Osmosis is the passage of water from a weak solution to a strong solution through a semi permeable membrane.
A semi-permeable membrane is otherwise known as a selectively permeable membrane. It is a type of barrier that will allow the passage of some molecules but not others. In osmosis, water passes from one side of this membrane to the other. The water will move only in one direction--it will move from the side where the water is in highest concentration to the other side of the membrane. The liquid that the water moves into is a strong solution, so the water is acting to dilute the solution. If one side of the membrane has a solution of sugar and the other side has water then the water will flow into the sugar solution, diluting it. The pore size of the membrane is such that water molecules can readily move through but the larger solute molecules cannot. The water diffuses from one side to another until the concentration is the same on both sides of the membrane. Once the concentration of the solute and the water is the same on both sides the process of osmosis stops. Osmosis will occur only across a diffusion gradient.
Osmosis is a passive process as opposed to an active one. Osmosis requires no energy to drive it. Osmosis generally requires a living cell membrane but it will also happen with suitable non-living membranes such as Visking dialysis tubing.
All solutions have an osmotic potential. The osmotic potential is the amount of net movement that can occur when the solution is compared to pure water. The osmotic pressure of a solution is the pressure that has to be exerted to halt osmosis.
Osmosis occurs in natural systems in cells and in organs. In humans, osmosis occurs in the kidneys to recover the water form waste materials of the body. In plants, osmosis occurs for example at root hairs, allowing the uptake of water from the soil. Individual cells can prevent water rushing into them by osmoregulation. If water were to rush into cells unchecked then the cells would most likely burst. Plant cells are surrounded by a rigid cellulose cell wall. If there is a strong solution external to the cell, water will flow out from the cell into the strong solution. This will result in the cytoplasm and the cell membrane pulling away from the cell wall, a process known as plasmolysis. At this point, the cell is incapable of supporting weight; it is nearly empty of water and it is said to be flaccid. If this situation occurs for too long the cell will dry out and die, but this does not frequently occur in nature. When there is a weaker solution outside the cell water will flow into the cell, allowing the cell to become more supportive. There will be a greater pressure inside the cell and it will become turgid. If water continues to flow into the cell, the cell becomes increasingly turgid. Eventually, as the cell presses against the rigid cell wall, osmosis slows down and eventually stops. This is due to the pressure exerted by the wall of the plant cell. This is called wall pressure and it halts osmosis. A fully turgid cell gives the maximum support for the plant. Generally, in a healthy plant the cells alternate between being flaccid and fully turgid.
Similar reactions can be seen in animal cells although they lack the rigid cell wall. In some simple organisms, such as amoeba, water is actively excreted from the cell in vacuoles. Red blood cells will burst if placed in fresh water and if they are placed in too strong a solution they will shrink. The kidneys regulate the concentration of water in the blood plasma.
If a cell is surrounded by water, the water will flow into the cell. The osmotic pressure of the external solution is lower than that of the internal solution. In this situation, the external solution is said to be hypotonic to the cell. If water flows from the cell into the stronger solution outside, the external solution is said to be hypertonic. If both solutions are of the same concentration and there is no net flow of water then the solutions are said to be isotonic to each other. The flow of water into a cell is endomosis, the flow out is exomosis.
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