Osmosis

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Osmosis is the net movement of water through a selective permeable membrane from a region of low solute potential to a region of high solute potential (or equivalently, from a region of high solvent potential to a region of low solvent potential). The partially permeable membrane must be permeable to the solvent, but not to the solute, resulting in a pressure gradient across the membrane. 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 solvent 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.

1 Basic explanation of osmosis
2 Examples of osmosis
3 Reverse osmosis
4 Forward osmosis
5 See also
6 Animal cell osmosis and plant cell osmosis
7 External links

Basic explanation of osmosis

Consider a permeable membrane, such as visking tubing, with apertures small enough to allow water molecules, but not larger molecules, to pass through. Suppose the membrane is in a volume of pure water. At a molecular scale, every time a water molecule hits the membrane, it has a defined likelihood of passing through. In this case, since the circumstances on both sides of the membrane are equivalent, there is no net flow of water through it. However, if there is a solution on the other side, that side will have fewer water molecules and thus fewer collisions with the membrane. This will result in a net 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. Osmosis can also be explained via the notion of entropy, from statistical mechanics. As above, suppose a permeable membrane separates equal amounts of pure solvent and a solution. Since a solution possesses more entropy than pure solvent, the second law of thermodynamics states that solvent molecules will flow into the solution until the entropy of the combined system is maximized. Notice that, as this happens, the solvent loses entropy while the solution gains entropy. Equilibrium, hence maximum entropy, is achieved when the entropy gradient becomes zero.

Examples of osmosis

Many plant cells perform osmosis. This is because the osmotic entry of water is opposed and eventually equaled 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 [pron. flaxid]. (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.

Reverse osmosis

Main article: 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. Recent advances in pressure exchange and the ongoing development of low pressure membranes have significantly reduced the costs of water produced by reverse osmosis. The reverse osmosis technique is commonly applied in desalination, water purification, water treatment, and food processing.

Forward osmosis

Osmosis may be used directly to achieve separation of water from a "feed" solution containing unwanted solutes. A "draw" solution of higher osmotic pressure than the feed solution is used to induce a net flow of water through a semi-permeable membrane, such that the feed solution becomes concentrated as the draw solution becomes dilute. The diluted draw solution may then be used directly (as with an ingestible solute like glucose), or sent to a secondary separation process for the removal of the draw solute. This secondary separation can be more efficient than a reverse osmosis process would be alone, depending on the draw solute used and the feedwater treated. Forward osmosis is an area of ongoing research, focusing on applications in desalination, water purification, water treatment, and food processing.

Animal cell osmosis and plant cell osmosis

Plant cells always have a strong cell wall surrounding them. When they take up water by osmosis they start to swell, but the cell wall prevents them from bursting. Plant cells become "turgid" when they are put in dilute solutions. Turgid means swollen and hard. The pressure inside the cell rises, eventually the internal pressure of the cell is so high that no more water can enter the cell. This liquid or hydrostatic pressure works against osmosis. Turgidity is very important to plants because this is what makes the green parts of the plant "stand up" into the sunlight. When plant cells are placed in concentrated sugar solutions they lose water by osmosis and they become "flaccid"; this is the exact opposite of "turgid". If you put plant cells into concentrated sugar solutions and look at them under a microscope you would see that the contents of the cells have shrunk and pulled away from the cell wall: they are said to be plasmolysed. When plant cells are placed in a solution which has exactly the same osmotic strength as the cells they are in a state between turgidity and flaccidity. We call this incipient plasmolysis. "Incipient" means "about to be". When you forget to water plants, you will see their leaves droop. Although their cells are not plasmolsysed, they are not turgid and so they do not hold the leaves up into the sunlight. When animal cells are placed in sugar solutions things may be rather different because animal cells do not have cell walls. In very dilute solutions, animal cells swell up and burst: they do not become turgid because there is no cell wall to support the cell membrane. In concentrated solutions, water is sucked out of the cell by osmosis and the cell shrinks. In either case there is a problem. So animal cells must always be bathed in a solution having the same osmotic strength as their cytoplasm. This is one of the reasons why we have kidneys. The exact amount of water and salt removed from our blood by our kidneys is under the control of a part of the brain called the hypothalamus. The process of regulating the amounts of water and mineral salts in the blood is called osmoregulation. Animals which live on dry land must conserve water; so must animals which live in the sea (remember the sea is full of salt), but animals which live in freshwater have the opposite problem; they must get rid of excess water as fast as it gets into their bodies by osmosis. Osmosis is an important topic in biology because it provides the primary means by which water is transported into and out of cells.