Diffusion, Osmosis and Cell Membranes
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.
Role of the Cell Membrane
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.
Osmosis in Elodea Cells
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 Elodea cells.
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 not change
Factors Affecting Diffusion
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 warmer water.
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.
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.
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.
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.
The University of Arizona
Department of Biochemistry and Molecular Biophysics
General Biology Program for Secondary Teachers