Osmosis

Osmosis is the diffusion of a solvent through a selectively-permeable membrane from a section of high water concentration to a section of low water concentration. The selectively-permeable membrane must be permeable to the solvent, but not to the solute, resulting in a situation across the membrane which drives the diffusion, which permits the presence of osmosis. Osmosis is a natural phenomenon. However, it can be artificially opposed by increasing the pressure in the section of high solute concentration with respect to that in the low solute concentration. 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 turgor pressure. Osmotic pressure is a colligative property, meaning that the property depends on the concentration of the solute but not on its identity.

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

Basic explanation of osmosis

Consider and focus on a type of permeable membrane that has small enough apertures to allow water molecules to pass through it, however it does not enable larger solvents or solutes to pass through it. An example of this can be visking tubing. First, suppose such a membrane in a volume of pure water. It seems as if there is no flow from one side of the membrane to the other, but at a molecular scale, every time a water molecule hits the membrane, it has a defined likelihood of passing through; water is passing through the membrane, however the circumstances on both sides are equivalent and therefore nothing happens. If there is a solution on the other side, there will be fewer water molecules on that side and thus will collide with the wall less frequently. This will induce a flow of 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. Like above, if the pressure is artificially increased within a high solute solution section, with respect to the low solute solution, the pressure from the high solute solution would oppose the natural propensities of osmosis by 'forcing' the water molecules backwards by means of pressure and would serve as a counterbalance to osmosis: this makes it useful for aspects such as water purification.

Example of osmosis

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 flaccid. (This means the cell has become plasmolysed - the cell membrane has completely left the cell wall due to lack of water pressure on it (the opposite of turgid).

Osmosis can also be seen very effectively when potato slices 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. Additionally, 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 substances 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:

Where R is the gas constant, T is the temperature and x2 is the solute concentration in terms of mole fraction. Most real solutions approximate the 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 enables diffusion of water through the selectively-permeable membrane.

Osmotic pressure

As mentioned before, osmosis is 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, or pressure, 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 (dľ = dPV). Therefore, osmosis stops, when the increase in potential due to pressure equals the potential decrease from Equation 1, i.e.:

Where δP is the osmotic pressure and V is the molar volume of the solvent.

For the case of very low solute concentrations, -ln(1-x2) ≈ x2 and Equation 2 can be rearranged into the following expression for osmotic pressure:

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.

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