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Diffusion and Osmosis Through Plasma Membrane

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Diffusion and Osmosis Through Plasma Membrane
15 Jul 2021

Have you ever wondered how does a cell survives in a body?

Well, it needs gas, water and nutrients to survive. Waste produced by cells (also called cellular waste) also needs to be thrown out of the cell for survival. Carbon dioxide is an example of cellular waste.

Thus, cell needs a mechanism through which it can obtain nutrients or water and expel waste from its inside.

Apart from that, cell also needs to ensure that not every substance can enter it as it would harm the cell.

Plasma membrane is the gateway for the movement of selected materials like water or CO2 from inside and outside of the cell.

The plasma membrane allows the entry and exit of only selected materials in and out of the cell. It also prevents the movement of other materials. That is why it is called selectively permeable membrane.

For example, CO2 (carbon dioxide) which is a waste for cell, when accumulates in high concentration inside the cell whereas if in the external environment the quantity of CO2 is low. Then, due to difference in the concentration of the CO2, it moves from the inside of the cell to the external environment. This process of movement of substances from the high concentration to the low concentration is called Diffusion.


Osmosis is a special case of diffusion through selectively permeable membrane. Water follows the law of diffusion.

The movement of water molecules through the plasma membrane from in and out of the cell is called osmosis.

The movement of the water across the plasma membrane is affected by the amount of substance dissolved in the water. Thus, osmosis is the process of the passage of the water from region of high concentration to low concentration through the selectively permeable membrane until an equilibrium is reached.

Some examples of osmosis include:

  • Unicellular organisms living in fresh water ponds or oceans.
  • All the plant roots absorb water through osmosis.

Let us understand it through an example:

What will happen when we will put an animal cell or a plant cell in sugar or salt water?

There are three possible cases out of which any one may happen.

Hypotonic Solution:

  • If the salt/sugar solution has higher water concentration than the cell, i.e. the outside solution is very dilute, then the cell will gain water by osmosis. Such a solution is called hypotonic solution.
  • In simple terms, if the outside of cell, more water is present as compared to the inside of the cell, then the water will enter into the cell.
  • The cell will swell up.

Hypertonic solution:

  • If the salt/sugar solution has lower water concentration than the cell i.e. the outside solution is very concentration solution, then the cell will lose water by osmosis. Such a solution is called hypertonic solution.
  • In simple terms, if there is less water outside the cell then cell will transfer the water to the outside environment.
  • In this case, the cell will shrink.

Isotonic solution

  • If the concentration of water inside the cell and outside the cell is exactly the same, then there will be no movement of water across the plasma membrane. Such a solution where no movement of water takes place is called isotonic solution.
  • Though water crosses the plasma membrane but the amount of water going in is same as the amount of water going out, thus overall, there is no net movement of water.
  • In this case, the size of the cell will remain the same in the solution.

In conclusion, diffusion is important for exchange of gases and water in the life cycle of the cell. In addition to that, cell obtains nutrients through external environment through this process only.