Using Analogies in Microbiology: The Bacterial Cell as an Entertainment Venue to Illustrate the ATP-binding Cassette (ABC) Transport System

Written by Kristen Z. Swider, Capital Community College

Students in my microbiology class are relatively unfamiliar with the scientific concepts involved in the course and will often attempt to rely on memorization. However, due to the complex nature of the material, it is difficult to access information from the perspective of pure recall. As abstract concepts are discussed throughout a science course, many learners still operating in the concrete stage of development may be lost by a failure to attach understanding to anything of substance. As a result, the concepts are often missed during examinations. There is no requirement that an instructor complicate the approach in order to communicate scientific principles. For these reasons, the use of analogies to illustrate complex processes can enhance a student’s comprehension of the material and make connections that promote lifelong learning. Analogies may be presented to the learner as prepared elements of a lecture or they may be generated by the learners themselves. Self-generated analogies can and do occur spontaneously in discussion. Students are encouraged to develop and present analogies to the class. In either case, the interactive, social process of exploring analogies, whatever their source, contributes to the learning process.

How the ATP-binding Cassette (ABC) Transport System Works

– ATP-binding cassette (ABC) system: This involves substrate-specific binding proteins located in the bacterial periplasm, the gel-like substance between the bacterial cell wall and cytoplasmic membrane.

– The periplasmic-binding protein attaches temporarily to the substance to be transported and carries it to

– Meanwhile, ATP gets broken down into ADP, and phosphate, releasing energy. It is this energy that powers the transport of the substrate, by way of the membrane-binding transporter, across the membrane and into the cytoplasm.

– Examples of active transport by means of ABC systems include the transport of certain sugars and amino acids. There are hundreds of different ABC transport systems in bacteria.


The Bacterial Cell as an Entertainment Venue to Illustrate the ATP-Binding Cassette (ABC) Transport System

The players:

Bacterial cell: Entertainment Venue

Substrate: Patron

Periplasm: Outer arena area

Substrate-specific binding protein: Event ticket

Cytoplasmic membrane: Inner arena barrier with turnstiles

Membrane-spanning transport protein: Turnstile

Cytoplasm: Event location (inner arena)

ATP: energy needed to move the turn-stile and allow entry of the substrate (Patron)

– The bacterial cell is the entertainment venue, with the cell wall being the outer boundary of the arena property. Once the patron reaches the arena, he/she can easily migrate through the cell wall to the inner arena (periplasm) since a “ticket” is not yet needed.

– In order for the patron to gain entry into the main arena area of the venue (cytoplasm), he/she must pick up a ticket at a will call/box office. Here in the periplasm, a patron will pick up a pre-prepared ticket (periplasmic binding protein) just before the entering the event.

– Before entering the main arena area, the patron with the ticket (transportable substance and periplasmic binding protein complex) must enter the arena through the turnstile (membrane-spanning transport protein). A turnstile is a form of gate which allows one person to pass at a time. A turnstile can restrict passage only to patrons who provide a coin or a ticket. It can also be made so as to enforce one-way traffic of people.

– Once at the turnstile, the ticket (periplasmic binding protein) gets left behind, and the transportable substrate (patron) can enter the cell via a turnstile.

– As the substrate (patron) moves through the turnstile, energy is required, and ATP is broken down. The patron (substrate) is now in the arena and can be used by the cell.

Using Waves at the Beach to Describe Concentration Gradients

Written by Jennifer A. Metzler, Ball State University

When discussing passive versus active transport and the difference between an input of cellular energy, I ask students to imagine they are at the beach or at a wave pool. Since passive transport is going down a concentration gradient, I tell them to liken it to having the waves at their back and moving into shore. It is not a problem for them at all and they do not need to expend any energy as they are going with the flow. With active transport going against the concentration gradient, I tell them to imagine turning around and having the waves hit their chest and try to move away from shore. In this case they must expend energy as they are going against the flow of the waves.

Using Food and Drink to Describe Osmosis

Written by Jennifer Wiatrowski, Pasco-Hernando Community College

I use two different analogies to relate osmosis to the real world:

a. I use two large beakers (beakers A and B), a jug of water and lemonade mix or fruit punch mix (these are dry powders). I then tell the class I am going to mix two classes of a refreshing beverage (nice in a hot climate like Florida). I tell them that I will make the first beverage very sweet, and pour in a large amount of lemonade or punch mix. The second beverage will not be nearly as sweet and I pour in a small amount of the powder. I then fill each of the beakers to equal volumes. I then ask the class if the two beakers have equivalent solutions. (They say no!). So, although the two beakers appear to have equal volumes, the amount of water is varies between the beakers. This shows students that the amount of water in a given space is influenced by the amount of solute. I then ask the students to imagine connecting the beakers together with a selectively permeable membrane and then ask them which way the water would flow  (from beaker A to B or B to A?)

b. I then talk about Martha Stewart. Martha always says that you should never toss your salad until your guests arrive or you are ready to serve it (I was recently informed by my students that “toss the salad” has taken on additional meaning……so this gets a good laugh out of the class). So, at the beginning of class, I take a bag of salad and some dressing and mix them. I then put it aside for awhile. In the meantime we talk about hypertonic and hypotonic. I then ask to think about the scenario with the salad and the dressing. They eventually reason that the salad is hypotonic to the dressing (or the dressing is hypertonic to the salad) and the consequence of this is a watery salad as the dressing pulls water out of the greens. This would make Martha HORRIFIED.  To finish, we take look at the salad we mixed earlier.