At the conclusion of the lab, the student should be able to:
- define the following terms: diffusion, osmosis, equilibrium, tonicity, turgor pressure, plasmolysis
- describe what drives simple diffusion (why do the molecules move?)
- list the factors that may affect the speed of simple diffusion
- list which molecules, in general, can freely diffuse across the plasma membrane of a cell
- describe what drives osmosis (why do water molecules move?)
- explain why water moves out of a cell when the cell is placed in a hypertonic solution
- explain why water moves into a cell when the cell is placed in a hypotonic solution
- describe what physically happens to a cell if water leaves the cell
- describe what physically happens to a cell if water enters the cell
Understanding the concepts of diffusion and osmosis is critical for conceptualizing how substances move across cell membranes. Diffusion can occur across a semipermeable membrane; however diffusion also occurs where no barrier (or membrane) is present. A number of factors can affect the rate of diffusion, including temperature, molecular weight, concentration gradient, electrical charge, and distance. Water can also move by the same mechanism. This diffusion of water is called osmosis.
In this lab you will explore the processes of diffusion and osmosis. We will examine the effects of movement across membranes in dialysis tubing, by definition, a semi-permeable membrane made of cellulose. We will also examine these principles in living plant cells.
Part 1. Diffusion Across a Semi-Permeable Membrane: Dialysis
- Cut a piece of dialysis tubing, approximately 10 cm.
- Soak the dialysis tubing for about 5 minutes prior to using.
- Tie off one end of the tubing with dental floss.
- Use a pipette and fill the bag with a 1% starch solution leaving enough room to tie the other end of the tubing.
- Tie the other end of the tubing closed with dental floss.
- Fill a 250 mL beaker with distilled water.
- Add Lugol’s iodine to the distilled water in the beaker until the water is a uniform pale yellow color.
- Place the dialysis tubing bag in the beaker.
- The molecular formula for Lugol’s solution is I2KI (atomic mass = 127). Starch consists of long chains of glucose (atomic mass of each glucose = 180). Iodine turns a deep blue in the presence of starch. Formulate a hypothesis for each of the following. Remember to provide a reasonable explanation for your predictions.
- The movement of starch
- The movement of iodine
- The color of the solution in the bag after 30 minutes
- The color of the solution in the beaker after 30 minutes
- Add the dialysis bag to the beaker and allow the experiment to run for 30 minutes. Record the colors of both the dialysis bag and the beaker.
|Table 1: Dialysis Tubing Data|
|Dialysis tubing contents||Beaker contents|
|Pre-experimental contents||1 % Starch solution||Dilute iodine water|
- Is there evidence of the diffusion of starch molecules? If so, in which direction did starch molecules diffuse?
- Is there evidence of the diffusion of iodine molecules? If so, in which direction did iodine molecules diffuse.
- What can you say about the permeability of the dialysis membrane? (What particles could move through and what particles could not?)
- What is the difference between a semi-permeable and a selectively permeable membrane
Part 2. Plasmolysis—Observing Osmosis in a Living System, Elodea
If a plant cell is immersed in a solution that has a higher solute concentration than that of the cell, water will leave/enter (circle one) the cell. The loss of water from the cell will cause the cell to lose the pressure exerted by the fluid in the plant cell’s vacuole, which is called turgor pressure. Macroscopically, you can see the effects of loss of turgor in wilted houseplants or limp lettuce. Microscopically, increased loss of water and loss of turgor become visible as a withdrawal of the protoplast from the cell wall (plasmolysis) and as a decrease in the size of the vacuole (Figure 1).
- Obtain a leaf from the tip of an Elodea Place it in a drop of water on a slide, cover it with a coverslip, and examine the material first at scanning, then low power objective and then at high power objective.
- Locate a region of health. Note the location of the chloroplasts. Sketch a few cells. For the next step, DO NOT move the slide.
- While touching one corner of the coverslip with a piece of Kimwipe to draw off the water, add a drop of 40% salt solution to the opposite corner of the coverslip. Do this simultaneously. Be sure that the salt solution moves under the coverslip. Wait about 5 minutes, then examine as before. Sketch these cells next to your sketch of cells in step two, note the location of the chloroplasts. Label it 40% salt solution.
- What happened to the cells in the salt solution?
- Assuming that the cells have not been killed, what should happen if the salt solution were to be replaced by water?
- Are plant cells normally hypertonic, hypotonic, or isotonic to their environment? Why?
- Can plant cells burst? Explain.
- Review your hypothesis for each experiment. Was your original hypothesis supported or rejected for each experiment. Explain why or why not. This should be based on the best information collected from the experiment. Explain how you arrived at this conclusion.
- If it was incorrect, give the correct answer, again based on the best information collected from the experiment.
Sources of Error
- Identify and explain two things that people may have done incorrectly that would have caused them to get different answers from the rest of the class. Be specific.