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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.

Dynamic Equilibrium

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.

Osmosis
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.

Osmosis and Blood Cells
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.

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