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In this unit of instruction, the students
will explore some of the properties and processes of the cell
membrane, and in the process acquire an understanding of some
basic concepts of physical science. Upon completion of this unit,
each student should be able to:
Teaching this Unit
To accomplish the objectives, the students
will perform a number of laboratory exercises intended to lead
them, through exploration and analysis, to a thorough understanding
of the importance of the cell membrane and the physical processes
of diffusion and osmosis.
Begin the first lesson with a discussion
of the necessity for the cell membrane as a physical barrier demarcating
the boundary of the cytoplasm and protecting the contents of the
cell from the less-organized surroundings. In this discussion,
lead the students to an exploration of the nature of the interaction
of the cell with its surroundings. The most obvious of these interactions
are the acquisition of nutrients, gases, and water, and the elimination
of wastes. Maintaining the proper balance among these components
is an essential role of the cell membrane. The selective permeability
of the membrane should be stressed, but a discussion of the biochemical
structure of the membrane is not warranted except for very advanced
students. The students should also
be aware that the cell membrane is an essential component of the
cell's response to environmental stimuli.
Having completed this discussion, the
students should then move to the first activity (Osmosis in Elodea Cells),
an exploration of the effects of solutions of various concentrations
on the leaves of the common aquarium plant, Elodea. The
pre-laboratory discussion should focus on the techniques
of slide preparation in order to allow the students to discover
for themselves the effects of the various conditions. Mention
should be made of the role of the experimental control (the tap
water) in the experiment. Encourage the students to make careful
sketches of their observations and to attempt to label as many
structures as they can identify. The students will likely need
help in identifying suitable regions of the Elodea to observe,
which may be accomplished by projecting a slide of a typical view
of the leaf. The post-laboratory discussion should center on the
students' explanations of the changes that they observed in the
From this exercise, the students move
to an exploration of the concept of dynamic equilibrium, a concept
essential to a proper understanding of diffusion and osmosis,
as well as many other processes that the students will encounter
later in their science education. The exercise (Dynamic Equilibrium)
is a simple one, but it requires that the students simultaneously
transfer water from one container to another. The pre-laboratory
discussion should focus on the procedure and the data collection
only, again to allow the students to discover the concept on their
own. The post-laboratory discussion should emphasize the dynamic
nature of the equilibrium that is achieved and the idea that while
the position of the equilibrium (the final water levels) may not
be the same, once equilibrium has been achieved for a closed system
(no gain or loss of water), the position of the equilibrium will
In the third activity (Factors Affecting Diffusion),
the students will observe the effects of two factors that affect
the rate at which substances diffuse in a given medium. Diffusion
involves the movement of molecules from a region of high concentration
to a region of lower concentration. That the regions are distinctly
different implies that the system is highly ordered. Entropy is
the tendency for a system to reach a state of maximum disorder
or to change from a highly-ordered state to a disorganized state.
Thus diffusion is driven by entropy.
In the first part, the effect of temperature
on the rate of diffusion is determined by varying the temperature
of a sample of water in to which a crystal of potassium permanganate
has been placed. The concept associated with this phenomenon is
based on the kinetic theory of matter, in which idealized molecules
move in continuous, random motion at a rate that is directly proportional
to the average kinetic energy of the system. Thus, the higher
the temperature of the system (a measure of the average kinetic
energy) the greater the velocity of the molecules, according to
the formula KE = mv2/2, where KE is the kinetic energy,
M is the mass of the molecule, and v is the velocity of the molecule.
As an opening activity, uncap a bottle
of perfume or other aromatic substance near the center of the
room. Ask the students to raise their hands when they smell the
substance. It should be apparent that those students that are
nearest the substance will smell it first. Discuss the molecular
nature of matter and introduce the basic concepts of the kinetic
theory as it relates to the movement of the perfume molecules
through the air. Ask the students what will happen to the distribution
of the perfume molecules in a closed room. (A state of dynamic
equilibrium will be achieved when the molecules are evenly distributed
throughout the room.) Discuss the procedures for the exercise,
and note the hazardous nature of the potassium permanganate. Instruct
the students to use the radii of the circles on the polar graph
paper to measure the distance that the potassium permanganate
molecules migrate. After the students have completed this part
of the exercise, discuss why the molecules moved faster in the
The second part of this exercise demonstrates
the variation in the rate of diffusion of different molecules.
