Osmosis

Osmosis is the diffusion of a solvent through a selectively-permeable membrane from a region of low solute concentration to a region of high solute concentration, or in other words, from a high water concentration to a low water concentration. The higher the solute concentration the lower the water concentration will be. The selectively-permeable membrane is permeable to the solvent, but not to the solute, resulting in a chemical potential difference across the membrane which drives the diffusion. That is, the solvent flows from the side of the membrane where the solution is weakest to the side where it is strongest, until the solution on both sides of the membrane is the same strength (that is, until the chemical potential is equal on both sides).

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Computer simulation of process of Osmosis

Osmosis is an important topic in biology because it provides the primary means by which water is transported into and out of cells.

Explanation

Consider and focus on a permeable membrane that allows water to pass through it, but not a larger solvent. First, suppose such a membrane divides two volumes of pure water. It seems as if there is no flow from one side of the membrane to the other, but at a micro scale, every time a water molecule hits the membrane, it has a defined probability of passing through; water is passing through the membrane, however the circumstances on both sides are equivalent and therefore nothing happens. Now imagine the same membrane separates a volume of pure water from a volume of a solution. A water molecule hits the membrane but because there are fewer water molecules per volume in the solution, the water molecules on that side will collide with the wall less frequently. As a result, there will be a net flow of fresh water to the side with the solution. Assuming the membrane does not break, this net flow will slow and finally stop as the pressure on the solution side becomes such that the diffusion in each direction is equal.

Example of osmosis

A practical example of this can be seen in red blood cells. These contain a high concentration of solutes including salts and protein. When the cells are placed in a hypotonic solution, water rushes in to the area of high solute concentration, bursting the cell. This bursting of the cell is refered to as cytolysis.

Many plant cells use osmosis. This is because the osmotic entry of water is opposed and eventually equalled by the pressure exerted by the cell wall, creating a steady state. In fact, osmotic pressure is the main cause of support in plant leaves.

When a plant cell is placed in a hypertonic solution, the water in the cells moves to an area higher in solute concentration, and the cell shrinks and so becomes frigid. (In regard to the red blood cells mentioned earlier, this phenomenon is known as crenation.) This means the cell has become plasmolysed - the cell membrane has completely left the cell wall due to lack of water pressure on it.

Osmosis can also be seen very effectively when potato chips are added to a high concentration of salt solution. The water from inside the potato moves to the salt solution, causing the potato to shrink and to lose its 'turgor pressure'. The more concentrated the salt solution, the bigger the difference in size and weight of the potato chip.

In unusual environments, osmosis can be very harmful to organisms. For example, freshwater and saltwater aquarium fish placed in water with a different salt level (than they are adapted to) will die quickly, and in the case of saltwater fish rather dramatically. In addition, note the use of table salt to kill leeches and slugs.

Chemical potential

When a solute is dissolved in a solvent, the random mixing of the two species results in an increase in the entropy of the system, which corresponds to a reduction in the chemical potential. For the case of an ideal solution the reduction in chemical potential corresponds to:

$RT\ln(1-x_{2})\qquad (1)$

where $R$ is the gas constant, $T$ is the temperature and $x_{2}$ is the solute concentration in terms of mole fraction. Most real solutions approximate ideal behavior for low solvent concentrations (At higher concentrations interactions between solute and solute cause deviations from Equation 1). This reduced potential creates a driving force and it is this force which drives diffusion of water through the selectively-permeable membrane .

Osmotic pressure

As mentioned before, osmosis can be opposed by increasing the pressure in the region of high solute concentration with respect to that in the low solute concentration region. The force per unit area required to prevent the passage of water through a selectively-permeable membrane and into a solution of greater concentration is equivalent to the osmotic pressure of the solution, or turgor. Osmotic pressure is a colligative property, meaning that the property depends on the concentration of the solute but not on its identity.

Increasing the pressure increases the chemical potential of the system in proportion to the molar volume ($\delta \mu =\delta PV$). Therefore, osmosis stops, when the increase in potential due to pressure equals the potential decrease from Equation 1, i.e.:

$\delta PV=-RT\ln(1-x_{2})\qquad (2)$

Where $\delta P$ is the osmotic pressure and $V$ is the molar volume of the solvent.

For the case of very low solute concentrations, -ln(1-$x_{2}$) ≈ $x_{2}$ and Equation 2 can be rearranged into the following expression for osmotic pressure:

$\delta P=RTx_{2}/V\qquad (3)$

Reverse osmosis

The osmosis process can be driven in reverse with solvent moving from a region of high solute concentration to a region of low solute concentration by applying a pressure in excess of the osmotic pressure. This reverse osmosis technique is commonly applied to purify water. Sometimes the term "forward osmosis" is used for osmosis, particularly when used for rehydrating dried food using contaminated water.

See also

• Diffusion
• Active transport
• Reverse osmosis
• Osmotic pressure
• Plasmolysis