General

Osmosis


Definition:

The one-sided diffusion of a substance (usually water) through a semipermeable membrane is called osmosis (Greek osmos = penetration). A semipermeable membrane is characterized by the fact that it is permeable only to certain substances. Water can always pass through the membrane, the substances dissolved therein, e.g. Sugar or salts but not.
The osmosis is based on the tendency of the particles to create a concentration balance between the inner and outer space of the membrane. Therefore, the water always flows from the place of higher water potential (less dissolved particles) towards the lower water potential (more dissolved particles). Osmotic pressure persists until it balances on both sides of the membrane. From this point in time, the same amount of water flows in both directions (isotonic state).
Cherries are great for illustrating the process of osmosis, because their outer skin functions as a semi-permeable membrane. In late summer, when the cherries ripen, the sugar content inside the cell is highest. A rain shower can then burst the cherries. This happens as follows: Drops of water on a cherry stream into a cell interior of a cherry because the concentration of dissolved sugar particles in the interior is much higher, than on the outside. A balance of the concentration can be achieved only by inflowing water (to some extent by dilution of the cell juice with water), because the sugar molecules are too large to leave the semipermeable membrane outwards. As a result, the cherry sucks so long with water (osmotic pressure) until it comes to concentration compensation (equal to many dissolved particles inside and outside the membrane). However, as the cherry's cells can not expand at will, the cherry eventually bursts.
Incidentally, osmosis is also responsible for the wrinkled skin when bathing. The concentration of dissolved salt particles is much higher in the skin cells than in the bath water. As a result, water flows into the cells and causes the skin to swell up.