Diffusion in Elodea and Yeast Cells Purpose: To study the effects of osmosis in the cells of elodea leaves placed in salt water and distilled water and the effect that boiling has on the cell membranes of yeast cells. Background: Do you sometimes mix up the meanings of osmosis and diffusion? Many students do! Let’s try to clear up the confusion about diffusion. First there is simple diffusion. If you drop a lump of sugar into a cup of hot tea, the sugar will dissolve and its molecules will spread evenly throughout the liquid. Should you pour a few drops of perfume into a saucer, the drops will evaporate and the perfume molecules will spread throughout the room. What causes this simple diffusion (spreading out) of molecules of a gas or a liquid? Scientists tell us that molecules are constantly moving in a helter-skelter fashion and therefore have kinetic energy (energy of movement). This movement results in frequent collisions that bump the molecules away from their source. The molecules are always scattered from an area of more concentration to one of less concentration. Diffusion also may occur through a membrane. A membrane is a thin sheet of solid material. If molecules of a substance dissolved in a liquid pass through a membrane, the membrane is considered permeable to that substance. A membrane may allow some kids of molecules to pass through it while preventing other kinds. Such a membrane is selectively permeable. The living cell membrane is selectively permeable. It allows certain substances dissolved in water (either inside or outside the cell) to pass through it. Its permeability can also change as needed. Unlike a nonliving membrane it may permit the molecules of a substance to pass through it at certain times but not at others. Although a great many substances can diffuse through cell membranes, some, like many salts, do so rather slowly. Others, such as carbon dioxide and water, pass through with ease. We call the diffusion of water molecules through a selectively permeable membrane by a special name: osmosis. The osmotic flow of water molecules into a cell causes a pressure, called the osmotic pressure , inside the cell. As you might expect, such pressure may cause the cell to swell. The osmotic flow of water out of the cell lowers the osmotic pressure, and the cell may shrink (plasmolysis); a rise in osmotic pressure as water flows into the cell would cause swelling and possibly bursting (deplasmolysis). If the concentration of water molecules on one side of a membrane is greater than the concentration on the other side, water molecules will flow from the side of higher concentration to that of lower concentration. Materials: elodea sprigs in water microscope microscope slides cover slips dissecting needle strips of filter paper 5% salt solution distilled water yeast suspension with Congo red solution boiled yeast suspension with Congo red solution Pre-lab Questions: 1. Define diffusion. 2. What is a selectively permeable membrane? 3. Define osmosis. 4. What causes osmotic pressure in a cell? 5. Define plasmolysis. 6. Define deplasmolysis. Procedures: A. Take a young leaf from near the tip of an elodea sprig. Place the leaf on a microscope slide and add a drop of tap water, and apply a cover slip. Focus under low power so that one edge of the leaf is in the center of the field. B. Switch to high power. On the data sheet sketch three cells of the elodea leaf edge and label the cell wall and chloroplasts. C. Place a drop of 5% salt solution at one edge of the cover slip. With a strip of paper towel at the opposite edge, draw the tap water out from under the cover slip and the salt water in to replace it. Do this while you are viewing the cells under high power, so that you can observe any cell changes as they occur. Repeat this procedure to be sure of replaci