Osmosis is the phenomenon of water flow through a semi-permeable membrane that blocks the transport of salts or other solutes through it. Osmosis is a fundamental effect in all biological systems. It is applied towater purification and desalination, waste material treatment, and many other chemical and biochemical laboratory and industrial processes.
When two water (or other solvent) volumes are separated by a semi-permeable membrane, water will flow from the volume of low solute concentration, to the volume of high solute concentration. The flow may be stopped, or even reversed by applying external pressure on the volume of higher concentration. In such a case the phenomenon is called reverse osmosis.
There are some key terms related to osmosis which may be helpful to know when thinking about how osmosis works. The fluid which passes through the membrane is known as a solvent, while the dissolved substance in the fluid is a solute. Together, the solvent and dissolved solute make up a solution. When a solution has low levels of a solute, it is considered to be hypotonic, while solutions with high solute levels are known as hypertonic.
In a classic example of osmosis, plants use osmosis to absorb water & nutrients from the soil. The solution in the roots of the plants is hypertonic, drawing in water from the surrounding hypotonic soil. Roots are designed as selectively permeable membranes, admitting not only water, but some useful solutes, such as minerals the plant needs for survival. Osmosis also plays a critical role in plant and animal cells, with fluids flowing in and out of the cell wall to bring in nutrients and carry out waste.