Diffusion and Osmosis
OBJECTIVES:
1. To explore how different molecules move by diffusion and osmosis
through semi-permeable membranes.
2. To understand how concentration affects the movement of substances
through semi-permeable membranes.
3. To understand how the processes of diffusion and osmosis through semipermeable membranes affect homeostasis in Cells
I. BACKGROUND MATERIAL
All cells are separated from their external environment by a lipid membrane.
Cell membranes regulate the molecular traffic in and out of the cell. As a
result of their supra-molecular structure (many molecules ordered into a
higher level or organization), these membranes exhibit the property of
selective permeability. Selective permeability refers to the ability of
certain molecules to pass through the membrane while other molecules are
blocked by the membrane, or must use special protein pores to pass through
the membrane. In this lab, we will examine the property of selective
permeability, utilizing dialysis tubing and a set of solutions designed to model
the osmotic situations cells handle. NOTE: Since these processes only
operate efficiently over short distances, the basic living thing – a cell, is
limited in the size it can attain. This is a basic consideration in the evolution
of life.
TERMS
Diffusion: The random movement of molecules or particles, resulting in the
net movement of a substance from a region of high concentration to a region
of low concentration.
Osmosis: The diffusion of water across a semi-permeable membrane.
Solute: A substance that is dissolved in a solution.
Solution: A homogeneous, liquid mixture of two or more substances.
Semi-permeable Membrane: A membrane that allows some
molecules, but not others, to pass through it.
Homeostasis: The inherent tendency in an organism to maintain physiological
balance.
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II. FORMING HYPOTHESES
1. Make observations about the natural world.
2. Ask questions about those observations.
3. Formulate a reasonable testable hypothesis to explain observations.
4. Collect data to test the hypothesis.
5. Evaluate the hypothesis by comparing it to the collected data.
Observation:
All living cells are surrounded by lipid membranes. These lipid membranes
are semi-permeable. Which type of molecule can pass through the
membrane is determined by the types of pore proteins present. This
semi-permeability establishes concentration gradients across the
membrane, altering diffusion and osmosis rates.
Dialysis tubing is a membrane that is selective based on the size of solutes,
and can be used as a model for cellular membrane.
Questions:
How does the molecular size of a solute affect diffusion through the dialysis
tubing?
How does the concentration of solute affect the rate of diffusion and
osmosis?
What happens if one solute can pass through the membrane, but another
cannot?
How do semi-permeable membranes and the processes of diffusion and
osmosis contribute to homeostasis in cells?
Hypotheses:
1. Large starch molecules are too large to pass through the semi-permeable
membrane, but glucose and iodine are small enough to pass through the
membrane.
2. The concentration of solute on either side of the membrane has no
affect on the rate of osmosis because it does not affect the movement
of water.
3. Since diffusion and osmosis are physical properties, they must influence
homeostasis in living cells
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Experiment: We will utilize solutions of starch, glucose, sucrose, and iodine
to test hypotheses 1 and 2, using dialysis tubing as a model of the
membrane of cells. Below, we lay out basic techniques that you can
use to structure experimental protocols.
Results: Record your results and compare them to the hypotheses. Do your
data support or reject each hypothesis? Do your results differ from/
agree with your classmates? What inferences can you make from your
own results and from compiled results?
III METHODS
Hypothesis 1: Large starch molecules are too large to pass through the
semi-permeable membrane, but glucose and iodine are small enough to
pass through the membrane.
Before you can make predictions about the results of your
experiment, you need to know a little chemistry. When glucose or starch is
dissolved in water they are colorless (or milky white). Starch and iodine
react when in solution and turn a solution blue. Additionally, we can test for
the presence of glucose in solution with a urinalysis test strip.
In this experiment, you will fill a dialysis tube with starch and glucose
and submerge it in a beaker full of water and iodine solution. Think about
this experimental design, and make predictions about what results you might
expect to see from your experiment, based on the hypothesis you are
testing. Be sure to record your observations in Table 2.
