The cell and its extracellular environments are fluid in that concentrations of substances dissolved in it vary. Depending on varying concentration of the substances dissolved in water, be it extracellular or intracellular, dissolved substances or water move from one compartment to another directed by physical principles of gradients. To accomplish this, the cell utilizes the processes of osmosis and diffusion as appropriate. These phenomena can be visualized outside the cell by simulating these in vitro. Since cell membrane is a semipermeable membrane and exchanges occur through this, these experiments must utilize a construct of the semipermeable membrane that can be made with a commercially available dialysis tubing and bag. These membranes have microscopic pores through which small molecules like water can pass, but larger molecules, such as sugar cannot pass through them. Thus through this molecular net, solutions of different solute concentrations can be observed to pass in a measurable fashion. This experiment has been designed to prove the hypothesis of the mechanisms of osmosis and diffusion through such a semipermeable membrane.
Diffusion is defined as the movement of molecules from a site of higher concentration to that of a lower concentration. Technically speaking, this difference in concentration creates a concentration gradient, and the steepness of the gradient determines the rate of diffusion. In an attempt to find out the reason as to why it occurs, it can be attributed to a great deal of space between the molecules of all substances that are candidates for diffusion. Liquids have moderate intermolecular space. Moreover, all molecules are in a state of constant random movement so that they collide and intermingle. It is expected that in solids diffusion would occur slowly for tight packing of the molecules. Liquids and gases would diffuse freely because their molecules are spaced widely. Thus, it can be stated that any solute will tend to uniformly occupy the entire space available to it. This movement, known as diffusion, is due to the spontaneous Brownian random movement that all molecules experience. The net result of diffusion is the movement of substances according to their difference in concentrations, from regions of high concentration to regions of low concentration. Diffusion is an effective way for substances to move short distances. Diffusion across a membrane has no preferential direction; it can occur from the outside of the cell toward the inside or from the inside of the cell toward the outside. It is however determined the extend of permeability through a membrane, which in the case of a living cell is the protein-lipid-protein plasma membrane, and therefore, diffusion across the membrane usually implies that the diffusing solute enters the lipid bilayer to cross it, the solute's solubility in a lipid solvent compared with its solubility in water is important in determining its permeability through it. Hydrophilic substances, such as ions and sugars, do not interact well with the lipid component of the membrane, hence would diffuse across the membrane more slowly. This is, however, not applicable in physical membrane such as a dialysis membrane (Campbell NA, Reece JB, Mitchell LG., 1999).
If equal volumes of pure water and a strong sugar solution are taken, the pure water will have more water molecules and would have higher water concentration that a sugar