The Cell Membrane: A Thought Experiment

A number of years ago, I developed a method for teaching students about the basic structure of the cell membrane that has proven to be both enjoyable and memorable, and after receiving positive feedback upon sharing this activity with my friends at the Center for BioMolecular Modeling at the Milwaukee School of Engineering, I figured that it is time that a share it with a larger community of individuals.

Simply, the method is a “thought experiment” in which students graphically hypothesize the arrangement of a collection of molecules unknown to them (phospholipids) in three sequential situations, in order to personally discover the self-assembly of phospholipids in generating a simplified cell membrane.

Once students have discovered this basic image of a cell for themselves it becomes easier for them to supplement it generating a more complex conception of the cell.

In fact, I usually draw an analogy between the cellular container and the test tubes and beakers used in chemistry.  Student readily understand that cells, “as places where chemical reactions occur” require:

• a container (the goal of this series of thought experiments),
• reactants,
• points of entry and exit (the membrane and supplemental proteins),
• a solvent (the water inside this simple cell),
• catalysts as biological enzymes,
• energy,
• and a controlling region or some means of regulation.

A summary of the “thought experiment” follows but for those that want more details, including images, or would rather read offline, I have created a document explaining this Cell Membrane Thought Experiment.

Background

Prior to conducting this thought experiment students have a good understanding of the chemical structure and biological importance of water, and have previously introduced to the importance of carbon and the basic structure of organic molecules.

I begin the series of “thought experiments” by drawing a simple representative phospholipid on the board, consisting of a circular head and two lines or tails extending from head.  I inform the students that this is a large organic molecule.  Large enough that one rarely sees it represented in an chemical diagram.  Furthermore, I label the head and tail of the molecule hydrophilic and hydrophobic, respectively.  We discuss the roots words and then begin…

Experiment 1

• I have the students take out a piece of paper and draw a beaker that is 2/3 full of water.
• Next, I walk around the room and give them a number of imaginary phospholipids.
• Finally, I challenge them to graphically represent how these molecules will arrange themselves if dropped into their beaker.

This thought experiment is rather easy for the majority of students.  I walk around the room checking their progress and have a student share their answer and logic on the board.  At this point all the students feel confident, and I have one share their logic verbally for all to hear.

Experiment 2

• I have the students draw a second beaker filled with water, and I give them more imaginary phospholipids.
• In this situation, I tell the students that they again have to represent the arrangement of the molecules after being dropped into the beaker, but with the added stipulation that the molecules have to be  submerged entirely below the surface of the water and are not allow to touch the sides of the beaker.  I remind them of the properties of the molecules and tell them that if the tails of their molecules are touching water they aren’t happy, and that that such arrangements are incorrect.

In contrast to the previous situation, the majority of students are now dumbfounded.  I let them struggle, giving them plenty of wait time.   A few students generally arrive at an answer though, and when I notice their correct answers I immediately turn their paper over so that others can’t copy.   There are actually two distinct graphic answers for this situation.  Some students draw a sphere of heads while others cut their sphere showing a cross-section of their arrangement showing the tails inside.

So that more students feel the joy of success, I often hint that the molecules must “work together” to solve the problem, and for even better results I may later add that “they protect each other”.  Finally, I elect an excited student to draw and share their logic from the board.

Experiment 3

• I have student draw one final beaker of water, and give them their last pile of phospholipids.
• Like the previous situation,  I tell the students that the arrangement of molecules they draw has to be entirely submerged but with the added stipulation that the structure has to be a container for water as well.

Students easily see that they have to modify the previous structure since the tails don’t like water, but some need time to think about it.  Some will quickly come to an answer through, applying the logic from the previous situation.  For this situation I again walk around the room turning over correct answers.  At some point I will have a different student share their drawing and logic.

Finale

After we finish this series of “thought experiments”, I remind the students that we don’t know identity of the molecules that we have been playing with, and that I never even shared an biological objective for the activity.  So, for closure I tell them that I will give extra credit to the first person who identifies the molecule that we have been representing and/or names the three dimensional structure that we graphically represented in the third experiment.  The students happily compete to discover “phospholipids” and the “cell” or “cell membrane” (sometimes they find the “lipid bilayer” but I usually direct them to find a more common name for the structure).

Extentions

Other cellularly important topics that such a simple model helps to elucidate include:

• Origins of Life – Self-assembly is a concept that is important …  The Discover article The First Cell …
• Cell Size – this model helps to focus students on the relationship between the membrane surface area and the cytoplasmic volume.
• Cellular Evolution – this model helps students to appreciate the hypothesized origin of the eukaryotic nucleus and associated cytomembrane system from infolding of the phospholipid bilayer, eukaryotic cells grow larger in size than prokaryotic cells.
• Protein Folding – the model helps students in their application of the concepts of hydrophilic and hydrophobic to their understanding of the nature of some amino acid sidechains, protein folding, and the structure of globular and membrane proteins that result from these characteristics.

I have also created a Chime tutorial covering this same information.  Chime is a little bit dated, since you will have to acquire and install a plug in as directed on the site (if you can’t anymore let me know), before being able to observe and interact with the imagery on the website.

Let me know if your student enjoy this “thought experiment” as much as mine!