For two gases at the same temperature, the average kinetic energy
of the molecules is the same. Thus, from the formula given above,
if the molecules of the two gases have different masses, they
should be moving at different velocities, with the more massive
molecules moving more slowly. In this exercise, the students should
be able to determine that one of the gases (the ammonia) moves
farther in the same amount of time, and hence should have molecules
with less mass than the molecules of hydrochloric acid. Be sure
to warn the students of the dangers of the ammonium hydroxide
and hydrochloric acid and to drop the solutions on to the cotton
balls simultaneously, and explain that it will take several minutes
(depending on the temperature) for the precipitate (ammonium chloride)
to form on the inside of the tube. If more than one trial is necessary,
the tubes must be thoroughly cleaned and dried between trials.
In the fourth activity (Osmosis),
the students will investigate osmosis. Osmosis occurs when a membrane
separates two solutions of different concentrations. The membrane
allows the solvent to pass through, but not the solute. On the
side of the membrane with a lower concentration of solute, the
water molecules strike the membrane and pass through it more frequently
than on the side with the higher concentration of solute molecules,
probably since the solute particles attract the solvent molecules
and interfere with their direction of travel and because the solute
molecules physically block the solvent molecules from striking
and passing through the membrane. This difference in the rate
of diffusion through the membrane causes the solvent to accumulate
on the side of the membrane with the higher initial concentration
until the solution concentrations on both sides of the membrane
are equal and an equilibrium is established. Equilibrium can also
occur when the pressure due to the added column of liquid in the
high-concentration side is great enough to increase the flow of
the solvent to the other side. This pressure is called the osmotic
pressure, and it is capable of rupturing cells placed in solutions
with low concentrations of solutes.
This is an important mechanism by which the cell membrane regulates the flow of ions and large molecules into and out of the cell, and is due to the biochemical structure of the cell membrane. The basic components of the membrane are the phospholipid molecules, which have a hydrophilic and a hydrophobic end. The hydrophobic ends of the molecules group together, forming a bilayer of the molecules, with the hydrophilic ends in contact with the cytoplasm and the extracellular environment. Protein and glycoprotein molecules are embedded in this bilayer, and perform various roles in regulating the movement of materials into and out of the cell, and in cell response and communication.
Small, nonpolar molecules like N2,
O2 , and CO2 are able to pass readily through
the membrane, so their concentrations inside the cell depend on
their diffusion into and out of the cell. Polar molecules and
ions have a more difficult time passing through the hydrophobic
region in the center of the bilayer, and so do not pass through
the membrane as easily, although water is able to diffuse into
and out of the cell with little difficulty. Large molecules are
unable to pass through the phospholipid bilayer, and so must rely
on other, energy-consuming mechanisms to cross the membrane.
In this exercise the students will
use a semipermeable membrane to test the osmotic potential (ability
to generate osmotic pressure) of starch solutions of various concentrations.
The apparatus is simple to construct, and relies on a commercially-available
dialysis membrane to separate the solutions. An iodine solution
is added to the low-concentration side to show that the smaller
molecules can pass readily through the membrane, but that the
larger starch molecules cannot.
Begin the exercise by reminding the
students of the experiment with the Elodea cells. Discuss
the semipermeable nature of the cell membrane, and introduce the
concept of solution concentration. Warn the students of the toxic
nature of the iodine solution. After the exercise, review the
group data and discuss the trends.
