Molecules, the tiny pieces which make up all matter, tend to move from where they are more concentrated to where they are less concentrated. This equalizing movement, called diffusion, occurs because the random motions of the molecules make them move around. If there are more molecules of one kind in a certain region than outside that region, more molecules will move out of that region than move into it. Osmosis is the DIFFUSION of water through a membrane that will not allow other, larger molecules to pass through it. Such a membrane is called semipermeable.
Many chemists and physicists include in osmosis the passage of any gas, liquid, or dissolved solid through a semipermeable membrane. Other scientists restrict the term to the passage of liquids and dissolved substances, such as food and minerals, through membrane. Most biologists, however, state that osmosis is the movement of water through a semipermeable membrane from where water is more concentrated to where it is less concentrated.
The classic experiment which shows osmosis uses parchment as the semipermeable membrane. The parchment allows water, but not molasses, to pass through.
The force by which water moves into the solutions is called osmotic pressure. Osmotic pressure depends upon the concentration of water inside and outside the membrane and upon temperature. The greater the difference in concentrations and the higher the temperature, the greater the osmotic pressure. This pressure is frequently measured in pounds per square inch. Osmotic pressure is one of the important forces which makes sap in plants rise.
Living cells have a membrane surrounding them through which osmosis can take place, but this membrane can allow or not allow molecules to pass into and out of the cell much more selectively than a simple semipermeable membrane can.