Predictions:
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Table 1. Part I Materials
Materials needed:
Beaker
Potassium iodide solution (IKI)
Glucose Test Strips
Dialysis tubing
Glucose/starch solution
Clamps
Amount needed per group:
1
A few drops
3
125mm
10-15ml
2
Experimental procedure:
1. Fill a beaker about 1/2 full of water and add a several drops of Iodinepotassium iodide (IKI) solution, enough to make the solution yellow.
2. Use a glucose test strip to test for the presence of glucose in the
beaker. Dip the strip into the solution, remove immediately, and wait
30 seconds before comparing the color with the scale on the bottle.
Record the concentration in Table 2.
3. Take one piece of soaked dialysis tubing, fold the end over on itself about
1”, and then close with a clamp
4. When you are ready to fill the dialysis tube, rub the open end of the tube
between your fingers to open it up. Use a pipette to fill the tube with
10-15ml of the glucose/starch solution.
5. Use one glucose test strip to test for the presence of glucose in the
solution you have just put into the dialysis tube.
6. Clamp the open end of the dialysis tube and then place it into the beaker
of water and iodine. Monitor the experiment for changes. Once the
results are apparent, record the color of the liquids in the beaker, and
the tube. Repeat the glucose test of the solution in the beaker.
Table 2. Data Table for Part I:
Beaker
Contents
Color
Start
After 30 min.
Color Records
Beaker
Contents
Glucose Test
Strip
of Liquid Contents.
Solution In
Dialysis Tube
Dialysis Tube Glucose Test
Color
Strip
Don’t test
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Evaluation: Compare your results in table 2 to your predictions for this
part of the lab. Do these results cause you to reject your original
hypothesis?
If your data are not in accord with your predictions, how could you revise
the hypothesis to match these results, and what new experiment would you
use to test the revised hypothesis?
Thought questions:
1. If the dialysis tubing makes a good model for the cell membrane, what do
the results of your experiment mean for a cell that needs to maintain
homeostasis?
2. It the tube represents a cell, what do the contents of the beaker
represent, i.e. the solution outside of the dialysis tubing?
3. If a molecule is able to pass out of the dialysis tubing, do you think it
would be able to pass the other way? (In other words, is the dialysis tubing a
one-way path?) Describe how you could alter the above experiment to
determine if the dialysis tubing is semi-permeable one-way or two-ways.
Hypothesis 2: The concentration of solute on either side of the
membrane has no effect on the rate of osmosis because it does not
effect the movement of water.
To test this hypothesis, we have provided you with an experimental
apparatus that can be used to quantify (measure) changes in volume that
result from the process of osmosis. It consists of a glass pipette, a stopper,
some dialysis tube, a beaker, and a ring stand. Your TA will demonstrate
how to use this set up. How can this apparatus be used to test a hypothesis
that has to do with the rate of osmosis? You also have several solutions of
sucrose to use to test this hypothesis. See Table 3 for a list of materials
available for this experiment. Sucrose (common table sugar) is a molecule
composed of one glucose molecule, and one fructose molecule.
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Table 3. Materials for testing Hypothesis 2
Materials:
Amount per group:
0.2 M sucrose solution
20ml
0.4 M sucrose solution
20ml
0.8 M sucrose solution
20ml
1.6 M sucrose solution
20ml
Dialysis tubing
125mm
Osmosis apparatus
1
Food coloring
As needed
Experimental procedure:
For this experiment, the procedure is not provided, and relies on you
to establish it. Discuss with your group, and the class, the hypothesis you
are testing, the materials you have available, and develop an experiment that
tests the hypothesis. When an experiment has been agreed upon by the
class, you can then develop predictions of the experimental results.
Describe your experimental protocol, and predictions below.
Experimental Procedure:
Prediction(s) for experiment 2:
Use the space below to record your results.
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Evaluation:
Do the results of this experiment match your predictions and support
your hypothesis? Use experimental evidence to support your answers.
