Osmosis is the spontaneous net movement of solvent molecules through a semi-permeable membrane into a region of higher solute concentration, in the direction that tends to equalize the solute concentrations on the 2 sides. It may also be sued to describe a physical process in which any solvent may move across a semi-permeable membrane separating 2 solutions of different concentration. In this picture, the process of osmosis is over a semi-permeable. The purple dots represent particles driving the osmotic gradient. Osmotic pressure is defined as an external pressure required to be applied so that there is no net movement of the solvent across the cell membrane. osmotic pressure is a colligative property, meaning that the osmotic pressure depend on the molar concentration of the solute but not on its identity. In general, these membranes are impermeable to large and polar molecules 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, and nitric oxide.
Osmosis is the movement of a solvent across a semi-permeable membrane towards a higher concentration of solute. In biological system, the solvent is usually water, but osmosis can occur in other liquids also. It can even also occur in supercritical liquids and even gases. When a cell is submerged in water, the water molecules pass through the cell membrane from an area of low solute concentration to an area of high solute concentration. For example, if the cell is submerged in salt water, then the water molecules move out of the cell. If the cell is submerged in fresh water, then the water molecules move into the cell. When the membrane has a volume of pure water on both sides, water molecules pass in and out in each direction at exactly the same rate. there is no net flow of water through the cell membrane. The mechanism for driving osmosis has been commonly represented in biology and chemistry text as either the dilution of water by solute ( resulting in the lower concentration of water on the higher solute concentration side of the membrane and therefore a diffusion of water on the concentration gradient )or by a solutes attraction to water ( resulting in less free water on the higher solute concentration side of the membrane and therefore the net movement of water toward the solute ). Both of these nations have been conclusively refuted. The diffusion model of osmosis Is rendered untenable by the fact that osmosis can drive water across a membrane toward a higher concentration of water. The bound water model is refuted by the fact that osmosis is dependent on the size if the solute molecules, a colligative property, or how hydrophilic they are. it is hard to explain osmosis without a mechanical or a thermodynamic explanation, but basically, there is interaction between the solute and water that counteracts the pressure that otherwise free solute molecule would exert. Many thermodynamic explanations go through the concept of chemical potential and how the function of the water on the solution side differs from that of pure water to the higher pressure and the pressure of solute counteracting such the chemical potential remains uncharged. The virtual theorem demonstrates the attraction between the molecules ( water and solutes) reduces the pressure and then the pressure exerted by water molecules on each other in solution is less than in pure water allowing pure water to force the solution until the pressure reaches equilibrium.