The tendency for aqueous solutions [solvents] to migrate across a semi-permeable membrane from a higher water potential to a lower water potential is known as diffusion. Semi-permeable membranes are selective, allowing only specific particles to flow through and preventing others from invading. Water always flows from lower solute concentration [dilute solution] to higher solute concentration until a balance is produced. Dilute solutions are highly concentrated in water and low in solute. Hence, a low concentration of solute results in a high water potential. The general motion is from more water to less water or from less solute to more solute.
- See Water Potential for more information.
During osmosis, pressure is generated due to the difference in solute potentials of two environments separated by a semi-permeable membrane. This pressure provides the force for the water to move from low solute concentration to high solute concentration. Once the water pressure reaches the osmotic pressure, osmosis stops. The osmotic pressure is measured by the van't Hoff equation: Π = icRT, where "ic" represents osmolarity. Osmolarity is a product of the molar concentration of solute and the van't Hoff factor, i, a measurement of the extent of dissociation of the solute. The van't Hoff factor is equal to how many particles the solute dissociates in to. For example, NaCl dissociates into Na+ and Cl-, so the van't Hoff factor is 2. Glucose does not dissociate, so it's van't Hoff factor is 1. If there are multiple solutes, the total osmolarity is equal to the sum of the osmolarities of the individual solutes. 
Reverse osmosis is the process by which excess pressure is placed on one end of a semipermeable barrier in order to drive a solution from an area of high solute concentration to that of a low solute concentration. Opposite from general osmosis, the solvent does not go down a concentration gradient. In this case, the cell membrane serves as a filter. However, the key difference between osmosis and filtration is that in reverse osmosis, separation is done by diffusive mechanisms rather than size exclusion or staining. Reverse osmosis has been used industrially for water treatment. Salt water collected from the ocean is transformed to pure water by setting an external pressure equal to the air pressure at sea level. Water purification is only one industrial use of reverse osmosis. Metals and chemicals are also recycled through this process.
Example of the environmental use of reverse osmosis: County Sanitation Districts
Types of Tonicity
- The scenario when the solute concentration inside and outside of a cell (or semi-permeable membrane) is in equilibrium. There is no net movement of water if the cell is in the isotonic environment. Since there is equilibrium between the water inside and outside the cell, no net movement of water occurs.
- When the solute concentration outside of a cell is higher than the solute concentration inside of a cell. Hence the cell shrinks since the water from inside of the cell rushes outside of the cell in order to equilibrate the solute concentrations inside and outside of the cell. Fish such as salmon have to excrete salt from their gills to prevent themselves from being in a hypertonic solution.
- When the solute concentration outside of a cell is lower than the solute concentration inside of a cell. Hence the cell swells because the water from outside of the cell flows inside of the cell in order to make the solute concentrations equal. This may cause the cell to burst. To prevent bursting, cells have developed mechanisms to cope with its environment. Freshwater protists that inhabit hypotonic environments have organelles such as contractive vacuoles to pump water out from the cell. For bacteria and plants, a plasma membrane is surrounded by a rigid and non-expandable cell wall that has the strength to resist osmotic pressure and prevent osmotic lysis. Lastly, for multi-cellular animals to prevent bursting, blood plasma and interstitial fluid are used to maintain the osmolarity level close to that of cytosol.
The Role of Osmosis in Living Organisms
Living cells may be thought of as microscopically small bags of solutions contained within semi permeable membrane that allows water to flow in or out. In order for the cell to survive, the concentration of solutes within the cell cannot be changed dramatically. Water passes through the membrane in both directions to generate equilibrium between the cell and its surroundings. If the cells are in a highly concentrated solution, the water in the cell would flow out to maintain the equilibrium between the exterior and interior of the cell. This may cause the cell to shrink due to loss of water and die of dehydration. Oppositely, if the cells are in a more diluted solution, water will enter the cell and cause it to burst and be destroyed. In the molecular level, storage cells maintain osmotic pressures by storing energy in forms of macromolecules, such as polysaccharides, rather than micro molecules, such as glucose. By storing macromolecules, the osmotic pressures are diminished, thus preventing storage cells from bursting. The reason behind this is that osmolarity depends on the amount of solutes in the cell rather than the solute's size and mass. Therefore, the storage of macromolecules prevents a dramatic increase in osmotic pressure.
To survive through harsh conditions in nature, organisms have developed various methods to maintain their solute concentration within safe levels. For example, organisms that live in saltwater have much higher cell solute concentrations than organisms living in fresh water; other animals replace lost water and solutes by drinking and eating, or by removing the excessive water/solutes to decrease the solute concentration through excretion of urine.
Osmosis also plays an important role in plants. It contributes to the movement of water through parts of the plant. As the minerals and other solutions from soil are taken up by root cells to leaf cells, the solute concentrations in the plant cell will increase. It brings the differences of osmotic pressure between the cell and the exterior environment. As a result, water will be drawn upward and spread through the plants cell. When too much water is taken up into the cells, water is evaporated from leaves by regulating the size of the openings in the leaf surfaces to remove the excess water. In addition, osmolarity plays a role in plant rigidity. In a plant cell, the vacuole holds much of the plant volume and solute concentration. Because of this high solute concentration, the osmotic pressure causes water to enter the vacuole. However, because of the immutable cell wall, the cells do not burst from such a hypotonic solution. Instead, the cell stiffens, thus increasing the rigidity of the plant and its tissues.
The Role of Osmosis in Studying Organelles
Within the cell, there are organelles that have semipermeable membranes that allow the intake and output of water. Naturally, these organelles, such as mitochondria, chloroplasts, and lysosomes, exist in the cytoplasm where the solute concentration is higher. Keeping that in mind, in order to study organelles and isolate them from the cell, precautions must be taken in order to create an isotonic solution and prevent the organelles from absorbing too much water and bursting. Processes such as Differential Centrifugation depend on this precaution in order to obtain successful separation.