Movement of molecules into and out of cells is partly controlled by the semipermeable nature of the cell membrane. The terms "semipermeable" means that the membrane allows some molecules through but not others. Movement of water across the cell membrane is a special case of diffusion and called osmosis. As in other cases of diffusion, water moves from an area of high concentration to an area of lower concentration. Thus when water maoves across a cell membrane, it moves from a dilute colution (high concentration of water molecules ) to a less dilute one (lower concentration of water molecules). Osmosis can lead to a variety of changes in a cell. Cells placed in dilute solutions experience an inflow of water and swell up. The internal pressure exerted on the cell membrane is called osmotic pressure. The surrounding dilute medium is said to be hypotonic to the cyptoplasm inside the cell. If osmotic pressure builds up high enough the cell may bursst or lyse. On the other hand, cells in concentrated solutions lose water and may shrivel up; in this case the surrounding medium is said to by hypertonic to the cell. When the concentrations of water on both sides of a semipermeable membrane are equal, the cell and its surround medium are said to be isotonic. Osmosis does not require energy input from the cell. Random molecular motion "powers" the process. However, depending on the concentration of water in their surrounding, many cells must actively pump water out of the cell to avoid bursting. They may also move other molecules across their membranes and therby affect the direction or rate of osmosis. This lab uses a model to demonstrate the osmosis process. The cel model is made of dialysis tubing, which is a semipermeable memebrane with submicroscopic pores that only allow the passagie of small molecules. This laboratory builds on the concepts of osmosis by modeling hypo-, iso-, and hypertonic solutions. It is important to review these concepts before beginning.

Scientists conduct investigations for a wide variety of reasons. For example, they may wish to discover new aspects of the natural world, explain recently observed phenomena, or test the conclusions of prior investigations or the predictions of current theories. Cells have particular structures that underlie their functions. Every cell is surrounded by a membrane that separates it from the outside world. Inside the cell is a concentrated mixture of thousands of different molecules which form a variety of specialized structures that carry out such cell functions as energy production, transport of molecules, waste disposal, synthesis of new molecules, and the storage of genetic material.

* Students will need to have a basic understanding of the priciples of osmosis along with knowledge of hypo-, iso-, and hypertonic stolutions. * Students will use an electronic balance to measure mass. * Students will use basic math skills in the analysis of data. * Students will follow standard precautions. * Students will work cooperatively in groups. * Students will use computers to type of laboratoy activities.

* Review concepts of hypo-, iso-, and hypertonic solutions. * Demonstrate how to "open" dialysis tubing. * Prepare stock solutions for students prior to the start of the activity. * Display Table 2 on the board or on an overhead transparency so students can compile and share their data.

Completion of laboratory writeup and post-lab discussion.

Osmosis Across A Semipermeable Membrane Purpose: To demonstrate how osmosis in a model cell is affected by the concentration outside the model. Background: Movement of molecules into and out of cells is partly controlled by the semipermeable nature of the cell membrane. The term 3semipermeable2 means that the membrane allows some molecules through but not others. Movement of water across the cell membrane is a special case of diffusion called osmosis. As in other cases of diffusion, water moves from an area of high concentration to an area of lower concentration. Thus when water moves across a cell membrane, it moves from a dilute solution (high concentration of water molecules) to a less dilute one (lower concentration of water molecules). Osmosis can lead to a variety of changes in a cell. Cells placed in dilute solutions experience an inflow of water and swell up. The internal pressure exerted on the cell membrane is called osmotic pressure. The surrounding dilute medium is said to be hypotonic to the cytoplasm inside the cell. If osmotic pressure builds up high enough the cell may burst or lyse. On the other hand, cells in concentrated solutions lose water and may shrivel up; in this case the surrounding medium is said to be hypertonic to the cell. When the concentrations of water on both sides of a semipermeable membrane are equal, the cell and its surrounding medium are said to be isotonic. Osmosis does not require energy input from the cell. Random molecular motion 3powers2 the process. However, depending on the concentration of water in their surroundings, many cells must actively pump water out of the cell to avoid bursting. They may also move other molecules across their membranes and thereby affect the direction or rate of osmosis. In this lab, we will use a model to demonstrate the osmotic process. The cell model is made of dialysis tubing, which is a semipermeable membrane with submicroscopic pores that only allow the passage of small molecules. Materials: dialysis tubing sugar solution distilled water beakers balance string scissors Procedures: A. Cut off three pieces of dialysis tubing that measure 10 cm each. B. Moisten each piece with water to make them flexible. Open the tubing by rubbing it between your fingers. Tie a knot in one end of the tubing. C. Open the free end of Tube 1 and fill it about 1/3 full with 50% concentrated sugar solution. Twist the free end and tie it securely with a piece of string. DO NOT squeeze the tube or it will leak. D. Rinse off the tube, blot it dry on a paper towel and mass it on a balance. Record its mass on your data table under 3Pre-test Mass2. E. Fill Tube 2 about 1/2 full with a solution of 50% sugar solution. Tie off the free end as before, rinse, blot dry and mass the tube. Record the mass of the tube in your data table. F. Fill Tube 3 completely full with distilled water. Tie off the free end as before, rinse, blot dry and mass the tube. Record the mass of the tube in your data table. G. Prepare three beakers large enough to hold the tubes with the following solutions: Beaker 1: 150 ml distilled water Beaker 2: 150 ml 50% sugar solution Beaker 3: 150 ml 50% sugar solution H. Place Tube 1 in Beaker 1, Tube 2 in Beaker 2 and Tube 3 in Beaker 3. Allow the tubes to sit in the beakers for 50 minutes. I. After 50 minutes remove the tubes, rinse them off, gently blot them dry and mass them. Record the mass in the 3Post-test Mass2 column. J. Calculate the mass change of each tube by subtracting (the post-mass from the pre-mass). Calculate the percentage mass gained or lost by each tube by diving the mass change by the Pre-test Mass and multiplying by 100. Use a + or - sign to indicate the direction of change. Record the values in your data table. K. Prepare a class data<

*Electronic balances *Computers (if lab reports typed) *Calculators