Molecular Movement: Diffusion and Osmosis
Laboratory Investigation


  • to observe the physical processes of diffusion and osmosis in living materials
  • to observe the physical processes of diffusion and osmosis in an artificial system
  • to recognize that diffusion and osmosis are passive transport mechanisms that do not require an outside energy input to occur
  • to determine how temperature, and concentration of solutes, affect diffusion rate
  • Introduction

    The movement of molecular materials within and between cells is accomplished by transport mechanisms of two types. One group of mechanisms require an input of energy by the system (cell or organism) and are known as ACTIVE TRANSPORT MECHANISMS. In active transport molecules are moving against a concentration gradient trying to find a place within an already crowded area. This is somewhat like moving into a crowded bus or crowded room. Some active transport mechanisms involve only the cell membrane while others require a movement by larger areas of the cell.

    The second group of mechanisms are known as PASSIVE TRANSPORT MECHANISMS. In these situations molecules are moving from areas where they are in high concentration into areas where that same type of molecule is in low concentration. No additional expenditure of energy by the cell or organism is required to move the molecules. It is sometimes suggested that they are moving down a concentration gradient. Examples of passive transport are DIFFUSION, OSMOSIS and FACILITATED DIFFUSION. OSMOSIS is a special case of diffusion involving movement of solvent molecules. Since water is the solvent of the cells, osmosis is often thought of as diffusion of water. Observe the following simulation of simple diffusion. Go to Dr. Patlak's Homepage. Go to the LabBook Module mentioned on that page.

    Cellular membranes are selective of the molecules that can move across them. Size, shape, mass, and charge of moving molecules are all considerations as to whether the molecules may cross the membrane.

    Part I Diffusion and Osmosis in Living Materials

    There are a number of simple investigations that can be done at home and involve foods such as celery, carrots, onions, and potatoes. You can use jars, water bottles or glasses as containers. It is important that the slices or cubes be of equal size. If you have a small food scale, weight the slices or cubes and try to get them of approximately equal size.

    1. One simple investigation involves putting slices of any of these foods into containers of distilled water, tap water, and an equal sized slice into solutions of salt water or sugar water. Use distilled water to make your solutions. (DI water may be purchased at your local pharmacy). Make up different solutions of salt and sugar by adding larger or smaller amounts. Observe the overall changes and also the rate of change. Leave the slice in the solutions for several hours.
    2. Here's a small variation on the use of the carrot.
    3. Try putting a stalk of celery into salt water, another into distilled water and a third into water with iodine or food coloring in it.
    4. If you place the stalk of a white carnation in a food-colored solution or a solution of ink, you will see diffusion in action.
    5. Osmosis in Elodea and Red Onion
      1. Sprigs of Elodea or red onion may be placed in distilled water, tap water and salt solutions. If microscopes and slides are available follow the procedures you used in your first microscope lab. Make a slide of Elodea or red onion, cover with coverslip and add your test solution to one side of the coverslip using a paper towel to draw the solution across the surface.
        In the absence of microscopes and slides we will observe the organisms by using the embedded links below.
      2. Elodea in tap water -normal
        Elodea in salt solution
      3. Red onion - normal and in salt solution
    Part II Osmosis and Diffusion in Model Systems

    There are two types of model systems that are routinely used to demonstrate osmosis and diffusion.
    1. Plate Diffusion - petri dishes filled with agar are often used to show the effects of temperature on diffusion rate of different types of molecules. This is similiar to a technique used in immunology to demonstrate antibody activity. Note the setup at this linked site. To do the same at home, take a bottom section of a 24 oz. water bottle to serve as your petri dish. Use Knox (unflavored) gelatin as your agar. Cover the plate with foil. Use various types of food coloring to show the effects. Use a wide-bore straw to create an opening in the gelatin. Place one "petri dish" in the refrigerator. Place another at room temperature. At the end of a set number of hours, measure the distance travelled by the dyes in each setting.
    2. The Model Cell or Osmometer - the second type of model system involves dialysis tubing filled with various solutions. The following links illustrate the setup and results.
      1. Pictures of a dialysis setup using a sugar solution. Go to Dr. Cullen's Faculty homepage. Click on courses, then Biology 107. Click on labs and go to Osmosis and Dialysis pictures. Dialysis tubing in an iodine solution
    There are a number of variations that can be attempted. There are also ways to make these investigations more quantitative.


    1. In the Elodea investigation, which solutions (if any) were hypertonic, isotonic, or hypotonic?
    2. Explain what is meant by each of these terms.
    3. Is there a difference in the distance travelled by the different dyes?
    4. What would account for these differences?
    5. What effect does temperature have on the distance travelled by the dyes?