Role-Playing Activity to Demonstrate Diffusion Across a Cell Membrane.

INTRODUCTION

The diffusion of molecules across the cell membrane can be difficult for students to differentiate and remember, particularly for non-STEM majors and/or are students in introductory biology classes (1). Role-playing is a simple, inexpensive method for students to actively learn complex concepts while also developing cooperative social skills (2), and it has been used to teach various biological topics (3–5). This role-playing activity was designed for an introductory biology course for non-STEM majors to achieve the following learning outcomes: 1) To explain how and in which direction small, nonpolar molecules—like gases—move across the cell membrane; 2) To explain how and in which direction small, polar molecules—like water—move across the cell membrane; and 3) To correctly apply the terms: “hypertonic,” “hypotonic,” and “isotonic.” Students were taught about diffusion, osmosis, and active transport before doing this activity.

PROCEDURE

Preparation

Thirty index cards are each labeled with the name and chemical formulae of the following molecules (10 cards for each molecule): oxygen, carbon dioxide, and water. Two volunteers are selected from the class to act as channel proteins and are asked to research which of the distributed molecules would require channel proteins. They are given eight minutes to conduct their research before the activity begins. They must either say “stop” or use their arms to prevent the students from passing through their proteins if the student carries the incorrect molecule or if the molecules are moving in the wrong direction. The index cards are distributed to the remaining students; each student should receive at least one index card. All of the students with index cards are given eight minutes to research how their molecule(s) cross the cell membrane; students form groups with other similar “molecules” and discuss their findings with each other. To guide the students in their search, they are given the following questions:

How does your molecule move across the cell membrane?

Is a protein involved? If so, what kind of protein?

Is ATP required? Why or why not?

Do the terms “hypotonic,” “hypertonic,” or “isotonic” apply to your molecule? If so, how?

Movement of small, nonpolar molecules (Part 1)

After about eight minutes, the two “channel proteins” stand in the middle of the room facing each other with their hands touching each other’s above their heads (forming an arch), representing the pore within the protein that molecules can move through. The part of the classroom to the right of the students is designated as “outside of the cell” while the part of the classroom on the left is “inside of the cell” (Fig. 1). The students carrying the cards labeled with “oxygen” are asked to stand “outside of the cell” and are then asked how they would move into the cell and instructed to act accordingly. The students should realize that oxygen does not require a protein to cross the cell membrane and that oxygen molecules can pass around the channel proteins, along their concentration gradient, until the oxygen concentration is equal on both sides of the cell membrane. These students are asked to sit and then the carbon dioxide students are called up and told to stand “inside the cell”; the students are then asked to act out the diffusion of carbon dioxide across the cell membrane. The students move around the channel proteins until the number of carbon dioxide “molecules” is equal on both sides of the cell membrane. Those students are then asked to sit.

FIGURE 1

Diagram showing how the students are arranged in the classroom during Part 2 of the activity. The grey circles represent two students acting as channel proteins, the white circles represent students acting as water molecules. In this scenario, there are more water molecules outside the cell than inside the cell (the cell is in a hypotonic solution) so water molecules will move into the cell though the channel proteins until the number of water molecules is equal on both sides of the cell membrane (represented by the dotted line).

Movement of small, polar molecules (Part 2)

Next, all of the students holding water cards are asked to stand outside of the cell. Students are told that solutes are normally present on both sides of the cell membrane and both the amounts of water and solute molecules affect concentrations and tonicity; since osmosis is the diffusion of water molecules across the cell membrane, the difference in amounts of water molecules inside and outside the cell dictates the direction of diffusion. Students are asked whether the solution outside the cell is hypotonic, hypertonic, or isotonic compared with inside the cell and why. Students then pass between the two students acting as channel proteins until the number of students is equal on both sides of the cell membrane, thus illustrating isotonicity. Then, all the students are told to stand inside the cell and are asked whether the solution outside the cell is now hypertonic, hypotonic, or isotonic compared with inside the cell. Students are then again asked to move through the “channel proteins” to demonstrate osmosis.

DISCUSSION

After the activity is completed, the students are each asked to write down the answers to the four questions they were given at the beginning of the activity (shown above) for each of the molecules involved in the activity: oxygen, carbon dioxide, and water. The students are quizzed about the movement of those three molecules during the next class session. Anecdotally, the students were able to correctly answer questions about diffusion and osmosis after completing the activity.

CONCLUSION

This role-playing activity is a simple, cost-effective, fun way to teach the basic principles of cell membrane transport for small non-polar and polar molecules. While rigorous assessment of this activity still needs to be undertaken, students seem to enjoy the activity and are able to adequately answer short-response questions about the diffusion of these molecules during the following class period. This activity was designed for a class of 24 students but can be adjusted for larger classes by dividing the class into smaller groups and having the groups do the activity simultaneously. When we do this in my classroom, each step of the activity is enhanced for the students with an explanation of how the scenario applies to our own cells and how that affects movement of the molecules. For example, students are told that carbon dioxide is continuously produced by mitochondria through cellular respiration, so its concentration within the cell is always higher than outside the cell and carbon dioxide is therefore always diffusing out of our cells into our bloodstream. This activity can also be adjusted to illustrate other forms of cellular transport including cotransport (e.g., sodium-potassium pump) and bulk transport (e.g., phagocytosis).