Osmosis, the spontaneous flow of a liquid or gaseous substance through a semipermeable membrane. A semipermeable membrane is a material through which only certains kinds of molecules can readily pass. Parchment, which allows water molecules to pass through it but effectively blocks the passage of sugar molecules contained in the water, is an example of a semipermeable membrane.

In the situation most often encountered, the semipermeable membrane separates one liquid solution from another. The direction of flow is determined by the composition of the two solutions. In most cases, flow occurs in both directions, the greater flow being from the less concentrated solution to the more concentrated solution. The amount of flow that occurs is determined by the nature of the membrane.

Osmosis is extremely important in all processes in which water moves into and out of cells, because all living cell membranes are semipermeable. Osmosis keeps the amount of water in all the cells of an animal or a plant approximately equal and helps to prevent certain cells from drying out. Water is drawn from the soil into the roots of a plant by osmosis. When there is an adequate water supply, osmosis helps to prevent the wilting of plants that is caused by loss of water.

The effect of osmosis in a root is shown in the accompanying illustration. A hole, one inch (2.5 cm) deep, is cut into the top of the carrot and nearly filled with a concentrated solution of sugar and water. The carrot is then placed in a glass of water with its top one-half inch (1.3 cm) above the surface. Within a few hours, water from the glass will be drawn into the hole by osmosis. The level of the solution in the hole will rise until the solution overflows.

Osmotic Apparatus

The movement of the substances involved in osmosis is shown in the illustration Principle of Osmosis. Between the two halves of the U-shaped tube is a semipermeable membrane, such as a piece of animal bladder, parchment, or cellophane. Arm A contains a solution of sugar and water and arm B contains pure water. The water molecules on both sides of the membrane hit the membrane and pass through. (The membrane is permeable to water.) The sugar molecules also hit the membrane, but, because of the composition of the membrane, only a few (the ones traveling at very high speeds) pass through. (The membrane is impermeable to sugar.)

More water molecules touch the membrane on the side in contact with the pure water (side B) than on the side in contact with both sugar and water (side A). More water moves from side B, the pure-water side, to side A than from side A to side B. This net movement of water through the semipermeable membrane into the sugar-and-water solution constitutes osmosis.

As water moves into the solution, the volume of the solution (illustration 2, side A) increases. The increased volume eventually builds up so much pressure that the force of the pressure causes as much water to leave the solution (side A) as enters. This pressure, called osmotic pressure, causes the flow of water to be equal in both directions.