What is Osmosis? A Simple Explanation

Ever wondered what is osmosis? Did you come across the terms like hypertonic, hypotonic and isotonic solutions? This small and simple write-up explains everything you need to know for now.

What is Osmosis?

Osmosis can be defined as the movement of water from where it is in higher concentration to where it is in lower concentration through a semi-permeable membrane.

A semi-permeable membrane is any membrane that allows only certain substances to pass through it.

To make it easier for you, imagine there is a cup which has water in it. At exactly half the cup, there is a semi-permeable membrane.

The water molecules randomly move here and there and there is equilibrium between the two sides of the cup. There will be no net movement of water molecules because there is equal concentration of water present on both the sides.

Now, imagine adding 10 or 15 grams of salt in one side and 5 grams of salt on the other side. Here, water is the solvent and the salt are the solute.

Note: In a given solution, a solute is something which is present in a lower concentration. The solvent is the one which is present in a higher concentration.

Water can move through the semipermeable membrane but salt cannot. As you can see, the solute concentration on both the sides of membrane is not equal. The concentration of the solvent is equal on both sides of the membrane.

As osmosis is also a type of diffusion, the water molecules must evenly distribute themselves on both the sides of the cup (the solute disturbed the equilibrium present earlier). Therefore, water molecules move from the lower solute side to the higher solute side.

This movement of water molecules from one side to the other side of the cup continues until there is zero difference in the concentration gradient of water.


It is a value that describes the solute concentration in a solution.

Osmolarity is the total amount of solute that is dissolved in a given amount of solution.

There are three terms to relate the osmolarity of extracellular fluid to the osmolarity of a cell. They are hypotonic, hypertonic, and isotonic.

Hypotonic solution – The extracellular fluid has low solute concentration than the solute concentration in the cell. The water concentration or the solvent concentration is low in cells and high in extracellular fluid. So, the water moves inside the cell. This may cause the cell to lyse or burst.

Hypertonic solution – The extracellular fluid has low water concentration than the water concentration in the cell. The solute concentration is low in cells and high in extracellular fluid. So, the water moves from the cell into the extracellular fluid. This may cause the cell to shrink or crenate.

Isotonic solution – Here, the solute and water concentration are equal in both the extracellular fluid and inside the cell. There is no movement of water molecules either inside or outside the cell. The cells will be normal in this case.

Note: It must be noted that these three terms are to be used in the comparative sense meaning ‘A’ is shorter than ‘B’. We cannot say that A is shorter. Whenever we must use these terms, the terms must be used in comparisons.

How the cells control osmosis?

The cells in freshwater are always at a risk of getting burst by allowing more water inside. The cells developed a technique to tackle this problem. Freshwater protists have a contractile vacuole.

This contractile vacuole absorbs excess water from the cytoplasm of the cell. Later when the vacuole is filled with water, the water is removed from the vacuole out of the cell through a pore.

Uses of Osmosis

  1. It prevents the cells from rupturing the plasma membrane.
  2. It helps in transfusion.
  3. It helps the cells to maintain the blood volume and fluid balance.

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