Cell membranes are selectively-permeable barriers that separate the inner contents of the cell from its environment. Cells allow different types of materials to cross the cell membrane. For example, nutrients must enter the cell while waste products must exit.
The cell membrane is passively permeable—certain molecules and ions enter and exit freely via diffusion. This relatively slow process relies on the random motion of the molecules to ultimately balance out the concentration differential on either side of the membrane so that the concentration inside the cell and the concentration outside the cell are appropriately matched.
A schematic diagram of this process is shown below, where N is equal to the number of molecules on each of the two sides of the membrane, and t represents different points in time (t0 being the start point):
The cell is also capable of actively transporting molecules and ions across its membrane, as well as regulating the net transport of particles that ordinarily move via passive diffusion.
A similar phenomenon is that of osmosis. Water is a very small molecule and it readily diffuses across the cell membrane. When particles are unable to cross the membrane to equalize their concentration on either side, the water in which they are dissolved can cross the cell membrane instead; this equalizes the concentrations of the dissolved particles within and without the cell. The net force driving this transport of water is called the osmotic pressure.
Laboratory Technique
Dialysis Tubing
In this lab exercise, you'll explore a model of a semi-permeable cell membrane constructed from commercially-available dialysis tubing used to clean the blood of patients with kidney failure. The tubing material is carefully manufactured to be permeable only to those molecules smaller than a certain size, the MWCO or molecular weight cutoff. The MWCO regulates the passage of dissolved particles through the tubing.
First, you will perform experiments on the diffusion of molecules across the semi-permeable dialysis tubing. Semi-permeable dialysis bags are constructed from cut lengths of the tubing which are tied off at either end. They are filled with one solution and then immersed in a beaker containing a second solution. The closed bag acts as a model of a cellular membrane for the purposes of studying diffusion and osmosis.

Procedures
Experiment 1: Membrane Size Selectivity
1. I placed a diffusion bag and a clean 250 mL beaker from the Containers shelf onto the workbench.
2. Then I Filled the bag with 90 mL of water and 10 mL of iodine solution from the Materials shelf.
3. Then I Filled the beaker with 150 mL of Water and 50mL of 2% starch solution.
4. Then I moved the diffusion bag into the beaker.

5. Then I Watched the beaker and bag for signs of color change. After about a minute, move the dialysis bag out of the beaker and onto the workbench.
What color is the liquid in the beaker? Blackish purple What color is the liquid in the dialysis bag? Yellow
Do your results support your predictions? Yes Record your observations and conclusions in your notes.
6. Discard the beaker and dialysis bag into the recycling bin under the workbench. Experiment 2: Relative Concentration and Osmosis
1. Then I Placed four diffusion bags from the Containers shelf onto the workbench.
2. Then I Added 20% sucrose solution and water from the Materials shelf to each bag as indicated in the chart below:
Sucrose Solutions
Diffusion Bag
20% sucrose solution (mL)
Water (mL)
1
100
0
2
75
25
3
50
50
4
25
75
3.
Note that diffusion bag 1, in which the sucrose is undiluted, contains 20% sucrose solution. The percent sucrose in other bags can be calculated using the equation:
4.
% sucrose = (x mL sucrose solution / 100 mL total) * 20% sucrose
For diffusion bag 2, x = 75 mL, so the equation can be solved as follows:
% sucrose = (75 mL sucrose solution / 100 mL total) * 20% sucrose = 15% sucrose
5.
Calculate the