Osmosis is a movement of water from region of high water concentration to region of low water concentration through a semi-permeable membrane until a state of dynamic equilibrium is reached (Figure 1).
It is a passive process. Water molecules are free to pass across the cell membrane in both directions. As the concentration of water is more outside the cell, it enters the cell causing it to swell.
Osmotic Pressure
Osmotic pressure is a pressure applied by a solution to prevent the inward flow of water across a semipermeable membrane.
It occurs due to the tendency of a pure solvent to move across a semi-permeable membrane.
The osmotic pressure (∏) of a dilute solution can be calculated using the formula.
Π = iMRT
Where, i = vant hoff factor
M = molarity of the solution
R = gas constant
T = temperature
Osmotic coefficient (
) is a quantity, characterizing the deviation of solvent from an ideal behavior referenced to Raoult’s law. The osmotic coefficient on a molality basis is defined as,
or
Where, 
and 
are the chemical potentials of the solvent as a pure substance and in solution, respectively. MA is its molar mass, XA its amount fraction, R the gas constant and T the temperature.
Osmosis is of great importance in biological processes. Transport of water and other molecules across membranes is essential to many processes in living organisms (Figure 1 and 2).
Comparison of Osmosis and Diffusion
The energy that drives the process is usually denoted as osmotic pressure. It differs from diffusion (table 1and figure 4)
| Osmosis | Diffusion |
| It is a spontaneous movement of water molecules across a semi-permeable membrane from region of high solute concentration down the concentration gradient | It is a spontaneous movement of particles from area of high concentration to area of low concentration. It describes the spread of particles through random motion |
| Osmosis occurs when the medium surrounding the cell has a higher water concentration than the cell. Consequently, the cell gains water | Diffusion process occurs mainly in gaseous molecules |
| Passage of water is selective and is determined by the type of membrane | It doesn’t require a semi-permeable membrane |
Tonicity
Tonicity is a measure of osmotic pressure of two solutions separated by a semipermeable membrane.
Hypertonic solution is a solution containing more solute than another solution to which it is compared.
Hypotonic solution is a solution containing less solute than another solution to which it is compared.
While, the isotonic solution is a solution containing the same concentration of solute as another mixture to which it is compared (Figure 5).
Applications of Osmosis
Osmosis is physical phenomenon that has been extensively studied by scientists in various disciplines of science and engineering.
One of the most important applications of osmotic effect involves living cells. Applications of this process are many and are varied in nature.
Desalination
It is used for desalination process for emergency water supply on lifeboats, along with continuous process for seawater desalination (Figure 6).
Digestive System
Major examples of osmosis essential for life processes are absorption of water by alimentary canal i.e., stomach, small intestine and the colon.
Excretory System
Absorption of water by the proximal and distal convoluted tubules of the nephron in kidneys and re-absorption of tissue fluid into blood capillaries.
Food Industry
Osmosis has been used for preservation of food since long time ago. It is very useful in food processing and pharmaceutical industry.
Other Induatries
It is used in wastewater treatment and concentration of diluted industrial wastewater for its purification. Osmotic process is used for the concentration of this landfill leachate.
Space Science
Osmosis is used in direct potable reuse for advanced life support systems. Reverse osmosis (RO) has potential for the removal of ionic and organic pollutants for recycling space mission wastewater.
Reverse Osmosis (RO)
RO is a movement of water across the membrane against the concentration gradient, from lower concentration to higher concentration.
For example, if there is a semipermeable membrane in a cell with fresh water on one side and a concentrated aqueous solution on the other side.
In the case of osmosis, the fresh water crosses the membrane to dilute the concentrated solution.
Whereas in reverse osmosis, pressure is exerted on the side with the concentrated solution to force the water molecules across the membrane to the side of fresh water (Figure 7).
Reverse osmosis is most commonly known for its use in purification of drinking water (Figure 7).
It has several other applications such as concentration of food liquids (fruit juices), water treatment, concentration of maple syrup, to prevent formation of minerals on the surface of electrodes, reef aquarium as artificial seawater mixtures, or production of deionized water.
