Difference Between Endosmosis and Exosmosis, Practice Problems and FAQs

Have you seen a leech sucking the blood of an animal? Leeches are sanguivorous (blood feeding) parasites that suck the blood of different animals and as they suck more and more blood, they swell up. But have you seen how a leech is killed? Salt is spread onto its body and it immediately starts shrinking and detaches from the host body. Do you know why this happens?

This is because salt causes water to move out of the leech’s body and causes it to eventually shrink and die. This is due to a process called exosmosis which allows water to diffuse out of the cell, into a concentrated medium, through the semipermeable cell membrane.

Similarly, water can also enter the cell, from a dilute medium, by diffusion across the semipermeable cell membrane. This process is called endosmosis.

In this article we will discuss the difference between these two processes.

Table of Content

• What is osmosis
• Endosmosis
• Exosmosis
• Difference between Endosmosis and Exosmosis
• Practice Problems
• FAQs

What is osmosis?

Osmosis is the movement or diffusion of water from its region of higher concentration to its region of lower concentration across a semipermeable membrane. In other words, osmosis is the diffusion of water across a semipermeable membrane from a region of lower solute concentration (dilute solution or pure solvent) to a region of higher solute concentration (concentrated solution).

The measure of free energy and concentration of water per unit volume of a system is known as the water potential. Higher the concentration of water in a system, higher is the water potential. Thus, pure water is considered to have the maximum water potential and addition of solutes in it reduces its water potential. Higher the concentration of solutes in a solution, less is its water potential. Thus, water moves from a region of higher water potential to a region of lower water potential during osmosis.

Ideally osmosis should continue until there is an equilibrium in the concentration of water molecules on both sides of the semipermeable membrane but actually it continues till the point when the pressure generated due to entry of water in a solution is not high enough to prevent further entry of water. This minimum pressure that is needed to prevent osmosis is known as osmotic pressure. Pure water has no osmotic pressure and the measure of osmotic pressure increases as the solute concentration increases. Water moves from a solution with less osmotic pressure to a solution with higher osmotic pressure.

Osmosis can be of two types based on the direction of movement of water - endosmosis and exosmosis. Let us now take a closer look at these processes.

Endosmosis

The movement of water into the cell by osmosis is known as endosmosis. This occurs when the cell is placed in a solution that has relatively lower concentration of solute compared to the cell cytoplasm. Such a solution is known as a hypotonic solution. As the cell has relatively higher solute concentration compared to the surrounding solution, it thereby has lower water concentration and water potential compared to the hypotonic solution outside. Thus, water flows from its region of higher potential, that is from the solution, to its region of lower potential, that is into the cell.

When water enters the cell by endosmosis, the cell starts swelling and a pressure develops within the cell due to the entry of water. This causes the cell contents to exert a pressure known as turgor pressure or pressure potential on the cell membrane towards the external environment. If the cell is surrounded only by the cell membrane, then excessive turgor pressure can cause the cell to burst. In cells having a cell wall surrounding the cell membrane, such as plant cells, the turgor pressure is exerted on the cell wall which prevents it from bursting. The cell wall also exerts an equal and opposite pressure on the cell contents which is known as the wall pressure. When the turgor pressure becomes equal to the osmotic pressure, endosmosis stops and the cell is said to be a turgid cell. In turgid plant cells, the cell membrane is fully distended and pressed against the cell wall.

Fig: Endosmosis in a cell placed in hypotonic solution

Example of endosmosis

• The root hair cells of plants absorb water from the surrounding soil by endosmosis.
• Raisins soaked in water swell due to endosmosis.

Exosmosis

Exosmosis is the outward movement of water from the cell by the process of osmosis. This occurs when the cell is placed in a solution that has relatively higher solute concentration of solutes compared to the cell cytoplasm. Such a solution is known as a hypertonic solution. As the cell has relatively lower solute concentration compared to the surrounding solution, it has higher water concentration and water potential compared to the hypertonic solution. Thus, water from the cell diffuses outside into the hypertonic solution by exosmosis.

