Osmotic pressure
From Freepedia
Osmotic pressure or turgor (also called turgor pressure) is the pressure produced by a solution in a space that is enclosed by a differentially permeable membrane.
When a biological cell is in a hypotonic environment (the cell interior contains a lower concentration of water than its exterior), water flows across the cell membrane into the cell, causing it to expand. The membrane (or, in plant cells, the cell wall) restricts the expansion, which causes an increase in pressure. The resulting pressure is called turgor. This pressure is what prevents more water from flowing into the cell, thus creating an pressure equilibrium between water flowing down the concentration gradient and the taut membrane pushing back. In this example, the equilibrium prevents the cell from ever becoming isotonic to its environment. Cells not adapted to hypotonic environments, with the flow of water into them but no strong membrane or cell wall, will burst. The osmotic pressure π of a dilute solution can be calculated using the formula
- <math>\pi = MRT \,</math>
where
- M is the molarity
- R is the gas constant
- T is the absolute temperature
Note the similarity of the above formula to the ideal gas law.
Applications
Osmotic pressure is the basis of reverse osmosis, a process commonly used to purify water. The water to be purified is placed in a chamber and put under an amount of pressure greater than the osmotic pressure exerted by the water and the solutes dissolved in it. Part of the chamber opens to a differentially permeable membrane that lets water molecules through, but not the solute particles. The osmotic pressure of ocean water is about 27 atm. Reverse osmosis desalinators use pressures around 50 atm to produce fresh water from ocean salt water.
Osmotic pressure is neccessary for most plants. It is the resulting turgor that allows herbaceous plants to stand upright, and how plants regulate the aperture of their stomata.
