One of the principal ways molecules get in and out of cells is diffusion.
Diffusion is defined as the net movement of molecules from areas of higher concentration to areas of lower concentration. For example, if you place a crystal of a purple dye in a glass of water, the purple color spreads out as the dye molecules diffuse through the water. Another example of diffusion is inside the air sacs of your lungs, where the net movement of oxygen is out of the air and into your blood. Because oxygen molecules move in random directions, there will always be some molecules moving in the “wrong” direction. But overall, more oxygen molecules move out of the air than out of the blood, simply because there are more oxygen molecules in the air than in the blood. The diffusion of water molecules is so important to living cells that it is given a special name, osmosis. Osmosis is the net movement of water molecules across a semi-permeable membrane. [Semipermeable membranes allow small molecules to pass through them, but not larger molecules like sugar and dye.] When the concentration of water molecules is higher outside than inside the cell, the net movement of water will be into the cell and the cell will swell. When the concentration of water molecules is lower outside, the net movement of water will be out of the cell and the cell will shrivel. The cell membrane of a living cell is similar to a semipermeable membrane in that it permits some substances to cross it but not others. For example, the cell membrane allows small, uncharged molecules (water, oxygen and carbon dioxide) to diffuse freely, but blocks the diffusion of molecules that are large (e.g., sugar, proteins) or are electrically charged (e.g., sodium and chloride ions). In the exercise below, you will make a simple “model” cell from (kidney) dialysis tubing – plastic tubing manufactured with holes just large enough to be permeable to water but not permeable to large molecules like sucrose (table sugar). Now suppose you filled the dialysis bag with water and a little sucrose, and then placed the bag in a beaker of water. Water will begin to move by osmosis into the bag. But why? Think of it this way: the concentration of water inside the bag is lowered by the presence of the sucrose molecules. (In a sense, the dissolved sugar molecules “dilute” the water, lowering the water’s concentration.) Because the concentration of water is higher outside than inside, water diffuses “down” its concentration gradient and into the bag. Unlike water, sucrose is unable to flow down its concentration gradient, because sucrose molecules are too large to get through the pores and out of the bag. Because sucrose molecules can’t get out, their presence will keep the concentration of water molecules always lower inside the bag compared to the pure water outside the bag. So water will continue to move into the bag (a) until the bag bursts, or (b) until the internal fluid pressure gets high enough to prevent more water from diffusing in. Notice that because of the presence of sucrose inside the bag, the concentration of water inside can never become the same as the concentration of the water outside. With this background, you can now understand the formal definition of osmosis: the passive movement [= diffusion] of water across a semi-permeable membrane in response to differences in pressure and solute concentrations on either side of the membrane. (We define solute as any dissolved substance. Sucrose is the solute in the above example.)