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We rely on osmosis to survive. It is a naturally occurring process that happens automatically in our bodies where a solvent moves through a semipermeable membrane into a solution of higher solute concentration. This tends to equalize the solute concentration either side of the membrane. This occurs in the human body in certain cells and organs in the distribution of nutrients and the breaking down of waste product.

A semipermeable membrane is a membrane that will pass some atoms or molecules however will prevent others from passing. An example of semipermeable membranes in the human body are the cells, as they need to keep certain harmful atoms out and let other nourishing ones in. If a blood cell is dissolved in a isotonic liquid it will not penetrate the atoms membrane, however place the cell in a hypertonic solution then the cell will lose liquid and shrink.

To understand the process of osmosis it is helpful to picture a tube with a semipermeable membrane covering the end which is submerged in a solution. In the tube is a solution of salt and water and the osmosis takes place as the osmotic pressure builds up in the tube driving the water level in the tube up as salt particles block one side of the membrane, as they cannot fit out of the tiny perforations.

To elaborate further, picture a u-bend with a semipermeable membrane separating the two sides where the water level is the same however the left hand side has a high solute concentration of salt. The salt molecules will oscillate and perform elastic collisions with other particles. This creates a thermodynamic process where the unseen microscopic kinetic energy is converted into macroscopic visual movement, which is the water level rising.

Osmosis is a reversible thermodynamic process. In reverse osmosis the principal of osmotic pressure is used to drive certain molecules through a semipermeable membrane whilst keeping other molecules trapped. This process is used in the filtration of water. Taking for example the concept filtering seawater, the salt molecules would be stuck in the membrane as only the pure water molecules can fit through the tiny holes.

Reverse osmosis will occur when the direction of the water flow through the membrane is changed and this is due to the external pressure on the solution. Again using the u-bend model, when pressure is applied to the left side of the semipermeable membrane, despite the oscillation caused by the microscopic thermo-kinetic collisions, the water particles will be forced through the membrane leaving behind the salt particles.

Although reverse osmosis is possible, reverse diffusion is not. If we take a cup of tea for an example, when sugar is spooned into the cup there is no way of reversing this as the process of diffusion is not reversible. Reverse osmosis is an essential process in water purification as it can trap all molecules apart from water providing valuable water purity.