If the results do not match your predictions, what revised hypothesis would
explain what you did see?
How could you test this revised hypothesis?
Thought Questions:
1. What do the results of your experiment tell you about how the
environment might affect a cell’s ability to maintain homeostasis?
2. How might the concentration of a solute influence the rate of osmosis?
3. Do the results of your experiment tell you anything about the influence of
concentration on the rate of diffusion in general?
4. How might you test whether the concentration of a solute alters its rate
of diffusion?
5. What keeps the solution from flowing out of the top of the pipette?
Hypothesis 3: Since diffusion and osmosis are physical properties, they
must influence homeostasis in living cells.
To test this hypothesis, you will use cells of red onion, and your own
cheek epithelial cells. You have a solution of 15% salt (NaCl), and de-ionized
water. The procedure to perform these experiments is provided below.
Actual cell membranes have been found to be semi-permeable just like the
dialysis membrane you used for the previous experiments. However, they
don’t exclude molecules only based on size; the charge of the molecule is also
important, permitting hydrophobic (uncharged) but not hydrophilic (charged)
molecules or ions to pass. When salt (NaCl) is put in to solution with water,
it separates into 2 charged atoms one of Na+ and one of Cl-.
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For this experiment, you will make slides of both Onion cells, and your
cheek epithelial cells. The onion cells will be soaked in 15% salt solution.
The cheek cells will be soaked in both de-ionized water (pure water) and 15%
salt water. Based on this experimental procedure, list your predictions.
Predictions:
Table 4. Available materials
Materials needed:
Red Onion Slice
Razor Blade
Slides
Cover Slip
Microscope
De-ionized water
15% NaCl solution
Tooth picks
Amount needed
1-2 cm2
1
3
As needed
1
As needed
As needed
As needed
Experimental procedure: Onion cells
1. Cut a small square out of a layer of the red onion (1-2 cm2).
2. Slice off/peel the thin red layer on the outside of the square using a
razor. The thinner the layer, the better, so take your time.
3. Place the sample on a slide and try to keep it from folding over. Place a
cover slip over the sample.
4. Place the slide on the microscope and focus on the layer of cells that are
filled with red pigment. Try to find an area where you can clearly see
individual cells. Make a new slide if you need too!
5. Draw the cellular structure of these cells in your notes (note your
magnification). If you look closely, you can see cell nuclei too.
6. Without removing the slide from the scope, drip a drop or two of
15%NaCl onto the edge of the cover slip. It should be pulled under the
coverslip by capillary action, and cover your onion layer.
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7. Continue to watch the cells for the next few minutes. You may want to
move your slide around to see other areas of the slide since some parts of
the tissue may react differently. Record your observations.
Cheek epithelial cells:
1. Take 2 slides. Place a small drop of DI water on one, and a small drop of
15% salt water on the other.
2. Using a toothpick, scrape the inside of your cheek several times. Swirl
the tip of the pick in the drop on one slide, and then repeat this for the
other slide using a new toothpick. Place a cover slip on each slide and
observe under the scope.
3. Make notes about the appearance of the cells and then let the slides sit
for 5 minutes and re-examine.
Evaluation:
Based on the results of these experiments, did you support your
prediction(s) ? Did your experiment support the hypothesis being tested?
Use experimental evidence to support your answers.
Thought Questions:
1. Did the cells gain, loose, or maintain volume in each solution? What does
this tell you about the solute concentration in the cell relative to each
solution? Refer to experimental evidence to support your answer.
2. What do the results of these experiments tell you about the challenges
cells face in trying to maintain homeostasis?
3. What do you think would happen to a single-celled freshwater organism if
it were suddenly thrown in the ocean? Refer to experimental evidence to
support your answer.
4. Early life evolved in the sea before moving onto land. What are the
challenges faced by a single-celled aquatic organism that tries to live on
land? Please explain with reference to the following terms: osmosis,
diffusion, solute concentration, and semi-permeable membrane.
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