As a final exercise (Osmosis and Blood Cells),
the students design and test their own hypotheses regarding the
effects of various solution concentrations on red blood cells
. In preparation for this, discuss the elements of good experimental
design, including the control. Review the elements of the laboratory
report and , if necessary, review the results of the Elodea
exercise. The students should be able to construct an experiment
very similar to the one performed on the Elodea . As a
post-laboratory exercise, discuss mechanisms by which cells counter
the effects of unfavorable osmotic environments, and the use of
salt and sugar curing in the preservation of foods.
It is recommended that the students
work in groups of two or three when performing the activities
and designing the experiment, in order to improve interaction
and the discussion of observations and results.
This unit should take ten days to complete, depending on the level of ability of the students. The following is a recommended schedule:
The following is a list of instructions
for obtaining and preparing the materials needed for the activities
in this unit. The list of materials can be found in the student
directions. Most of the materials can be obtained from scientific
or biological supply companies. Exceptions and alternatives for
some materials are noted below.
Osmosis in Elodea Cells
can be obtained from a local pet store or aquarium supply store.
For the 5% salt solution, dissolve 10 grams of sodium chloride
(table salt) in 190 mL of distilled water. Adjust the proportions
for the 10% solution. Dispense these solutions in clearly labeled
25-mL plastic dropping bottles.
Small plastic aquaria or terraria (approximately
25cmx40cmx25cm) or large plastic food containers (approximately
4 L capacity) can be used. The changes in the water level are
most obvious if clear containers are used. Alternatively, 1- or
2-liter beakers can be used. Provide paper cups that are sized
so that they may be dipped easily into the larger containers.
Again, small beakers may be used instead.
Factors Affecting Diffusion
Polar-coordinate graph paper may be
obtained from a bookstore or office supply company. Cool several
liters of water in a refrigerator overnight, and provide several
liters of warm water by heating it on a hot plate to approximately
Prepare 250 mL of 1M HCl by dissolving
22 mL of concentrated hydrochloric acid into 228 mL of distilled
water. Dispense into clearly labeled 25 mL dropping bottles for
student use. Prepare 250 mL of NH4OH by dissolving
67 mL of concentrated ammonium hydroxide into 183 mL of distilled
water. Again, dispense into clearly labeled small plastic dropping
bottles for ease of use. Make sure that the students wear aprons
and goggles when handling these materials, and instruct them in
proper safety considerations.
The tubing for the osmosis apparatus will have to be prepared. Cut two pieces of 1/2 inch tygon tubing (available at aquarium supply stores). Fire polish the ends of a 10 cm piece of 1 cm glass tubing. The students should then be able to assemble the apparatus on their own. Remind them to wet the glass tubing before inserting it into the tygon tubing.
Prepare 1 inch dialysis tubing by cutting it into 3 cm pieces, then slitting them along both sides to make rectangular pieces. The should soak in water for 15-30 minutes before the lab begins.
The students should prepare the starch solutions. Assign concentrations of 0, 2, 5, and 10% to the different groups. Advise the students to mix the starch with a very small amount of water to form a paste, then add the balance of the water to dilute. Alternatively, prepare a concentrated starch solution and have the students dilute it to the proper concentration. This will require some explanation, and may be more difficult for the students to understand. Be sure to use a soluble starch such as potato or corn starch.
Lugol's iodine is available from the supply companies, and works well as an indicator of starch. It is toxic, however, and the students must be careful when using it.
Advise the students to pour the solutions
into the tubes very carefully, so as not to trap air bubbles,
and to mark the initial liquid levels and the time.
Osmosis and Blood Cells
Whole blood may be obtained from a
slaughterhouse or veterinarian. Physiological saline may be purchased
or prepared using recipes from The Sourcebook for the Biological
For the structure of the cell membrane and a discussion of osmosis:
For a discussion of equilibrium, diffusion, and osmosis:
For the preparation of solutions and alternative techniques:
Science Education Connection|
Department of Biochemistry
The University of Arizona
Wednesday, February 12, 1997