According to WikipediA
OSMOSIS is the net movement of water across a partially permeable membrane from a region of high solvent potential to an area of low solvent potential, up a solute concentration gradient.It is a physical process in which a solvent moves, without input of energy, across a semi permeable membrane (permeable to the solvent, but not the solute) separating two solutions of different concentrations. Osmosis releases energy, and can be made to do work, as when a growing tree-root splits a stone.
Net movement of solvent is from the less-concentrated (hypotonic) to the more-concentrated (hypertonic) solution, which tends to reduce the difference in concentrations. This effect can be countered by increasing the pressure of the hypertonic solution, with respect to the hypotonic. The osmotic pressure is defined to be the pressure required to maintain an equilibrium, with no net movement of solvent. Osmotic pressure is a colligative property, meaning that the property depends on the molar concentration of the solute but not on its identity. Osmosis is the result of diffusion across a semi-permeable membrane.
Osmosis is important in biological systems as many biological membranes are semipermeable. In general, these membranes are impermeable to organic solutes with large molecules, such as polysaccharides, while permeable to water and small, uncharged solutes. Permeability may depend on solubility properties, charge, or chemistry as well as solute size. Water molecules travel through the plasma cell membrane, tonoplast (vacuole) or protoplast in two ways. Either by diffusing across the phospholipid bilayer directly, or via aquaporins (small transmembrane proteins similar to those in facilitated diffusion and in creating ion channels). Osmosis provides the primary means by which water is transported into and out of cells. The turgor pressure of a cell is largely maintained by osmosis, across the cell membrane, between the cell interior and its relatively hypotonic environment.
Consider a permeable membrane, such as visking tubing, with apertures big enough to allow water (solvent) molecules, but not larger solute molecules, to pass through. When this membrane is immersed in liquid it is constantly hit by molecules of the liquid, in motion due to their thermal kinetic energy. In this respect solute and solvent molecules are indistinguishable. At a molecular scale, every time a molecule hits the membrane it has a defined likelihood of passing through. Here, there is a difference: for water molecules this probability is non-zero; for solute molecules it is zero.
Suppose the membrane is in a volume of pure water. In this case, since the circumstances on both sides of the membrane are equivalent, water molecules pass in each direction at the same rate; there is no net flow of water through the membrane.
If there is a solution on one side, and pure water on the other, the membrane is still hit by molecules from both sides at the same rate. However, some of the molecules hitting the membrane from the solution side will be solute molecules, and these will not pass through the membrane. So water molecules pass through the membrane from this side at a slower rate. This will result in a net flow of water to the side with the solution. Assuming the membrane does not break, this net flow will slow and finally stop as the pressure on the solution side becomes such that the movement in each direction is equal: dynamic equilibrium. This could either be due to the water potential on both sides of the membrane being the same, or due to osmosis being inhibited by factors such as pressure potential or Osmotic pressure.
Osmosis can also be explained via the notion of entropy, from statistical mechanics. As above, suppose a permeable membrane separates equal amounts of pure solvent and a solution. Since a solution possesses more entropy than pure solvent, the second law of thermodynamics states that solvent molecules will flow into the solution until the entropy of the combined system is maximized. Notice that, as this happens, the solvent loses entropy while the solution gains entropy. Equilibrium, hence maximum entropy, is achieved when the entropy gradient becomes zero.
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