Red onions, Elodea, or decalcified chicken eggs? Selecting & sequencing representations for teaching diffusion & osmosis.
Diffusion and osmosis are important biological concepts that
students often struggle to understand. These are important concepts
because they are the basis for many complex biological processes, such
as photosynthesis and cellular respiration. We examine a wide variety of
representations used by experienced teachers to teach diffusion and
osmosis. To help teachers select appropriate representations for their
students, we briefly describe each representation and discuss its pros
and cons. After teachers select representations, we offer
recommendations for sequencing them. We recommend beginning with
macroscopic-level representations that easily allow students to
visualize the phenomenon, then moving to microscopic-level
representations (cell-level), and finally exploring the phenomenon at
the molecular level using virtual representations.
Key Words: Osmosis; diffusion; representations; models; sequencing.
Osmosis (Study and teaching)
Water chemistry (Study and teaching)
Birds (Eggs and nests)
|Publication:||Name: The American Biology Teacher Publisher: National Association of Biology Teachers Audience: Academic; Professional Format: Magazine/Journal Subject: Biological sciences; Education Copyright: COPYRIGHT 2012 National Association of Biology Teachers ISSN: 0002-7685|
|Issue:||Date: August, 2012 Source Volume: 74 Source Issue: 6|
|Geographic:||Geographic Scope: United States Geographic Code: 1USA United States|
Students often struggle to understand diffusion and osmosis and, as
a result, have difficulty predicting the direction of osmosis,
visualizing individual particles undergoing diffusion and osmosis, and
making sense of vocabulary terms. Diffusion and osmosis are challenging
concepts for students because visualizing the movement of individual
particles at the cellular level and predicting the direction of osmosis
requires students to understand and integrate concepts in physics,
chemistry, and biology (Odom & Barrow, 2007). Conceptual
understanding is important and provides a basis for explaining complex
biological processes, including photosynthesis, cellular respiration,
and homeostasis (Zuckerman, 1993; Odom, 1995). Here, we examine commonly
used demonstrations, laboratory activities, and innovative computer
simulations to offer guidelines for selecting and sequencing
representations for teaching diffusion and osmosis.
* Using Representations
Representations provide concrete models to support students' visualization of abstract processes. Hands-on representations offer students opportunities to make and test predictions, engage in problem solving, and integrate new understanding with their existing knowledge (Roth et al., 2005; Cook, 2006; Hubber et al., 2010). Selecting appropriate representations is important and requires teachers to have significant content knowledge as well as an understanding of what constitutes an effective representation (Roth et al., 2005). Below are guidelines for effectively using representations as teaching tools:
1. Use representations as demonstrations or student explorations during instruction to enhance understanding (Cook, 2006).
2. Engage students with animations to visualize dynamic phenomena (Cook, 2006).
3. Have students explore multiple representations of the same phenomena, stressing common features across the representations to avoid confusion (Cook, 2006).
4. Start with familiar, concrete representations (macroscopic level) that connect with students' prior knowledge (e.g., wilting lettuce) (Moreno et al., 2011).
5. Sequence representations from the most concrete (real objects) to the most abstract (formulas and textbook readings) (Olson, 2008).
6. After exploring the actual phenomenon, use virtual representations (i.e., simulations) to explore the phenomenon at the molecular level.
In the following sections, we apply these guidelines to examine commonly used and innovative representations for teaching diffusion and osmosis (see Table 1). We are not suggesting that teachers use all of the representations; our goal is to help teachers select and sequence representations. We recommend that the sequence begin with macroscopic representations, move on to microscopic, and ultimately focus on virtual representations to examine diffusion and osmosis at the molecular level. We provide the pros and cons for each representation in the table below to make that task easier.
* Representations for Diffusion
Diffusion is the tendency for molecules of any substance to spread out into available space, moving from regions of greater to lesser concentrations, and is ultimately driven by random molecular motion (Campbell & Reece, 2001). Our goal is to provide teachers with representations of diffusion that address the dynamic nature of the process and emphasize the role of random molecular motion. Diffusion is a critical concept and serves as a basis for understanding osmosis. We suggest initially building student understanding with concrete (i.e., macroscopic) followed by abstract (i.e., virtual) representations of diffusion prior to teaching osmosis (see Table 2).
* Macroscopic Representations
Diffusion of Food Dye in Water
Diffusion of food dye in water is easy for students to observe and provides a concrete experience with the phenomenon. Relative rates of diffusion can be contrasted if two beakers are used. One beaker should contain heated water and the other should contain cold water to emphasize the critical role of kinetic energy. Portrayed as a dynamic process affected by levels of kinetic energy within the system, this representation supports students' understanding of diffusion as a process driven by molecular motion and influenced by kinetic energy. It is important to note, however, that while diffusion is occurring, students are also observing advection or motion resulting from currents forming within the heated water. Diffusion alone cannot account for all the movement of the food dye in heated water. We suggest engaging students in a discussion focused on diffusion of food dye as well as the influence of larger-scale motion resulting from currents in the heated water.
