(from Greek, othismos: impulse) Osmosis is a term describing the movement of molecules in SOLUTION; osmotic is the adjectival form. Osmosis is particularly important in regard to the movement of molecules across a SEMIPERMEABLE MEMBRANE (that is, any membrane that is permeable to certain molecules but not others). A small amount of a solute (plain salt, sodium chloride [NaCl] for example) added to a solvent (water in a beaker for example) will, even without stirring, DIFFUSE and come to a more-or less even concentration throughout the solution. This represents osmotic movement. Of more interest is the situation that develops—OSMOTIC PRESSURE—when a semipermeable membrane is present.
If there was a semipermeable membrane dividing the beaker into two equal halves and NaCl was added to only one half, then for a brief period there would be NaCl solution on one side of the semipermeable membrane and only water on the other. Because molecules will always try to move down a CONCENTRATION GRADIENT (that is from a high concentration to a low one) water molecules are drawn across the membrane from the pure water side to the salty water side—that is from a high concentration (only water present) to a lower concentration (water and NaCl molecules present). The molecules of NaCl would make the reverse journey, though again, one that took molecules down a concentration gradient. The movement of solutes across membranes—or more properly, the force that solutes bring to bear on membranes—is known as osmotic pressure. (Note this is not the same thing as HYDROSTATIC PRESSURE: if one had a beaker with a semipermeable partition and poured 100 ml of water into one half and 50 ml into the other, very quickly the volume of water on either side would equalize at 75 ml each side. This is because of hydrostatic pressure: the pressure exerted by water, in this case, by the pull of gravity.)
