the state of being swollen and firm due to the internal pressure of water in plant cells. It is a result of the osmotic movement of water into the cell, causing the cell to expand and press against the cell wall. 

 

Types of Osmosis 

Endosmosis 

When a substance is placed in a hypotonic solution, the solvent molecules move inside the cell and the cell becomes turgid or undergoes de-plasmolysis. This is known as endosmosis. 

Exosmosis 

When a substance is placed in a hypertonic solution, the solvent molecules move outside the cell and the cell becomes flaccid or undergoes plasmolysis. This is known as exosmosis. 

 

                                                                    

                                                                                                                        Fig. 3 Types of osmosis 

 

Importance of Osmosis 

  • Osmosis helps in the transport of nutrients and the release of metabolic waste products. 
  • It maintains the turgidity of cells. 
  • The cell membrane of both plants and animals forms a selective barrier between the cell and its environment and does not allow toxic substances from the surroundings to enter the cell.  
  • Osmosis induces cell turgor which regulates the movement of plants and plant parts. 
  • The selective permeability allows the cell to regulate the flow of necessary substances into and out of the cell.  
  • In plants, osmosis is also responsible for absorbing water and minerals from the soil by using the semipermeable membrane of the root. 
  • The opening and closing of stomata are regulated by osmosis. 
  • Osmosis is one of the reasons to rise the water to such heights in trees against gravity. 

 

Learning Outcomes 

  • Students understand the concept of osmosis. 
  • Students understand the importance of osmosis and osmosis in different types of solution.  
  • Students will be able to do the experiment more accurately in the real lab once they understand the steps through the animation and simulation.