Diffusion of Cologne through a Latex Balloon
This representation is a great way to engage students with diffusion through a semipermeable membrane. Before inflating a balloon, add several drops of cologne, then inflate and seal the balloon. The cologne evaporates within the balloon, mixing with trapped air, and gradually diffuses through the latex membrane, resulting in a pervasive scent within the classroom. The representation provides an introduction to semipermeable membranes, making connections between the apparent odor of the cologne and passage of only certain materials through the latex membrane. Diffusion of gases is emphasized in this representation, and it is important to note that gases, like liquids, diffuse from regions of greater to lesser concentration. It is also important to note that air currents within the classroom may influence the diffusion of cologne particles.
* Microscopic Representations
Diffusion of India Ink
Place a single drop of India ink in several drops of water on a microscope slide. India ink consists of particles of carbon in suspension and provides an opportunity for students to observe the diffusion of particles over time. As the slide is warmed by the microscope light, students can see changes in the rate of diffusion. For this representation, pairs of students can use microscopes to observe diffusion or, if a digital microscope is available, the teacher may choose to project the slide for the whole class. India ink is available at art supply stores.
* Virtual Representations
Virtual Diffusion Representations at an Atomic Level
The Molecular Logic (MoLo) Project (Concord Consortium, 2001) is a collection of simulations created to support students' understanding of biological phenomena at the molecular level (http://molo.concord.org/). The site provides activities for students to explore and manipulate diffusion and investigate the relationship between kinetic energy and the rate of diffusion at the molecular level. We recommend the following MoLo diffusion activities:
* The Molecular Dynamics Introductory Activity Assessment (models/ DiffusionAssessment/diffusionAssessment2.cml) is an excellent activity that builds upon the cologne/balloon representation. In this activity, students manipulate the room temperature to observe changes in the random movement of cologne molecules in the air.
* Thermal (Brownian) Motion: Atoms and Molecules are Always Moving (http://molo.concord.org/database/activities/40.html). Students observe the effect of temperature change on Brownian movement at the molecular level. This brief activity includes a historical account of Robert Brown's discovery and connects Brownian movement to why refrigeration slows food spoilage.
MoLo requires a computer with an Internet connection and data projector, or a class set of laptops for students to work in pairs. The teacher should download the software in advance, in case there are Internet security issues to address at the building level. The MoLo searchable database reduces the amount of teacher time necessary to find appropriate representations. A distinct advantage of a virtual representation is that students can manipulate the virtual model to visualize molecular interactions and the effects of environmental conditions (e.g., temperature) on the rate of diffusion. Furthermore, the website can be used as a teacher-directed demonstration or student-directed virtual investigation. Student responses can be captured in two ways: students can print their responses or, with a free class registration, teachers can access an electronic file of student responses.
* Representations of Osmosis
Osmosis is the diffusion of water across a selectively permeable membrane driven by a variation in solute concentrations on either side of the membrane (Campbell & Reece, 2001). The semipermeable membrane allows diffusion of water molecules but prevents diffusion of solutes. The direction of osmosis is driven by relative concentrations of dissolved solids (e.g., tonicity) on either side of a semipermeable membrane. Hypotonic solutions contain only minimal solute concentrations and greater concentrations of water. Hypertonic solutions contain greater solute concentrations and lesser concentrations of water. Hence, water diffuses from hypotonic (areas of greater water concentration) regions to hypertonic (areas of lesser water concentration) regions. We recommend that these terms, if taught at all, be introduced after students develop a conceptual understanding of the phenomenon.
* Sequence of Representations for Osmosis
We recommend that teachers initially engage students with macroscopic representations of osmosis (see Table 3). Potato slices and lettuce leaves placed in saline or distilled water allow students to observe the phenomenon and note resulting variations in turgidity.
* Macroscopic Plant Representations of Osmosis
Cut equal-sized slices of a peeled raw potato. Record the initial mass of the slices before placing one slice in a hypotonic solution (0% NaCl), one slice in a hypertonic solution (5% NaCl), and the third slice in an isotonic solution (0.9% NaCl). Have students record their individual predictions, and then share their predictions and explanation with a classmate. Allow the potato slices to remain in each solution overnight before observing and massing the slices a second time. The laboratory works well as a teacher-led demonstration or as a student investigation.
Lettuce leaves are placed in solutions of varying salt concentration (0% NaCl; 5% NaCl; 0.9% NaCl). Students make observations of lettuce leaves before and after placing leaves in solutions of varying concentrations.
* Cellular-Level Representations of Osmosis
After investigating the wilting lettuce leaves or potato slices, we recommend engaging students with microscopic and macroscopic representations of osmosis at the cellular level. Elodea leaf cells, red onion cells, decalcified eggs, dialysis tubing, and plastic baggies are common microscopic and macroscopic representations at the cellular level. Effective representations allow students to manipulate the solute concentration within the environment while making and testing predictions of the resulting direction of osmosis. Challenge students to make and test predictions prior to exposing cells to hypertonic, hypotonic, or isotonic environments. Findings at the cellular level are used to explain changes in turgidity within the lettuce leaves and potato slices used earlier. We explore the pros and cons of each of these cellular-level models in the following sections.
