1. Overview

     In this particular interview I sought to explore a fellow student’s conceptions and misconceptions in her understanding of cellular membranes. My specific focus was towards identifying any misconceptions that the respondent would have in regards to a cell’s membrane structure, and how this structure relates to the different types of transport across this membrane.  

     While view of cell membranes is in most simple terms as a barrier, I felt that most people, to include many studying biology, do not have a full understanding of the complexity of the structure of a cell membrane. Further, they have a difficult time in understanding how this structure relates to the types of transport most commonly experienced across this barrier. I formed this belief in my work tutoring Biology students while at the University of Maryland-Baltimore County.  

     Forms of transport across a cellular membrane include Passive Transport (Diffusion, Facilitated Diffusion, and Osmosis) and Active Transport (which includes Carrier Mediated Transport for complex molecules and ions or vesicular transport known either as Exocytosis or Endocytosis). All forms of transport can be explained through the structural components of the cell membrane. My views from the outset at this interview were that the respondent’s answers to my questions would demonstrate a lack of clear understanding of the complexities of the cell membrane structure and how the structure facilitates the transport to occur across this membrane barrier critical to the cell.

2. Methodology

     I organized my questioning according to two levels of demonstrated understanding. One level was where basic subject matter questions were utilized and could be answered by anyone with a basic biology background. The questions would be provided in increasing complexity to see how respondent answered.  A second set of questions was developed to query more advanced concepts that the respondent might claim to have an understanding about. The list of questions was as follows.

Base-level Questions

1.      What defines a cell? What are some of the main parts?

2.      Now that we have discussed the main parts, I have this model here to represent the membrane. It is a very simple model and I have included some props to help you clarify your understanding of the cell and its surround membrane. Sound good? You can use what I have here to freely explain a concept if necessary.

3.      What is wrong with this model?

4.      Why is there a barrier?

5.      My next question is given this barrier—is there an exchange that happens across this membrane?

6.      What do you think can freely pass through this barrier? Just tell me in the most basic terms? How about the size of what can freely pass through? Give me an example.

7.      What can not pass through the barrier freely? Are some of those items critical to the cell’s functioning?

8.      Okay given that the cell needs these items, how might you view these items getting into the cell since they are too large to pass through freely

9.      Why are the exchanges critical to the cell? What function is served in this occurring?

10.  So how do you see these changes occurring—basically the different types of exchanges

11.  Is energy used in each case? When it is not—what is this called? And if energy is used what type of transport is this called.

Advanced-level Questions

12.  Describe the composition of a cell membrane—use model, magnetic tabs and markers as necessary. What are the key elements that compose the cellular membrane?  

13.  Why is a cell membrane important?  

14.  Describe the basic methods of transport across a cell membrane and the mechanisms that allow this to occur.  

a.       What are the methods? ( Passive and Active)

b.      How do they differ? ( ATP use—expense of energy)

c.       How do the methods relate to structure? facilitate bonding for carrier mediated Transport)

15.  Using the model—how would you explain the movement of basic substances such as O₂, CO₂ and water through the model?  

16.  How would you explain the movement of ions like Na⁺, K⁺, Ca⁺², Cl^- across a cell membrane. Explain it using the model. ( Use of protein Channels)  

17.  Explain how specialized proteins are moved into or out of the cell through the cellular membrane—large complex molecules?

Props and materials

I built a 9 inch by 12 inch frame covered with plastic to signify a cellular membrane.

This was provided to the respondent to facilitate discussion and explanations provided to my questions. Also provided were several sets of colored magnets to represent embedded proteins or receptors normally located in or on the membrane. I also provide color markers to allow respondent to draw structures on plastic surface to clarify explanations to my questions. Finally, a pad of paper was also provided if case any drawings were relevant to the discussion.

