Osmosis And Diffusion - Southern Biological

• Movement of materials across membranes occurs via diffusion, osmosis, active transport and/or endocytosis

BACKGROUND:

The cell membrane maintains the cell a separate entity; it holds the cell contents within, and acts as a barrier to the external environment. It is selectively permeable and has various mechanisms to allow for the exchange of gases and nutrients. These mechanisms allow for the intake of anything that is required and allows for the expulsion of waste and toxins. This membrane does not resemble a sheet or bag; rather, it is many molecules of Phospholipid Bilayers held together by the combined forces of attraction and repulsion. They are comprised of a Phosphate head; which is hydrophilic (water-loving), and a Lipid (fatty acid) tail which is hydrophobic (repelled by water). As the internal and external environments of a cell are aqueous, these molecules arrange themselves into two layers; one with the Phosphate heads oriented out into the external fluid, and the other with the heads oriented inwards into the internal fluid (the Cytoplasm). The Lipid tails are between the two layers of Phosphate heads; thereby, protected from the water, and the strength of this attraction/repulsion mechanism keeps the molecules together as though the membrane were a single entity.

In this practical, dialysis tubing is used as a surrogate cell membrane for a visual demonstration of osmosis and diffusion. A solution containing large molecules (Starch) and small molecules (Glucose) is placed inside the tubing; which is then placed in a solution containing iodine. Students are able to observe as the solution inside the tubing turns dark blue, while the surrounding solution it is submerged in does not. From this, students can use their prior knowledge of the Starch-Iodine complex to surmise that Iodine is able to pass through the membrane while starch is not. The Glucose-testing strips indicate that glucose has been able to pass out of the tubing and into the external fluid. Thus proving the tubing allows movement in both directions.

This inexpensive and simple experiment provides students with a clear visual result that effectively demonstrates how the size of a molecule can affect its ability to be transported into or out of a cell. It also illustrates the mechanics of diffusion and osmosis by which a cell will attempt to create homeostasis, or equilibrium between its inner and outer environments.

PREPARATION - BY LAB TECHNICIAN

1. Cut the dialysis tubing into 15cm lengths and soak for 15 minutes in a beaker filled with room temperature distilled water. Prepare one length of tubing per student or group. However, it is best to prepare extra strips for students, as some strips may tear or leak through handling.
2. To create the Starch solution, dissolve 2g of Starch in 100mL of boiling hot water (2% solution) on a hot plate until the Starch powder has been fully dissolved. Stir as required.
3. To create the Glucose solution, dissolve 30g of Glucose in 100mL water (30% solution) and continue stirring until the glucose has been fully dissolved.
4. Combine the Starch and Glucose solutions in a single beaker. Use a stirring rod to mix well.