Bio 13A Lab Manual
Biology 13A Lab #4: The Cell: Movement Across the Membrane
Lab #4 Table of Contents:
• Expected Learning Outcomes .
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• Introduction
.
.
.
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• Activity 1: Potatoes and Sailors
.
• Activity 2: Eggs and Osmosis .
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• Activity 3: Red Blood Cells and Osmosis
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Expected Learning
Outcomes
At the end of this lab, you
will be able to
• define relevant terms and
use vocabulary
appropriately in context;
• determine the effects of
salt water on plant and
animal cells; and
• observe the movement of
water across a membrane
in model cells (decalcified
eggs) and red blood cells.
Figure 4.1 Osmosis
Lab #4: The Cell: Movement Across the Membrane
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Bio 13A Lab Manual
Introduction
The plasma (cell) membrane is a
selectively permeable barrier. It
regulates the movement of
hundreds, if not thousands, of
different types of molecules into
and out of the cell. What goes
into and out of the cell defines
cell function; the plasma
membrane is literally the
boundary between life and death.
Movement of molecules is
influenced by diffusion, the
tendency for particles to spread
from higher concentrations to
lower concentrations until they
are evenly distributed. Molecules
such as O2 and CO2 pass easily
through the membrane because
they are small; other molecules
such as glucose diffuse across
through protein channels in the
membrane. Water also diffuses
across the membrane in a
process known as osmosis. This
occurs when two solutions are
separated by a selectively
permeable membrane, a barrier
that allows some material
through but excludes others. In
osmosis, water moves from areas
of low solute concentration to
areas of high solute
concentration of solutes. The
solutes don’t move across the
membrane, either because they
are too large or are not soluble in
lipids. Because systems always
tend toward equilibrium, this
movement of water molecules
Lab #4: The Cell: Movement Across the Membrane
occurs to even out the
concentrations on the two sides
of the membrane.
Osmosis has a major
impact on living cells and organs.
Organisms rarely exist in
environments with solute
concentrations that match their
cytoplasm; there are usually
more or fewer dissolved particles
in one of two compared solutions
separated by a membrane, such
as a cell and the media in which
it exists. A solution is hypotonic
if it has fewer dissolved particles
than the other solution,
hypertonic if it has more
particles than the other solution.
If the number of dissolved
particles is the same in the two
media, it is called isotonic.
If a cell is in a hypotonic
solution, water rushes into the
cell, causing it to expand. If
enough water comes in, the cell
bursts, or lyses. If a cell is in a
hypertonic solution, it loses
water, which causes it to shrink,
or crenate.
Osmosis underpins
homeostasis of physiological
features such as fluid balance
(associated with urine output)
and blood pressure (associated
with blood volume). An
understanding of kidney
function, for example, requires
an understanding of osmosis.
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Bio 13A Lab Manual
Check Your Understanding: Answer the following questions based on
your reading of the introduction.
1. Define diffusion and osmosis.
2. Define hypertonic, isotonic, and hypotonic.
3. Two solutions are separated by a semipermeable membrane. Solution
A has a 20% concentration of sugar and solution B has a 50% solution of
sugar, what direction would you expect the water to move, from A to B or
from B to A?
a. A to B
b. B to A
4. In the study of human physiology, why is it important to understand
osmosis?
Activity 1: Potatoes and Sailors
A shipwrecked sailor was stranded on a small desert island with no fresh
water to drink. She knew she could last without food for up to a month
but if she didn't have water to drink, she would be dead within a week.
She had managed to retrieve one of the most valuable treasures from her
sinking yacht, a gallon jug of water. This was a smart sailor and she
knew that no matter how thirsty she became, she could NOT drink the
seawater. She had read stories about shipwrecked sailors who died in
two days when their thirst became so overwhelming that they drank the
salty ocean water. As she waited for rescue, she pondered the question,
why does drinking seawater kill a sailor faster than not drinking any
water at all? Fortunately, she was rescued in a few days and lived to find
the answer to her question, and we will answer that question too!