3. Observations

1. The Respondent demonstrated good knowledge of cellular membrane structure and with prompting answer all of basic questions posed to her.  She was able to demonstrate

1)      Knowledge of cellular membrane structure as a bi-layer with embedded proteins. Her only lacking area was recognizing that some proteins are location the surface of the membrane

2)      She recognized that there are two types of transport, Passive and Active. She also demonstrated good knowledge that Active Transport expends energy which is derived from Adenosine triphosphate (ATP). ATP is produced by the cell. ATP transfers energy from chemical bonds to energy absorbing reactions within the cell, such as would be occurring in active transport

3)      With diffusion, respondent had to be prompted to draw out that basic molecules such as H₂0, CO₂ and  O₂ could freely flow through the cellular membrane

2. The respondent showed some difficulty in concept regarding protein receptors on the surface. Such receptors do not grab complex molecules and pull them in. They provide a structural form that facilitates binding to the complex molecule that it outside the cell. Once this complex molecule is bonded to the receptor, a signal results that causes the cell membrane to deform inward to draw complex molecule into the cell.

3. The respondent completely missed out on discussing Endocytosis and  Exocytosis as  means of active transport

4. Transcript and Analysis

a. Transcription

INTERVIEWER: What I am going to do—is that I have a prop….basically a piece of plastic on a frame—this will represent something. I have some color pens here that will help to facilitate if you want to draw something on this piece of plastic; then I have these little magnets if you want to simulate anything going on this piece of plastic. I am also providing you with a pad if you need to draw anything to clarify your explanations.  

INTERVIEWER: Since you are a Biology major I will ask to you in terms of structure what defines a cell?  

A:  A cell in my own personal view is small..it make up systems, organ systems, tissues…you mean the structure?  

INTERVIEWER: Yes, as you define what are some of the main parts? If you had to give the main parts.  

A: Well a cell has a nucleus. It is like a brain….. the center of command. It also has different organelles …like mitochondria that produces a lot of ATP (the powerhouse). It also has proteins and different things that help it function. It has a membrane or sometimes a wall.  

INTERVIEWER: That is what I have tried to represent with the model. Now that we have discussed, this is a simple model with props to help you explain. Based on what you know about membranes what is inherently wrong with my model?  

A: Well, the membrane is a bi-layer with lipids in there ( points at model). Outside is hydrophilic and inside of bi-layer is hydrophobic. Phospholipids. There can also be proteins that can be through the membrane like this. (attaches magnets to the plastic sheeting). They are randomly dispersed. They can help bring stuff into the cell and bring stuff out of the cell. There are also channels that can ions can go through and come out of.  

INTERVIEWER: I have this model with the proteins going through—why is there a barrier?  

A: It keeps stuff in the cell like the organelles. It keeps ions in the cell. Cells have a certain charge in them and can be stimulated with signals. It keeps all that stuff in. It also keeps stuff out like ions and proteins.  

INTERVIEWER: So it provides some level of organization?  

A: Yes.  

INTERVIEWER: Next question—I have this barrier—can things move across this barrier?  

A: Yes.  

INTERVIEWER: What do you think can freely pass through this barrier?  

A: Water  

INTERVIEWER: Okay water. Can you be a little more specific? Molecules or atoms of some smaller things?  

A: Smaller ones can go through; larger ones need to be passed through  

INTERVIEWER: Anything else?  

A: Smaller Ions  

INTERVIEWER: What type?  

A: Sodium Ions can pass through. They can also go through channels.  

INTERVIEWER: What else? Think for a moment what do you need for a cell to keep cell operating? You talked about that Mitochondria and the ATP. What does it use to make the ATP?  

A: ADP?  

INTERVIEWER: It’s a basic molecule.  

A: Oxygen (pleased with getting this answer)  

INTERVIEWER: There you go—I was just jogging your memory  

INTERVIEWER: What’s produced as a waste?  

A: CO2  

INTERVIEWER:  Can O₂ and CO₂ get through the cell?  

A: Yes  

INTERVIEWER: What can’t pass through the cell membrane?  

A: Bigger molecules like sugars, proteins.  

INTERVIEWER: Okay a cell may need some of those things. How do you think the cell can get those substances into the cell?  

A: Some of them can be passed through using energy…using the receptors. These tell what’s out there.  

INTERVIEWER: Okay if I am a sugar molecule of carbohydrate floating on the outside of the cell what happens?  

A: This receptor could sense it (points to receptors designated by magnets) ands sort I guess grab it I guess and pull it in.  

INTERVIEWER: Okay. Okay why is the exchange critical on these molecules?  

A: The cell could break it down and make energy.  

INTERVIEWER:  Okay from you Biology background what are the types of exchanges that occur on a cell membrane?  

