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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 it is required to allow the movement of water across a cell, and 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.

Cells can encounter potential problem when maintaining their intracellular osmolarity due to presence of metabolites, e.g. sugars, amino-acids and nucleotides. Metabolites are large in size and highly charged. They do not contribute directly to the osmolarity but they can attract many counterions, e.g. Na+ and K+ ions, which significantly contribute to osmolarity. To counter this problem, cells actively pump out Na+, so their intracellular fluid contains less amount of organic ions than the extracellular fluid. This maintains osmotic equilibriu[2].

An example of osmosis

Plants absorb water through their roots by osmosis.


  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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