Osmosis is the basis of metabolism in the living organisms: cells absorb water molecules through the walls, which are semipermeable membranes. Water passes from a less concentrated solution (extracellular fluids) to a more concentrated one inside the cell (intracellular fluids).

Accordingly, reverse osmosis is a reverse process: water flows from a more concentrated solution to a less concentrated one through the same membrane. Since this process is opposite to the natural mechanism, pressure must be applied: that’s where the most advanced filtration method begins.