Plant and Mammalian Physiology

Experiment 5

Aim of the Experiment

To study osmosis using a potato osmometer.

Principle

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 equalized 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:

1. Hypotonic Solution- These are solutions with low solute levels.

2. Hypertonic Solution- Solutions with high solute levels are known as hypertonic.

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

• Occurrence of Osmosis in Different Solution Types

1. 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.

2. 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 plasmolyzed.

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

Materials required

A large potato, knife, pins, 20% sugar solution, beaker, distilled water, knife petridish.

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.

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.

Questions:

1. Why the process of osmosis does not work in Boiled potato?

2. How Osmosis is different from Plasmolysis?