Osmotic Pressure

When you make kimchi, you sprinkle coarse salt on crisp, fresh cabbage. Before long, the cabbage goes limp and water pools on the bottom of the bowl. Nobody squeezed it. The water inside the cabbage simply moved out by itself.

The key is a difference in concentration. Inside the cabbage cells, there is plenty of water, so the liquid is dilute. Outside, where the salt sits, the liquid is packed with salt and very concentrated. Nature tends to even out the concentration on both sides, so water moves from the dilute side to the concentrated side.

The membrane around each cabbage cell lets water molecules pass, but it barely lets salt through once the salt has dissolved into ions. This kind of membrane is called a 'semipermeable membrane' (a membrane that lets water pass but mostly blocks dissolved substances). The salt can't get in, so only the water leaves the cabbage, and that is why the cabbage naturally wilts.

Osmosis is a natural game of chance that comes from water molecules constantly moving in every direction. On both sides of a semipermeable membrane, water molecules keep bumping into it, trying to get to the other side.

But on the concentrated side, where there is a lot of dissolved material (the solute), such as salt or sugar, water is less likely to cross the membrane than on the dilute side. That's because dissolved material lowers water's 'urge to escape.' You often hear that dissolved particles plug the holes in the membrane, but that alone doesn't fully explain osmosis.

So water crosses in both directions, but more water molecules go from the dilute side to the concentrated side. The result is a net flow of water from the dilute side to the concentrated side. If you push on the concentrated side just hard enough to stop that flow, the size of that push is the osmotic pressure.

Osmosis works at the very first step, when a plant takes in water from the soil through its roots. The force that pulls water up to the highest leaves comes mainly from transpiration, the evaporation of water from the leaves. The cells of the root hairs are more concentrated inside than the water in the surrounding soil, so water from the soil flows into the root cells on its own.

Osmosis is also why limp lettuce or spinach turns crisp and fresh again after a soak in cold plain water. The cells inside the leaves are more concentrated than the plain water outside, so water rushes into the cells and fills them tight.

On the other hand, drinking seawater when you're thirsty is dangerous. Seawater is much saltier than your body fluids, so your kidneys would need more water than you drank to flush out that salt. Your blood gets saltier, water is pulled out of your cells, and you end up even more dehydrated. In this way, osmotic pressure is a key principle that lets living things keep their water balance.