Loss of water from the cell due to exosmosis causes the cell to shrink and become flaccid. In plant cells, the presence of a cell wall prevents the entire cell from shrinking but the cell membrane and protoplasm shrinks and draws away from the cell wall. This phenomenon is known as plasmolysis and the cell is said to be plasmolysed.

Fig: Exosmosis in a cell placed in hypertonic solution

Example of exosmosis

• Grapes put in highly concentrated sugar solutions will shrink.
• Addition of salt in pickles helps to preserve them by killing bacterial cells by plasmolysis.

Difference between Endosmosis and Exosmosis

Practice Problems

Q1. A cell undergoing endosmosis has

a. Higher water potential than the surrounding solution.
b. Lower water potential than the surrounding solution.
c. Equal water potential as that of surrounding solution.
d. Zero water potential.

Solution: The movement of water into the cell by osmosis is known as endosmosis. This occurs when the cell is placed in a hypotonic solution that has relatively lower concentration of solute compared to the cell cytoplasm. As the cell has relatively higher solute concentration compared to the surrounding solution, it thereby has lower water concentration and water potential compared to the hypotonic solution outside. Thus, water flows from its region of higher potential, that is from the solution, to its region of lower potential, that is into the cell. Thus the correct option is b.

Q2. A plant cell is placed in a solution having higher osmotic pressure than itself. What is the fate of the cell?

a. It will shrink and shrivel
b. It will swell
c. No changes
d. It will be plasmolysed.

Solution: Osmotic pressure is the minimum amount of pressure that is needed to prevent osmosis. Higher the solute concentration of a solution, higher is the osmotic pressure. Thus placing a plant cell in a solution having higher osmotic pressure than itself indicates that the solute concentration of the surrounding solution is higher than that of the cell. Thus, we can say that the cell is placed in a hypertonic solution and will hence experience exosmosis, that is the exit of water from the cell into the solution by osmosis. This will cause the protoplasm of the plant cell to draw away from the cell wall and hence the cell will be plasmolysed. The presence of the cell wall prevents the plant cell from shrinking. Thus, the correct option is d.

Q3. Differentiate between endosmosis and exosmosis based on the following criteria:

a. Concentration gradient
b. Tonicity of solution
c. Osmotic pressure of the cell
d. Water potential of the cell

Answer:

Q4. In a turgid cell

A. turgor pressure is equal to osmotic pressure.
B. turgor pressure is more than osmotic pressure.
C. turgor pressure is less than osmotic pressure.
D. turgor pressure becomes zero.

Solution: When water enters the cell by endosmosis, the cell starts swelling and a pressure develops within the cell due to the entry of water. This causes the cell contents to exert a pressure known as turgor pressure or pressure potential on the cell membrane towards the external environment. When the turgor pressure becomes equal to the osmotic pressure, endosmosis stops and the cell is said to be a turgid cell. In turgid plant cells, the cell membrane is fully distended and pressed against the cell wall. Thus, the correct option is a.

FAQs

Q1. How can you revive a plasmolysed cell?
Answer: Placing a plasmolysed cell in a hypotonic solution results in endosmosis and hence the cell takes in water. The protoplasm is revived and gradually returns back to its original state as water keeps entering. Such a cell is called a de-plasmolysed cell.

Q2. Why does gargling with salt water cure sore throat?
Answer: Sore throat is caused by bacterial infections. Salt water being hypertonic, helps to kill the bacteria in the throat by plasmolysis. It also draws out water from the inflamed throat cells which retain water due to infection and swell up causing soreness. Loss of water from these cells helps to reduce soreness and provides relief.

Q3. What is solute potential?
Answer: The decrease in water potential of pure water upon the addition of a solute is known as solute potential or osmotic potential. Higher the solute concentration in a solution, greater is its solute potential and hence greater is the osmotic potential. Water flows from a region of lower osmotic potential to a region of higher osmotic potential during osmosis.

Q4. What is an isotonic solution?
Answer: A solution which has the same solute concentration and osmotic pressure as that of the cell is known as an isotonic solution. No net movement of water occurs when a cell is placed in an isotonic solution.