Osmosis Download Osm NEW!
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In physiology, osmosis (Greek for push) is the net movement of water across a semipermeable membrane. Across this membrane, water will tend to move from an area of high concentration to an area of low concentration. It is important to emphasize that ideal osmosis requires only the movement of pure water across the membrane without any movement of solute particles across the semipermeable membrane. Osmosis can still occur with some permeability of solute particles, but the osmotic effect becomes reduced with greater solute permeability across the semipermeable membrane. It is also true that, at a specific moment in time, water molecules can move towards either the higher or lower concentration solutions, but the net movement of water will be towards the higher solute concentration. The compartment with the highest solute and lowest water concentration has the greatest osmotic pressure. Osmotic pressure can be calculated with the van 't Hoff equation, which states that osmotic pressure depends on the number of solute particles, temperature, and how well a solute particle can move across a membrane. Its measured osmolality can describe the osmotic pressure of a solution. The osmolality of a solution describes how many particles are dissolved in the solution. The reflection coefficient of a semipermeable membrane describes how well solutes permeate the membrane. This coefficient ranges from 0 to 1. A reflection coefficient of 1 means a solute is impermeable. A reflection coefficient of 0 means a solute can freely permeable, and the solute can no generate osmotic pressure across the membrane. The compartment with the greatest osmotic pressure will pull water in and tend to equalize the solute concentration difference between the compartments. The physical driving force of osmosis is the increase in entropy generated by the movement of free water molecules. There is also thought that the interaction of solute particles with membrane pores is involved in generating a negative pressure, which is the osmotic pressure driving the flow of water. Reverse osmosis occurs when water is forced to flow in the opposite direction. In reverse osmosis, water flows into the compartment with lower osmotic pressure and higher water concentration. This flow is only possible with the application of an external force to the system. Reverse osmosis is commonly used to purify drinking water and requires the input of energy. The concept of osmosis should not be confused with diffusion. Diffusion is the net movement of particles from an area of high to low concentration. One can think of osmosis as a specific type of diffusion. Both osmosis and diffusion are passive processes and involve the movement of particles from an area of high to low concentration.
While diffusion is often depicted as the net movement of solutes between two solutions, osmosis is about the net movement of the solvent molecules (not the solute) (solvent such as water molecules). The differing concentration of water molecules between the two sides of the membrane is what drives the water to move so as to equalize the concentrations of the two areas.
In biology, osmosis is defined as the net movement of water molecules through a biological membrane (e.g. cell membrane) from an area of higher to an area of lower water potential. Other definitions of osmosis are as follows:
In chemistry, osmosis is defined similarly. It is the passage of a pure solvent from a solution of lesser to one of greater concentration of solutes when the two solutions are separated by a membrane that selectively prevents the passage of solute molecules while allowing the solvent molecules to pass through.
Etymology: The term osmosis is a Latinized form of now obsolete osmose. A derived word is osmotic, which is defined as pertaining to or of the nature of osmosis. Thus, osmotic pressure, for instance, is a pressure that arises due to or is relevant to osmosis.
Water molecules tend to move, and they move downhill, i.e. from an area of high water concentration (or fewer solutes) to an area of low water concentration (or greater solutes). If there is no net movement of water, it cannot be called osmosis. It should also incorporate a semipermeable membrane to move across. Without it, the process is only a case of diffusion and not osmosis.
In biological systems, osmosis is essential since many biological membranes are semipermeable, and it leads to different physiological effects. For example, when animal cells are exposed to a hypertonic surrounding (or lower water concentration) the water will leave the cells causing the cells to shrink. This condition is referred to as crenation. Conversely, when the animal cells are placed in a hypotonic surrounding (or higher water concentration), the water molecules will move into the cells causing them to swell. If osmosis continues and becomes excessive the cells will eventually burst.
The cell bursting due to too much water influx does not happen in plant cells. Plants are able to counter excessive osmosis through their cell walls and vacuoles. The cell wall exerts osmotic pressure that stabilizes the plant cell. In fact, osmotic pressure is what makes plants stay upright. The large vacuole inside the plant cell also helps through osmoregulation, a regulatory process where water potential is regulated so that the osmotic pressure inside the cell is kept within the optimal range.
Osmosis is a naturally occurring phenomenon and one of the most important processes in nature. It is a process where a weaker saline solution will tend to migrate to a strong saline solution. Examples of osmosis are when plant roots absorb water from the soil and our kidneys absorb water from our blood.
Below is a diagram which shows how osmosis works. A solution that is less concentrated will have a natural tendency to migrate to a solution with a higher concentration. For example, if you had a container full of water with a low salt concentration and another container full of water with a high salt concentration and they were separated by a semi-permeable membrane, then the water with the lower salt concentration would begin to migrate towards the water container with the higher salt concentration.
Reverse Osmosis is the process of Osmosis in reverse. Whereas Osmosis occurs naturally without energy required, to reverse the process of osmosis you need to apply energy to the more saline solution. A reverse osmosis membrane is a semi-permeable membrane that allows the passage of water molecules but not the majority of dissolved salts, organics, bacteria and pyrogens. However, you need to 'push' the water through the reverse osmosis membrane by applying pressure that is greater than the naturally occurring osmotic pressure in order to desalinate (demineralize or deionize) water in the process, allowing pure water through while holding back a majority of contaminants.
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