BIO 1120L-09

Osmosis and Diffusion via Semipermeable Membrane
Using Starch and Potato Cells

Osmosis is the tendency of molecules from a solvent to pass through a semipermeable membrane via passive transport, meaning that no energy needs to be provided for the movement of water molecules to happen. Cells that are living use active transport to change or maintain the concentration of many molecules inside of them to what's going on outside. All molecules travel from high concentration levels to areas that are lower in concentration. When a concentration goes to a lower concentration, it is known as diffusion. When the molecule moving down the gradient is water, it's known as osmosis. Water is a highly used solvent because it normally brings other molecules with it during osmosis and requires the semipermeable membrane. A semipermeable membrane allows small molecules to pass through it (like water), but refuses others (like the starch and potato used in this lab). Cell membranes have a phospholipid bilayer that is an effective barrier to water. Since water is slightly negatively charged, it makes it difficult to pass through the hydrophobic interior of said bilayer. Because of the charges, and being unable to pass through, the molecules pass through specialized protein pores that will only allow one specific kind of move across the membrane. Looking at a few solutions in this lab, one that has a lower concentration of solutes than another solution is said to be hypotonic. A solution with a higher concentration of solutes relative to another solution is hypertonic. When the solutions have the same concentration of solutes, they are isotonic. In this experiment, the size of atoms, water, and a complex carbohydrate (starch) is used to determine which atoms will move through the semipermeable membrane and see which atoms are too big to pass. The experiment was performed as described on pages 17-21 of the BIO 1120 laboratory manual. For the first part of the experiment, a plastic dialysis bag was filled with a teaspoon of cornstarch and 20 mL of distilled water was prepared along with a 200 mL beaker containing 150 mL water. Fifteen drops of potassium iodide was added to the beaker and the plastic bag was submerged into the potassium iodide solution. Notes were taken over the course of the rest of the lab to record any changes in the solution/dialysis bag. The second part of this lab experiment was followed exactly as stated in the BIO1120 lab manual. Seven test tubes were gathered and labeled and 5 mL of solutions with different molar concentrations of glucose (ranging 0 to 0.8M) were added to each of the tubes. A cork borer was used to make seven cylinders of potato and each cylinder was weighed after being patted dry. The cylinders were then cut in half and both halves placed into the test tubes. All samples were incubated for 45 minutes and swirled every 10-15 minutes. The tubes were then removed and the