Today we explore the reason that sea water can kill. We'll prepare
solutions of salt water to represent the sea and we'll cut up slices of
potato to represent the sailor. After all, potatoes are made of cells, as are
sailors! We will compare salt water to fresh water so we can see if there is
an effect caused by the salt in the water. The fresh water acts as a
control in this experiment.
Procedure:
1. Cut three equal sized slices of potato to represent the sailor. The best
way to do this is to core 2 long strips of potato 8 centimeters (cm) long.
Cut the potato slice into two equal pieces, 3-4 cm long. The goal is to
end up with three potato slices of exactly the same size. Enter the
length of the potato slices into Table 4.2 so you can compare change in
size of potato slices that are in different solutions.
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Bio 13A Lab Manual
2. Obtain 3 beakers and label and fill them as follows:
#1 50 ml distilled water
#2 50 ml 10% salt solution
#3 50 ml 20% salt solution
3. Place a slice of potato into each of three beakers. Cover the cups with
saran wrap using a rubber band to hold the wrap securely in place.
4. Make predictions about what you think will happen, and write the
predictions in Table 4.1.
Table 4.1 Predictions of Change in Potato Size in Different
Solutions
Grow/Shrink
Tonicity of Solution
Distilled H2O
20% Salt Solution
5. Leave the potatoes in the solution as long as possible during the lab.
Measure the length of the potato slices being careful to make a note of
which slice belongs to which beaker! Enter the change in size in Table
4.2. Use a plus (+) to indicate a slice has grown and a minus (–) to
indicate a slice has shrunk. (Example: +2 mm means grew 2 mm / –3
mm means shrunk 3 mm).
Table 4.2: Potato Data and Analysis
Initial
Final
Length
Length
(mm)
(mm)
Distilled
H2O
Change in
Length
(mm)
Tonicity
5% Salt
Solution
20% Salt
Solution
What happened to the potato slices? Why did it happen?
Lab #4: The Cell: Movement Across the Membrane
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Bio 13A Lab Manual
Explain how you determined the tonicity of the solutions. Was one
solution isotonic to the potato? If so, which one? How do you know it was
isotonic?
Why would a shipwrecked sailor die more quickly drinking salt water
than not drinking at all? What do you think would kill the sailor?
Are there less dramatic situations than shipwreck when the tonicity of
solutions surrounding cells in your body might change? How does your
body compensate to maintain homeostasis?
*Information for this lab was obtained from a website by Kevin C. Hartzog:
www.starsandseas.com/SAS%20Cells/SAS%20cellphysiol/Osmosis.htm
Activity 2: Eggs and Osmosis
In the following experiment, you will be using decalcified eggs as model
cells. The eggs have been treated with vinegar to remove the calcium
from the shell, leaving behind a membrane that is permeable to water
but not to other molecules (solutes).
Materials:
• 4 decalcified eggs
• 4 weigh boats, one for each egg
• Beakers containing solutions A,B, & C
• Beaker containing Solution X
• Paper towel
• Gram scale
* Before you begin the experiment, predict if the egg will get heavier
or lighter if the egg is immersed in a hypertonic solution and
explain why. Write your prediction here.
Lab #4: The Cell: Movement Across the Membrane
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Bio 13A Lab Manual
Activity 2A:
1. Obtain 4 decalcified eggs. Gently dry and weigh each egg before
immersing it in Solution A, B, or C. Dry the egg by gently rolling it
on a paper towel. Do not dry the egg for too long because the
paper towel will begin to draw water from inside the egg and will
change the weight of the egg. Record the weight of the egg.
2. Let the three eggs sock in solutions A,B, and C for 20 minutes. Go
on to Procedure B while you are waiting.
3. After 20 minutes, dry and weigh each egg and record your results.
4. The change in weight reflects the movement of water into or out of
the egg. Based on the movement of water, determine if Solutions
A,B, and C are hypo-, iso-, or hypertonic.
In Table 4.3, enter the presoak weight for eggs A,B, and C then enter
their weights after the 20 minute soak. Compare the presoak and
postsoak weights and determine and record the tonicity of the solutions.
Table 4.3: Determining the Tonicity of Solution A, B, & C
Weight Before
Soaking
Weight After
Soaking
Difference in
Weight
Tonicity of
Solution
(hyper-,hypo,
or iso-)
Egg in Solution A
Egg in Solution B
Egg in Solution C
Answer the following questions.
