The osmosis process occurs when two fluids, separated by a semipermeable membrane, have different concentrations. The fluid with a weaker concentration tends to pass through a semipermeable membrane to the side of other fluid with a stronger concentration. The semipermeable membrane only allows small cells to pass through. For example, the water molecules passes through the surface semipermeable membrane of the cell.
The Visking tubing apparatus, usually called dialysis tubing, demonstrates the osmosis process. The Visking tubing is a cellophane semipermeable membrane filled up with concentrated sugar solution, or a mixture of starch and glucose. This Visking tubing represents the surface semipermeable membrane of the cell and the mixture represents the cytoplasm. If the Visking tubing is immersed in water, after a while, it will be filled up with water, because the small water molecules can pass through the tubing, while the larger sugar molecules cannot diffuse out from the tubing because the dimension of sugar molecules do not allow the passing through the tubing.
Hence, the sugar molecules do not diffuse from the concentrated solution to the dilute solution, while water molecules diffuse from the dilute solution to the concentrated one.
This osmosis process occurs in red blood cells. Hence, if the red blood cells are placed in strong salty solution, they shrink, while if they are placed in distilled water, the red blood cells expand its volume. To prevent the process of shrinking or expanding of red blood cells, the plasma must not be too concentrated or too dilute.