What is Osmosis ?

Osmosis :  Osmosis refers to the diffusion of water across a semi-permeable membrane from an area of higher concentration of water to an area of lower concentration of water. That is, water molecules follow the same principle as other particles; they move toward an area of lower water concentration.

In a solution, the larger particles of solute may be inhibited from diffusing by the presence of the membrane, which is said to be semipermeable for this reason. Semi-permeability refers to the fact that the membrane allows smaller-sized particles to pass, while not allowing larger-sized particles through. The terms "selectively permeable" and "differentially permeable" are sometimes also used to describe this phenomenon.

The solution with a high concentration of solute is called hypertonic; the prefix from the Greek word for "higher." The solution with the relatively lower concentration is considered hypotonic, the prefix is from the Greek word meaning "lower." When both sides are equal in concentration, the solutions are said to be isotonic. The pressure exerted by excess water entering the hypertonic solution is called osmotic pressure.

Hypertonic, hypotonic, and isotonic are are also used to describe the concentration of solute in the medium in which a cell resides relative to the concentration of solute in the cell's solution. If a cell is placed into a solution with a salt concentration equal to the cell's salt concentration (an isotonic solution), the solutions will have the same chemical, or water, potential, and water will diffuse into and out of the cell at the same rate. In the case where the salt concentration of the solution in the beaker is greater than the cell's salt concentration, the solution in the beaker is referred to as being hypertonic to the cell. The way to remember this word is to recall what it is to be hyperactive - or overly active.

If a cell is placed into a solution with higher salt concentration relative to the cell's salt concentration (a hypertonic solution), the solution will then have a lower chemical, or water, potential relative to the cell, and water will diffuse out of the cell. This process is called plasmolysis, as seen here with this red blood cell. The contents of the cell, including proteins and large solute particles are trapped within the cell membrane, and only water diffuses out, causing a shrinking and shriveling of the cell.

In the case where the salt concentration of the solution in the beaker is lower than the cell's salt concentration, we refer to this solution in the beaker as being hypotonic to the cell. The way to remember this word is to recall what a hypodermic needle is - one that goes beneath the skin.

The presence of solute in a solution, such as the situation in a cell's cytoplasm, decreases the chemical, or water, potential within. This forms a chemical potential gradient between the outside and inside of the cell in terms of water. Since the cell has a lower chemical potential relative to the water outside, the water will tend to diffuse into the cell. The red blood cell will continue to take up water, swell, and eventually burst from the continued influx of water. This process is referred to as hemolysis.

Just as in the case of the red blood cell, if a plant cell is placed into a solution with a salt concentration equal to the cell's salt concentration, the solutions are isotonic to each other. The solution in the beaker will have the same chemical, or water, potential relative to the cell, and water will diffuse into and out of the cell at the same rate.

However, if the salt concentration of the solution in the beaker is greater than the salt concentration of the cell, the solution in the beaker is referred to as being hypertonic to the cell. When a plant cell is placed into a hypertonic solution, the solution has a low chemical, or water, potential relative to the cell, and water diffuses out of the cell. This process is also called plasmolysis , as seen here with this plant cell. The contents of the cell, including proteins, chloroplasts, and large solute particles are trapped within the cell membrane, and only water diffuses out, causing a shrinking and shriveling of the cell. Notice that the cell wall remains unaffected.

When the salt concentration of the solution in the beaker is lower than the cell's salt concentration, the solution in the beaker is referred to as being hypotonic to the cell.

Recall that the presence of solute in a solution decreases the chemical, or water, potential. Since the cell contains solute in the form of proteins and salts, it has a low chemical potential relative to the water outside, and the water will tend to diffuse into the cell.

The plant cell will continue to take up water and swell, but will not burst from the continued influx of water because the cell wall provides a "back pressure," which acts against the build up of water pressure within brought about by osmosis. The cell becomes "turgid" because of the increased turgor pressure. This explains the difference between a healthy, upright plant and a wilted, droopy plant!