Topics: Osmosis, Semipermeable membrane, Diffusion Pages: 5 (1559 words) Published: June 19, 2013
Osmosis is the spontaneous net movement of solvent molecules through a partially permeable membrane into a region of higher solute concentration, in the direction that tends to equalize the solute concentrations on the two sides.[1][2][3] It may also be used to describe a physical process in which any solvent moves, without input of energy,[4] across a semipermeable membrane (permeable to thesolvent, but not the solute) separating two solutions of different concentrations.[5] Although osmosis does not require input of energy, it does use kinetic energy [6] and can be made to do work.[7] Osmosis is essential in biological systems, as biological membranes are semipermeable. In general, these membranes are impermeable to large and polarmolecules, such as ions, proteins, and polysaccharides, while being permeable to non-polar and/or hydrophobic molecules like lipids as well as to small molecules like oxygen, carbon dioxide, nitrogen, nitric oxide, etc. Permeability depends on solubility, charge, or chemistry, as well as solute size. Water molecules travel through the plasma membrane, tonoplast membrane (vacuole) or protoplast by diffusing across the phospholipid bilayer via aquaporins (small transmembrane proteins similar to those in facilitated diffusion and in creating ion channels Jean-Antoine Nollet first documented observation of osmosis in 1748.[9] The word "osmosis" descends from the words "endosmose" and "exosmose", which were coined by French physician René Joachim Henri Dutrochet (1776–1847) from the Greek words ένδον (endon : within), έξο (exo : outside), and ωσμος (osmos : push, impulsion).[10]

Basic explanations
Osmosis can occur when there is a partially permeable membrane, such as a cell membrane. When a cell is submerged in water, the water molecules pass through the cell membrane from an area of low solute concentration to high solute concentration (e.g. if the cell is submerged in saltwater, water molecules move out; if it is submerged in freshwater, however, water molecules move in); this is called osmosis. The cell membrane is selectively permeable, so only necessary materials are let into the cell and wastes are left out.[8] The word 'osmosis' is particular to the diffusion of water molecules into the cell. Osmosis can be explained using the concept of thermodynamic free energy: the less concentrated solution contains more free energy, so its solvent molecules will tend to diffuse to a place of lower free energy in order to equalize free energy. Since the semipermeable membrane only allows solvent molecules to pass through it, the result is a net flow of water to the side with the more concentrated solution. Assuming the membrane does not break, this net flow will slow and finally stop as the pressure on the more concentrated side lessens and the movement in each direction becomes equal: this state is called dynamic equilibrium. Osmotic pressure is the main cause of support in many plants. Also, osmosis is responsible for the ability of plant roots to draw water from the soil. Since there are many fine roots, they have a large surface area, and water enters the roots by osmosis. Osmosis can also be seen when potato slices are added to a high concentration of salt solution. In unusual environments, osmosis can be very harmful to organisms. For example, freshwater and saltwater aquarium fish placed in water of a different salinity than that to which they are adapted to will die quickly, and in the case of saltwater fish, dramatically. Another example of a harmful osmotic effect is the use of table salt to kill leeches and slugs. Suppose an animal or a plant cell is placed in a solution of sugar or salt in water. 1. If the medium is hypotonic — a dilute solution, with a higher water concentration than the cell — the cell will gain water through osmosis. 2. If the medium is isotonic — a...
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