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Osmosis is the movement of water molecules from a high water concentration (hypotonic solution) to a low water concentration (hypertonic solution) through a semi-permeable membrane. It can also be described as the movement of water from a low solute concentration to a high solute concentration through a semi-permeable membrane.

Osmosis is very important in the biological systems, as water moves across cell membrane by osmosis. Water movement is essential to maintain the osmolarity of the cell. 

Osmolarity is defined as the number of water particles per unit volume, it can be calculated using the formula:
Osm = M (concentration) x n (number of particles)

Water will always move from an area of low osmolarity to an area of high osmolarity.

An example of osmosis:
Plants absorb water through their roots by osmosis

Cells have a potential problem when maintaining their intracellular osmolarity due to metabolites e.g. sugars, amino-acids and nucleotides. These are large in size and highly charged, which attract many counterions. Thus significantly contribute to osmolarity. To control this problem cells actively pump out Na+, so their intracellular fluid contains a decreased number of inorganic ions than the extracellular fluid. This maintains osomotic equilibrium [2].


  1. Patlak, J., (2000) Osmosis, available at http://physioweb.uvm.edu/bodyfluids/osmosis.htm (last accessed 14/11/2011).
  2. (Alberts B et al.,2008:664)
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