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Learning about Osmosis – Biology Revision

Plant and animal cells need water and essential nutrients to carry out their metabolic processes. The substance move in and out of the cells by Active Transport and Passive Transport. The movement of substance in and out of the cell is very crucial for various metabolic process like Photosynthesis, Respiration etc. Passive Transport takes place either by Diffusion or Osmosis. Osmosis is a special case of diffusion. 

The movement of water from a less concentrated (dilute) region to a more concentrated region through a semi permeable membrane is called Osmosis. A partially permeable membrane is something which allows some molecules to pass through it but not all. They just allow some selected molecules to cross the membrane. Example of partially permeable membrane are cell membrane or plasma membrane in cell.

Image source : pmgbiology.com

Here we can see a partially permeable membrane and as explained these membrane allow some molecules to pass through it. In this case, it allows water molecules to pass through it.

Process of Osmosis

Water molecules are present on both sides of the membrane but on the left hand side we have a lot of water molecules and we call this a dilute solution. On the right hand side we have fewer water molecules because we also have dissolved sucrose (sugar) in it, hence this solution is called a concentrated solution. Now according to the definition Osmosis is the movement of molecules from a less concentrated to a more concentrated region, so the water molecules should move from the left hand side to the right hand side in the diagram above. Cells contain a lot of dissolved molecules which mean they are relatively concentrated. This is proved when we place a cell in water. It is observed that the water molecules tend to travel inside the cell and it becomes swollen.

Image source: nevinerk.weebly.com
There are three types of solution depending on their concentration which decides the shape of the cell.


Hypertonic solutions are those solution which has a relatively higher molecular concentration than the cell. In such cases the concentration of the cell becomes relatively lower than the solution so water tends to move outside the cell making it asmolysed or shrink.


Isotonic solutions are those where the concentration of cell and water medium both are balanced or same. Such case results in movement of water molecule in and out of the cell uniformly and it results in Flaccid cell.


The concentration of the solution is less than that of the cell. In this case water molecules move inside the cell making it Turgid.

Image source: slideshare.net

Mahima Laroyia, is an Oxford Postgraduate in Integrative Biosciences who has more than 8 years of tutoring experience in tutoring students Maths and Sciences from KS2 up to A-levels. She specializes in providing custom and tailor made study materials to school students to make Maths and Sciences easier and fun.Find me at: www.expertguidance.co.uk


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