1. Why does the weight of the eggs change?
2. What causes the water to move in a particular direction, into or out of
the egg?
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Bio 13A Lab Manual
Activity 2B:
1. Gently dry and weigh the egg. Record the results.
2. Immerse the egg in Solution X. Dry and reweigh this egg every two
minutes for about a half hour.
3. Plot your results on a graph to show the rate of water movement
into or out of the egg. Ask the instructor if you are unsure how to
draw your graph.
Table 4.4: Change in Weight of Egg X Over Time
Time
Weight
Change in
Time
(minutes)
(grams)
Weight (gms) (minutes)
0
16
2
18
4
20
6
22
8
24
10
26
12
28
14
30
Weight
(grams)
Change in
Weight (gms)
Figure 4.2: Rate of Movement Into or Out of the Egg (Weight in Grams on Y axis; Time
on X)
Time
1. Is egg X in a hypo-, iso-, or hypertonic solution? Explain your answer.
2. Explain the shape of the graph within the framework of osmosis.
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Bio 13A Lab Manual
Activity 3: Red Blood Cells and Osmosis
You will use microscopes to examine horse blood that is in solutions of
different tonicities. We will focus on the effect of different solutions on
the shape of red blood cells. Cells will swell and eventually burst open, or
lyse, if a large influx of water is allowed to occur. This is called hemolysis
if this happens to RBCs. If water leaves the cell by osmosis, the cell will
shrink and lose its normal shape. This is called crenation.
Procedure:
Note: You will be working with fresh horse blood. We work with
horse blood because it is unlikely that it will have viruses or other
pathogens harmful to humans. It is still important to minimize
contact with the blood. You must wear gloves, and dispose of used
slides in the Sharps containers.
1. Work in pairs. Set up a microscope at your bench. Collect the dropper,
horse blood, and 0.0% (hypotonic), 0.9% (isotonic), and 5%(hypertonic)
saline solutions.
2. Prepare a wet mount of horse blood, placing one drop of the supplied
blood in the middle of the slide. Mix well before taking your sample! The
RBCs have a tendency to sink to the bottom of the tube. Add the
coverslip.
3. Focus the specimen first at low power (40X magnification), then
increase the magnification to 400X. If you have experience using oil
immersion, you may observe the cells under higher magnification.
Reminder: Do not switch back to a lower powered objective lens if you
have placed immersion oil on your cover slip. This will get oil on the other
lenses and ruin their optics.
4. Observe the morphology of the RBCs and draw several cells in your
notes. This specimen is called the control. Why do think this is so?
5. While you are watching the cells, have your lab partner gently place 12 drops of 0.9% saline at the edge of the cover slip. Capillary action will
suck the saline under the cover slip and will initially cause everything to
move very rapidly. When the movement of the cells slows, observe the
blood cells and note any changes in their morphology. The changes that
occur can be rather subtle and require careful observation.
6. Draw your results. Compare your drawing to your control. Keep this
slide for later reference.
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Bio 13A Lab Manual
7. Prepare a new slide with a drop of horse blood. As before, focus on the
cells until you are using high magnification. Gently place 1-2 drops of
0.0% saline (distilled water) at the edge of the cover slip. Quickly observe
the blood cells over a period of a few minutes and note any changes in
their morphology. Draw your results. Keep this slide for reference.
8. Prepare another slide of horse blood. Focus the slide under the
microscope. Gently place 1-2 drops of 5% saline at the edge of the cover
slip. Observe the blood cells over a period of a few minutes and note any
changes in their morphology. Draw your results.
9. Refer to the previous slides if you are unsure how the various
specimens differ. Summarize your results and label your drawings as
"Hemolyzed", "Crenated", or "Unchanged".
10. Dispose of the intact slide preparations in the "Sharps" box. Be sure
to wash your hands.
Questions
Explain why homeostasis of fluid in the body must be maintained.
How might your observations relate to the administration of intravenous
fluids
*The egg and red blood cell exercises are modified from Denise Lim’s Physiology Lab Manual.
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