To study the phenomenon of osmosis

Aim

To study the phenomenon of osmosis.

Material Required

Potato tuber, conc. (10%) sugar solution, water, knife, petridish, alpins.

Theory

Osmosis is a process in which two solutions of different concentrations are separated by a semi-permeable membrane. The diffusion of water or solvent molecules takes place from the solution of lower concentration to the solution of higher concentration. Osmosis is of two kinds: Endosmosis and Exosmosis.

Procedure

Peel off the outer skin of the potato tuber by knife. Cut the peeled potato into two equal sized pieces. Make a big cavity in the centre of the potato almost up to the bottom. Fill the concentrated sugar solution in the cavity of one tuber and place this tuber in a petridish containing water. Insert an alpin in the inner side of the potato cavity at the level of sugar solution. Fill the water in the cavity of other tuber and place this tuber in a petridish containing concentrated sugar solution. Mark the level of water in the cavity by piercing a alpin. Leave the experiment as such for one hour.

Here is the demonstration of osmosis through potato Osmoscope:

Observation

We observe that the level of sugar solution kept in the potato cavity (A) increases while the level decreases in another potato tuber i.e., the cavity (B) filled with water.

Explanation

The level of the sugar solution in the cavity (A) increases from the initial level 'a' to final level 'b'. Here water moves from the petridish into the cavity through the semi-permeable membrane of potato. In this process of osmosis, movement of water molecules takes place into the tuber from outs ide. Therefore it is known as endosmosis. however, in the second cavity, (B) initial water level 'a' falls to level 'b'. This fall in the water level is due to movement of water molecules from inside the tuber to the outside. The outer solution in this case is hypertonic and hence the water moves out of the cavity of tuber. This phenomenon of osmosis is called exosmosis.

Result

The above experiment shows osmosis.

Precautions

1. Potato tuber should be peeled off.
2. Cavity should be carefully made.
3. The cavity should be deep enough to keep only a thin layer of tissue at the base.
4. The sugar solution should be sufficiently of high osmotic concentration as compared to the cell sap of potato cells.

Viva Questions and Answers

Question 1: What is osmosis ?
Answer: Osmosis is a process in which two solutions of different concentration are separated by a semi-permeable membrane. The diffusion of water or solvent molecules takes place from the solution of lower concentration to the solution of higher concentration.

Question 2: What is meant by semi-permeable membrane ?
Answer: A membrane through which only solvent molecules can pass is called semi-permeable membrane.

Question 3: What is diffusion ?
Answer: The movement of molecules or ions of a solid, liquid or gas from the region of their higher concentration to the region of their lower concentration is called diffusion.

Question 4: What is endosmosis ?
Answer: In this process of endosmosis, the water or solvent molecules enter the cell from outer medium through plasma membrane due to which the cell swells up.

Question 5: What is exosmosis ?
Answer: In this process of exosmosis, the water or solvent molecules moves from cell into the outer medium through plasma membrane due to which the cell shrinks.