Osmosis

What do crispy leafs of lettuce that become soggy in the marinade after some time, and ripe cherries that burst open in the rain, have in common?

Both processes have the same cause: osmosis!

When two vessels are separated by a semipermeable membrane (green), like in the picture, and one vessel is filled with pure water and the other vessel with a concentrated salt or sugar solution, you will notice that the surface of the concentrated liquid rises after some time.
The reason is that water flows from the side of the lower concentration through the membrane into the solution of the higher concentration. The concentrations try to equalise. This effect is called osmosis.

The special properties of a so-called semipermeable membrane are responsible for that effect: The small water molecules are able to pass through the membrane, but the particles of the dissolved substance are not.

Because of the water that flows from the less concentrated solution on the right into the more concentrated solution on the left, a pressure is generated on the left side. This pressure is called the osmotic pressure.

What Is the Significance for Plants?

Osmosis is very important for all plants, as the plant cells are stabilised in that way: The liquid in the cells has a higher concentration of dissolved substances than the liquid on the outside. The cell wall serves as a semipermeable membrane, through which water flows into the cells. Because of the osmotic pressure, the cells are tightly filled and firm.

What happens if the osmotic pressure becomes too high?

If the osmotic pressure becomes too high, the cells might burst. That’s what happens to ripe cherries in the rain: Due to the high concentration of sugar in the cherry, the cells take up water through the skin until the cherry bursts open.

...and what happens, if the osmotic pressure is too low?

When the liquid outside is more concentrated than the cell fluid, the opposite happens. Water is removed from the cells, they shrivel, and the plant withers. That’s what happens to a leaf of lettuce floating in a salty marinade.

You can easily observe the effect of osmosis in a simple experiment:

Cut two equal-sized cubes out of a potato with a length of side of approximately 1 cm. Put one cube into a glass with distilled water (you can buy it in drugstore), and the other into a glass of water, in which you have dissolved a spoonful of salt. Leave the potato cubes overnight and compare how their size and form has changed!