lab assignment 4 diffusion and osmosis

Optional Lab Activities

Osmosis and diffusion, lab objectives.

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

Introduction

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 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.

Lab Questions

• 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.

Overall Conclusions

• 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 .
• Biology 101 Labs. Authored by : Lynette Hauser. Provided by : Tidewater Community College. Located at : http://www.tcc.edu/ . License : CC BY: Attribution
• BIOL 211 - Majors Cellular [or Animal or Plant]. Authored by : Carey Schroyer and Diane Forson. Provided by : Open Course Library. Located at : http://opencourselibrary.org/biol-211-majors-cellular-or-animal-or-plant/ . License : CC BY: Attribution

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1.6: Diffusion and Osmosis

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• Page ID 24109

• Susan Burran and David DesRochers
• Dalton State College via GALILEO Open Learning Materials

(Adapted from biologycorner.com)

Introduction:

Diffusion is the process by which molecules spread from areas of high concentration to areas of low concentration. This movement, down the concentration gradient , continues until molecules are evenly distributed. Osmosis is a special type of diffusion: the diffusion of water through a semipermeable membrane . The concentration of water is inversely related to the concentration of solute: more solute corresponds to less water and less solute corresponds to more water. This is important because osmotic vocabulary describes the solute and not the water. Hypertonic solutions contain a high concentration of solute and little water, relative to hypotonic solutions that have a low concentration of solute and therefore a higher concentration of water. The term “ isotonic ” is used when two areas have an equal concentration of solute: no net osmosis is occurring.

Exercise 1: Diffusion Through a Gel

One factor that can affect the rate of diffusion is the size of the molecule. Larger molecules tend to move more slowly than smaller molecules. In this experiment, students will compare the diffusion rates of two dyes traveling through agar.

• Pre-punched agar plates
• Potassium permanganate
• Janus green
• Using the dropper, drop a single drop of potassium permanganate into one of the wells on the plate.
• Repeat with Janus green.
• Allow the plates to sit undisturbed for 30 minutes.
• Which dye do you think will have a faster diffusion rate? _____________________
• After 30 minutes, measure the radius of the dye front from the middle of the well and record your results.
• Calculate the diffusion rate (mm/hr) by dividing the dye front radius by 0.5.

1. Did your outcome match your expectation? Provide an explanation for your results.

2. What are other factors that can affect the rate of diffusion?

Exercise 2: Observation of Osmosis in a Plant Cell

Plants have cell walls that can prevent lysis if too much water flows into the cell. Plant cytoplasm tends to be hypertonic to the outside environment, which results in an inflow of water and a high amount of pressure ( turgor pressure ) inside the cell. When a plant is placed in a hypertonic environment, the water will leave the cell. This causes the cell to shrink and detaches the plasma membrane from the cell wall ( plasmolysis ). Turgor pressure can hold plants upright, while plasmolysis can cause plants to wilt.

Observe the two Elodea leaves under the microscope. One slide is a leaf in isotonic solution: you should be able to identify the chloroplasts and an empty space in the middle of the cells which is the vacuole. The next leaf has been soaked in a salt water solution; compare the cells to the first slide.

1. What is the difference between a hypertonic solution and a hypotonic solution?

2. What will happen to plant cells that are placed in a hypertonic solution?

3. What will happen to animal cells placed in hypotonic solution? Why should this be different from plant cells?

4. Why are dehydrated patients given saline intravenously instead of water?

Exercise 3: Osmosis Across a Membrane

• Dialysis bags (4 per group)
• Dental floss
• 15% sucrose solution
• 30% sucrose solution
• Triple beam balance
• Beakers (4 per bench)
• Graduated cylinder

1. Obtain 4 strips of dialysis tubing and tie a knot in one end of each using the dental floss.

2. Pour approximately 10ml of each solution into separate bags (see table below).

3. Remove most of the air from the bag (but leave a little bit of space) and tie the bag.

4. Blot the bags to remove any sugar that may have spilled; check the bags for leaks.

5. Record the weight of each baggie in the data table.

6. Place a bag in each beaker (be sure to keep track of which bag is in which beaker!). Fill the beakers with enough of the appropriate solution to cover your bags (refer to the above table).

7. Predict what you think will happen during the experiment.

8. Record weight every 10 minutes in the data table below.

9. After 30 minutes, remove the bags from the solution and record the final weight.

1. Did your results match your predictions? Propose an explanation for why your results (either overall or an individual bag) may have differed from what you were expecting.

2. Based on what you have observed, are the dialysis bags permeable to sucrose?