LOOK OUT FOR THAT PSEUDOPOD!
By Michael J.V. Lazaroff
This is an entirely original activity.
TYPE OF ENTRY
TYPE OF ACTIVITY
- hands-on activity
- group / cooperative learning
- review / reinforcement
- Life Science
- Advanced / A.P. Biology
- Anatomy and Physiology
- Special Needs - ESL, LEP, NEP, etc.
- Other Special Needs - Visually Impaired, Auditory Impaired
What question does this activity help students to answer?
Note to teachers:
All the teacher needs to know is the basid idea of phagocytosis, the formation of a food vacuole, the fusion of a lysosome with the food vacuole, and the process of exocytosis of the wastes.
- How does a cell take in large food particles?
- How does a lysosome work?
Required of students:
Cooperation, especially in terms of group problem solving, as well a drawing are all the students need to demonstrate to me.
Preparation time needed:
15 minutes tops! This consists mainly of blocking the activity out in your mind, and perhaps setting up some drawing materials.
Class time needed:
15 minutes tops. This can be extended if you want the students to do their drawings in in class. I tend to have them do the drawings as homework.
ABSTRACT OF ACTIVITY
The concept of phagocytosis is an interesting one to students, yet a hard one to visualize. A good microscope and live amoeba can, of course, show students the real thing, but specimens can be hard to find under the microscope, and even if found they are rarely found in the act of phagocytosis (though viewing cytoplasmic streaming is always a good idea, and not to be dismissed). If, however, a good microscope and a live protozoan culture are not available, one can demonstrate it beautifully if one merely acts it out as a class.
The activity involves the teacher leading the class as a whole through a walk through of the process, then providing them with two tasks to act out silently (which increases the need for cooperation). One of these is merely a repetition of what I led them through, the other is the process of exocytosis. It is important that the teacher give students an opportunity to apply the information to a similar situation. This activity tends to work best if it is done before using the microscope. If possible, try not to forgo the microscope as this activity does not easily demonstrate cytoplasmic streaming. Whether you use this in lieu of, or in addition to, microscope work, it is a powerful visualization tool.
Your students themselves are the only materials needed!
This activity was designed to allow students to act out a microscopic event in a compressed time frame; this is especially valuable given their handicap of living in a macroscopic world in which organisms - to their own eyes at least - capture their food with lightning speed. Once the students have been filled in on the basics of cell membrane structure, diffusion, and osmosis, the idea of how to deal with BIG food particles is introduced. I find that a colander is a good model for relative particle size - water and salt pass through the holes easily, but a TENNIS BALL is another thing altogether.
Half way through the period I get the students out of their seats and ask them to hold hands in a big circle with everyone facing the center. I tell them that they are playing the role of a cell membrane. Whenever students fail to hold hands I usually react with mock horror, "Look Out! You're leaking cytoplasm!" If this fails to get the desired response, I suggest gently that the student either link index fingers, or hold on to one another's shirt sleeves.
I usually take this opportunity to ask the students, "What's wrong with this picture?" (the lack of a bilayer) I usually ask the students at this point which side of the phospholipid is toward the outside (the hydrophilic side). Three students are then chosen to form a smaller circle (in the same fashion) inside the larger circle. They become the lysosome, which is basically a sack of digestive enzymes.
At this point I usually assign to myself the role of a bacterium - one could call oneself a piece of food, but students love comparing their teacher to a pathogen! (It seems so much more fitting!) Given the size of the particle (me!), it is impossible for me to break through the line formed by their joined hands. I then slowly direct the students to try and surround me WITHOUT letting go of their hands. After a moment's hesitation they start to form what I like to call pseudo-pseudopods. Once they have me surrounded I instruct those surrounding me to QUICKLY change hands to form a small circle around me, while the others QUICKLY change hands to maintain a large circular cell membrane.
At this point they have formed a food vacuole. This method illustrates a couple of interesting points: the food vacuole is surrounded by the same material as the cell membrane, and the portion that was once the outside of the cell membrane is now the inside of the vacuole membrane. This helps to illustrate the value of the lipid bilayer structure.
Simply mentioning the contents of the lysosome often leads students to ask what would happen if the lysosome dumped its contents into the cytoplasm, but they soon realize that would mean the death of the cell. I usually ask at this point how those enzymes then get into the food vacuole, but rather than ask for a verbal response, I ask the 3 students performing the role of the vacuole to act it out, preferably with as few words as possible. Their reward for fusing with the food vacuole and dumping their digestive enzymes inside, remember of course that the teacher is the bacterium inside the vacuole, is the blood curdling scream that results from being digested alive.
I usually then limp my way out of the circle and arrange the students in another big circle, with three different students as the lysosome, and a student as the bacterium. Their task is to SILENTLY (thus not only increasing the level of cooperation, but allowing me to assess better how well they learned the lesson), but using a different area of the cell membrane as the pseudopods, perform the same task as was just performed by the class.
Having accomplished this task they are asked to perform, once again SILENTLY, the act of exocytosis of the wastes that are left over from the intracellular digestion (this is often a good moment to ask them what kind of digestion it is - intracellular or extracellular).. Once again I ask them to use a portion of the cell membrane that
has not yet acted as a pseudopod. Their final task is to individually do a series of
drawings of phagocytosis, which I usually have them do in color, as homework.
Because of the physical nature of the activity, it works quite well with students for whom English is not their first language. It works well with the deaf, as long as one realizes that all students may not be facing the same direction, necessitating a repetition of the instructions.
The activity may also be used with the visually impaired, especially because of its tactile nature. With the visually impaired it may be helpful to use a large beach ball as the bacterium, as it is easier for the students to form a vacuole around something they can touch without embarrassment. If this is your choice, you might want to give them the task of forming the vacuole and then holding the beach ball up with their bodies (remember they are holding hands), taking care not to drop it, as they move to the lysosome.
I have had students come back years later to tell me that they remembered phagocytosis because of acting it out in my classes. It is a winner, it takes little or no preparation, and it is quite easy to do! Have fun with it!
METHOD OF EVALUATION / ASSESSMENT
I provide two grades. The first is a group grade for the whole class based on the quality of their group performance, and the level of their cooperation. I have, on occasion, given individual students higher or lower grades based on their individual level of cooperation within the group, but for the most part they all tend to work quite well together. The second grade is an individual one based on their drawings of phagocytosis down after the activity.
EXTENSION / REINFORCEMENT / ADDITIONAL IDEAS
The most obvious extension is the observation of the amoeba via the microscope. There are also some good laser disks that illustrate phagocytosis.
Students often feel that this is of limited usefulness in that they feel that this process is limited to the amoeba. This is usually a good opportunity to mention:
the work of macrophages in our bodies
the forming of food vacuoles at the end of a paramecium's gullet
as well as related processes:
acetylcholine into the synaptic cleft
mast cells and histamine
Given the high cellular energy cost of phagocytosis, it is a good idea to initiate a discussion of the value of extracellular digestion over intracellular digestion (the saving of energy). Often the best way of doing this is to first review the role of the lysosome in the process, and why the digestion does not take place in the cytoplasm. I usually ask the students, before introducing the idea of extracellular digestion, what other way an organism could digest its food, as well as why it needs to digest its food.
It is a good idea to point out to the students that we absorb our food the same way an amoeba does - through the phospholipid bilayer. Given this, and such similarities as gas exchange across a moist cell membrane, I often use this as an introduction to an underlying theme that I refer to throughout my discussion of body systems, indeed the whole class (and some the students seem to get a kick out of): "We are all glorified amoeba!"