Richard M. Cavagnol and Thomas ~ a r n e t t Natural Sciences Division Johnson County Community College Overland Park, Kansas 66210
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Simple Models for Tough Concepts
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One -~~ of the more challeneine - -asvects . of instructional interaction is the presentation of dynamic chemical concepts in a wav that is both believable and understandable. Phvsical models assist in the presentation and assimilation of these concevts. A series of models. devictinn such phenomena as the hehawor of charged pnrticlrs in an ionir liquid demonstrated with floarinr nlacnets 11 I. atomic and molecular trmding with magnets an> washers (2), atomic orbitals with cardboard models (3), organic structure using colored Styrofoam or plaster space-filling models (4, 5, 7), and the structure of biopolymers with pipecleaners (6),have been presented in previous editions of this Journal. Using simplicity as our theme, we have devised a series of models that are simple, inexpensive, and require very little time or skill to construct. They allow the student to visualize a whole spectrum of phenoiena from atomic structure to enzyme-substrate interactions. ~
Materials and Construction Our models are constructed from a variety of materials, from poker chips to cardboard. Glue-hacked magnetic tape is attached to the back of the particular model and the model is displayed on a metal blackboard ("greenhoards" or "whiteboards" are satisfactory). If metal lecture hoards are not available, metal-covered asbestos hot pads work well, with the additional advantage of being portable. The magnetic tape (Margie Magnetic Flexible Strip, Paterson, New Jersey, 075i2j and the other model components may be purchased in most variety stores (TG&Y, etc.)
Subatomic Particles The number and arrangement of protons, neutrons, and electrons that comprise a particular atom are represented in our system with colored poker chips (red, white, and blue) with a piece of magnetic tape secured to one side. To aid in identification of the subatomic particles, the letters "p", "n", and "en are applied to the appropriate colored chip. Valence elrctrms i i e pointed out and the honding phenomenademonstrated hy moving the mwlved elecrrmi. This presents a dynamic p i r n ~ r rof atornir strucrure and interarrion Wig. II.
Molecules The poker chips or any other pieces of colored plastic that have been "magnetized" with the tape are used later in the
Figure 1 . Poker chip model. Front side (upper row) blank for anachment of letters. Back side (lower raw) shows attachment of magnetic tape.
course to represent the elemental components of molecules. Beginning chemistry students often fmd it d i f f i d t to visualize the products of a reaction as the reactants that have been chemically rearranged. For example, the reaction between propane and oxygen to form carbon dioxide and water (C3H8 502 3C02 4H20) has been depicted as shown in Figure 2. The student is now a witness to the dynamic aspect of the chemical reaction by observing the reactants disappear and the products being formed.
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Solvation, Dissoclatlon, and Osmolarity In our discussions centering around the behavior of ionic solids in water, the magnetic poker chips are used to represent the two ion species. Water molecules are constructed from colored paper in the approximate steric configuration, strengthened with cardboard and magnetic tape affixed to the back (Fig. 3). Using a combination of cardboard cutouts to represent large organic acids and the poker chips to represent the hydrogen ion, the concept of dissociation in solution may be visualized. The concept of osmolarity (molarity X number of particles formed from the solvation of an ionic solid) is clarified to the student by showing one molecule composed of two atoms (poker chips) dissociating into two particles in solution. Bioorganic Molecules The selective pasingr of materials through cell membranes is one of the fundamental concepts of physiology. To illustrate the prinuple of ostno+ molerules of glucosp, sodium chloride, and water were constructed from colored paper and magnetized with the magnetic strips. A differentially permeable membrane was similarly constructed from colored paper and a maenet affixed. Pores in the membrane are laree enoueh " to allow the passage of the water molecules and the dissociated ions of sodium and chloride. but not laree enoueh to allow the passage of the glucose molecules. This model of a dynamic system allows the student to participate in the activity by moving the models and investigating their characteristics.
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Enzyme-Substrate Interactions The more advanced concepts of enzyme-substrate interactions have "come alive" for our students with the cardhonr&mngnct model system. The key to understanding the interaction hetween an enqme and i h substrate is the concept
Figure 2a. Propane (lelt)and five molecules of oxygen (right) represented by poker chip models. Carbons are blue. hydrogens are white, and oxygens are red. Figure 2b. Interaction of reactants resuits in breaking of bmds and "disappearance" of reactant components. Figure 2c. Reactant components chemically combine to form products. Shown are two molecules of water (right) and one molecule of carbon dioxide (lefl).
Volume 53, Number 10, October 1976 / 643
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Figure 3. Posterboard cut-aut rodel. Cardboard is glued to back of posterboard to reinforce the model and magnetic tape is attached to the cardboard.
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of specificity. To illustrate this, we took a piece of white pmter paper and penciled an irregular line across the face of the paper. The paper was then cut along this line and the two halves separated. The lower portion represented the enzyme, the upper portion the suhstrate. Functional groups that might represent binding sites and active sites were drawn in to sueeest how the enzyme and suhstrate might mutually interU& (Fig. 4). The suhstrate molecule was made more realistic by cutting it at the point of hydrolysis. The suhstrate molecule is deoicted toeether until it interacts with the enzyme. The enzyme-substrate interaction results in the formation of two oroducts. Molecules of different shapes were cut out and used kith the enzyme model to further reinforce the principle of specificity. Other ADDlications ..
The simple mpdels we initially developed for our chemistry classes have found application in other courses. In physiology and pharmacology, we have used the cardboard cut-outs to describe the drug-receptor and neurotransmitter-receptor interactions. A similar set of complementary models is currently being used to pictorially present the series of events in synthesis and the movement of chromosomes during mitosis. In microbiology, the antigen-antibody interaction is
644 / Journal of Chemical Education
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Figure 4a. Posterboard madel of enzymMubstrate interanion. prior to reaction. apoenzyme (protein portion), metai activator, and substrate move into proper orientation. Figure 4b. Activation of the enzyme by the metai activator exposes the active site, which splits the substrate into two products.
depicted with complementary cardboard models with the magnetic hacking. The model system is dynamic by nature and its application to different situations is limited only by the imagination of the user. We are currently examining other materials for construction of the models and other applications of the models in and out of the classroom. Literature Cited Angell, C. A . and Gruon, D. M., J. CHEM. EDUC., 41,194 (19661. Baker. W.L., J. CHEM. EOUC.,37,131 (1362). Kapsu8n.A. F.,J. CHEM. EDUC., 43,412 119661. Kellett,J.C.,andMsrtin,A. N.. J. CHEM. EDUC., 43,374 (1966) ( 5 ) Kenney,M. E.. J. CHEM. EDUC,39.13011962I. (6) Poi1ard.H. B.. J. CHEM EDUC.,43,327 (19661. (7) Strong, C. L.. Seieniiiic Arne,,, 110. (Feb 19731.
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