A: There’s active transport that requires energy. There is passive transport? There is facilitated diffusion (doesn’t require energy). And there’s osmosis  

INTERVIEWER: So these would be distinct categories?  

A: Yes  

INTERVIEWER: Is energy used in all cases?  

A: No just in active transport.  

INTERVIEWER: Okay active transport—please walk me through the process?  

A: You would need one of these proteins that is stuck in the membrane. They would bring in one of the larger molecules that couldn’t freely flow into the cell…or they could bring stuff out of the cell. They use ATP and break it down into ADP.  

INTERVIEWER; So is it only complex molecules that can be active transported? (trying to develop discussion about  Na⁺, K⁺, Cl^-)  

A: Ummmm  

INTERVIEWER; How about something like a nerve cell ( perfect example)  

A: Take in nerve transmitters from other cells and ions that cause hyper-polarization.  

b.      Analysis  

     In conducting this interview my claim that most people, to include many studying biology, do not have a full understanding of the complexity of the structure of a cell membrane seemed disproved when utilizing the basic level questions.  The respondent, a Biology Major, showed good knowledge on cellular membranes at a basic level. Her demonstrated knowledge was good and she displayed good foundational knowledge on concepts related to the cell membrane and its structure.  

     However, as we progressed into more advanced questioning, there was some confusion on the respondent’s part to exactly how the cell membrane structure and associated functions to move both complex molecules and ions by active transport across the membrane. With her explanation, complex molecules involved in active transport seemed more plucked out of the environment, rather that the physical process that occurs in the receptor proteins deforming, using energy in this process and moving the complex molecules across the membrane.  

INTERVIEWER: Okay if I am a sugar molecule of carbohydrate floating on the outside of the cell what happens?

A: This receptor could sense it (points to receptors designated by magnets) ands sort I guess grab it I guess and pull it in.

     Additionally there seemed to be confusion on her part as to how ions such as Na+ get across the membrane. They do not move freely across the membrane as with such molecules as H₂O, O₂ and CO₂. 

INTERVIEWER: what do you think can freely pass through this barrier?

A: Water

INTERVIEWER: Okay water. Can you be a little more specific? Molecules or atoms of some smaller things?

A: Smaller ones can go through; larger ones need to be passed through

INTERVIEWER: Anything else?

A: Smaller Ions

     Typically ions are moved across either through ion channels (the embedded proteins) which facilitate diffusion or by means of active transport ion pumps.¹ Such channels are gated and allow opening when various stimuli such as  voltage, mechanical stress or ligand  binding allow the ions through the membrane.²  Active Transport  Ion Pumps ( also embedded proteins) are typically founded in critical ion transport, such as the Na⁺/K⁺ transport in nerve cells critical to the nervous system or Ca²+, an ion require in many intracellular processes.³

     Clearly the complexity of these functions can only be understood with a firm grasp of the structure of the membrane and how that structure relates to function. I felt my claim was more than adequately validated when we went into the more advanced questioning.

5. Recommended Changes

After going through this process I would recommend the following changes:  

1. I would provide more variations on the models I presented.  Historically there were several versions of the model that addressed the bi-layer form of the cellular membrane. I think these could have been used to validate the accuracy of the student’s knowledge.  

2. I would present the student with a sample of various substances, molecules, ions, and atoms. The student would have to clearly identify how each of these items moves across the membrane, either by passive or active transport. I would have the respondent detail the structure that accomplishes each type of transport.  

3. In conducting the interview, it is expected to assume that the respondent may have no basic knowledge of the topic area when questioning. I think a better method would to be to provide a chart where I would gradate subject matter areas. The respondent could possibly use this as a kick-off in the questioning, so that not so much time would be wasted in areas he or she felt very comfortable about. Clearly, in this interview the respondent was very comfortable in answering the basic questions. Our time would have been better served in going into more advanced topic areas with my questioning  

d.       Finally, I felt that the sample size of  the interview made it very hard to really validate my claim. With more samples taken with common students or Biology students early in their studies, my claim could have been better validated.

Footnotes

¹ Affinity Bioreagents, Inc., Spotlight on Ion Transport , p. 1

² Ibid, p. 2

³ Ibid, p. 3