Theory & Procedure, Study of Osmosis | Additional Study Material for NEET PDF Download

Objective

Our objective is that to study osmosis using a potato osmometer.

Theory

What is Osmosis?

Osmosis is the process in which solvent molecules moves through a semi-permeable membrane from a region of higher concentration to a region of lower concentration until the amount of fluid is equalised on both sides of the semipermeable membrane.  

The fluid that passes through the semipermeable membrane is known as the solvent, while the dissolved substance in the fluid is known as the solute.  The mixture of solvent and solute makes up a solution.

Cause for Osmosis

Osmosis occurs due to the free energy of the solvent molecule in two regions. Pure water or solvent has more free energy compared to that present in a solution. Therefore during osmosis, solvent or water moves through the semipermeable membrane from a region of its high free energy to a region of its low free energy.

Importance of Osmosis

Osmosis plays a critical role in plant and animal cells.  It helps in the distribution of nutrients and in the release of waste products. The living cells of both plant and animals are surrounded by a semipermeable membrane known as the cell membrane. The membrane forms a selective barrier between the cell and its environment and does not allow toxic substances from the surroundings to enter into the cell. 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.

Types of Solutions

Hypotonic Solution

These are solutions with low solute levels.

Hypertonic Solution

Solutions with high solute levels are known as hypertonic.

Isotonic Solution

If both solutions have the same amount of solute concentration they are then known as isotonic solution.

Occurence of Osmosis in Different Solution Types

Hypotonic Solution

If we place living cells in a hypotonic solution the water moves into the cell because of the higher concentration of water than in the cell.  The cell then swells to become turgid.

Hypertonic Solution

If we place living cells in a hypertonic solution the water moves out of the cell because of the lower concentration of water than the cell. The cell then shrinks and becomes plamolyzed.

Isotonic Solution

If we place living cells in an isotonic solution, it won’t show any change because of the equal concentration of water on either side. The cell remains as it is. 

The phonomenon of osmosis can be demonstrated using an osmometer, a device that shows the occurrence of osmosis.

Learning Outcomes

• Students understand the concept of osmosis.
• Students understand the cause for osmosis.
• Students understand the importance of osmosis in plant and animal cells.
• Students will be able to do the experiment more accurately in the real lab once they understand the steps through the animation and simulation.

Materials Required

Real Lab Procedure

1. Place the peeled potato on the tile and using the knife, cut both ends of the potato to make it flat.
2. Use the knife to make a cavity at the centre of the potato from one of the flat sides almost up to the bottom.
3. Pour distilled water into the Petri dish until it is half full.
4. Now, place the potato in the Petri dish.
5. Fill half the cavity made in the potato with 20% sugar solution.
6. Mark the level of sugar solution in the cavity using a pin.
7. The potato now functions as an osmometer.
8. Leave the osmometer undisturbed for about two hours.
9. Mark the rise in the level of the sugar solution in the cavity with another pin.

Simulator Procedure (as performed through the Online Labs)

1. To increase or decrease the concentration of the sugar solution in the Petri dish, choose the concentration of sugar from the 'In Petri dish: Select the sugar concentration’ drop down list.
2. To increase or decrease the concentration of the sugar solution in the potato, choose the concentration of sugar from the ‘In potato: Select the sugar concentration’ drop down list.
3. Click on the Check box ‘Cross sectional view’ to view the cross section of the potato, that helps view the water level inside potato.
4. Drag a pin to mark the initial level of the sugar solution in the cavity.
5. Click on the start button to start the experiment.
6. A timer is shown and you need to wait till the timer stops.
7. Drag another pin and mark the rise in level of the sugar solution in the cavity.
8. Click on the information icon to see the inference.
9. You can redo the experiment by clicking on the ‘Reset’ button.

Observation

The level of sugar solution in the potato cavity rises after some time due to the entry of water into the sugar solution through the selectively permeable membrane of the cells of the potato.

Conclusion

The movement of water from the Petri dish to the potato cavity occurs because of the difference in the concentration of solvent molecules in the two regions: sugar solution in the potato cavity and pure water in the Petri dish.

Precautions

• The cavity should be deep enough to keep only a thin layer of tissues at the base of the potato.
• The sugar solution should be of sufficiently high osmotic concentration as compared to the cell sap of potato cells.