Elodea Leaf or Red Onion Cells
Elodea leaf cells or the pigmented epidermal layer of a red onion are excellent microscopic representations of osmosis at the cellular level (see Table 4). Elodea can be stored in an aquarium prior to use. We suggest that students take younger leaves from the tip of the Elodea branch. The pigmented epidermal layer of red onion should be carefully peeled for viewing. Students need to be able to make wet-mount slides and focus microscopes. Distilled water and a 20% sucrose solution (dissolve 20 g of sucrose in 100 mL of distilled water) provide the hypotonic and hypertonic solutions, respectively.
There are several challenges with these representations. First, students tend to focus on the tissue as a whole, rather than on individual cells; be sure to focus attention on single cells within the leaf. Second, students are often distracted by the chloroplasts in Elodea cells and require guidance to observe the effect of osmosis on the central vacuole. Third, review plant structures and remind students that the cell wall remains constant while the central vacuole will swell or shrink, depending on the direction of osmosis. Emphasize the storage of pigment within the central vacuole of red onion cells. Instruct students to draw their observations, noting differences between the cells within hypotonic and hypertonic environments. Pairs of students can observe cells through microscopes, or the teacher could use these representations as a demonstration using a digital microscope and projecting the images.
[FIGURE 1 OMITTED]
Decalcified Chicken Eggs
A chicken egg is an excellent macroscopic representation of osmosis in animal cells (see Table 5). When the shell of a chicken egg is removed, a large single cell surrounded by a semipermeable membrane remains intact. We suggest that students quantify changes in the decalcified eggs prior to and following exposure to hypertonic (corn syrup) and hypotonic (distilled water) environments by carefully massing the eggs, preferably with an electronic balance, and using water displacement to determine egg volume. Use corn syrup to create a hypertonic environment rather than a saline solution because sodium and chloride ions have the potential to denature the membrane and alter results. A layer of water forms on the surface of the corn syrup after ~24 hours (see Figure 1). Ask students to look for this layer prior to removing the egg. The eggs will vary dramatically; the egg exposed to distilled water will gain significant mass and volume while the egg in corn syrup will shrivel with the yolk readily visible (see Figure 2). After observing the eggs and quantifying changes in mass and volume, challenge students to predict how the eggs would change if placed in the opposite environment. Reversing the eggs demonstrates the impact of tonicity on the direction of osmosis. Remind students to handle eggs carefully; membranes are delicate, although they remain intact for several days.
[FIGURE 2 OMITTED]
Prepare the eggs prior to the lab: place eggs in vinegar (acetic acid) for approximately 24-36 hours to dissolve the shell. Carefully rinse the eggs in tap water to remove shell residue. The remaining membrane is permeable to water, allowing water to diffuse into or out of the egg. Additional teacher preparation requires providing distilled water and corn syrup for the hypotonic and hypertonic environments. This representation has many advantages, in that chicken eggs are easy for students to handle and observe, are readily available, and are inexpensive.
Dialysis Tubing & Baggies
Artificial cells made of dialysis tubing or baggies make excellent macroscopic representations for both diffusion and osmosis (Zrelak & McCallister, 2009). Dialysis tubing must be ordered from a biological supply house and may be expensive; however, inexpensive store-brand baggies provide a readily accessible replacement for dialysis tubing. (Test the brand beforehand to ensure that it is semipermeable.) Teacher preparation involves making a 5% glucose solution (dissolve 5 g of glucose in 100 cm of water), a 20% corn starch solution (dissolve 20 g of corn starch in 100 cm of water), and providing baggies or cutting dialysis tubing into approximately 20-cm lengths and placing the tubing in water prior to the investigation (see Table 6). Dialysis tubing and baggies are semi-permeable membranes that restrict passage to small molecules (water, iodine, and glucose) and prevent passage of corn starch (large polysaccharide molecules). Use string to tie off dialysis tubing or baggies after placing 5 mL of the glucose solution and 10 mL of the starch solution in the tubing/baggie. The direction of osmosis into the bag is obvious as iodine (a small molecule) diffuses through the membrane with distilled water and reacts with the starch, turning contents into a dark blue or black. Use glucose test strips to test for the presence of glucose in the distilled water and iodine solution prior to the immersion of the dialysis tubing or baggie and at the close of the investigation. Students will note a positive test for glucose at the close of the investigation, indicating that glucose molecules diffused from greater to lesser concentrations.
The strengths of these representations include the following: (a) they highlight the nature of a semipermeable membrane as water, glucose, and iodine easily pass through the membrane but starch remains within the artificial cell; (b) diffusion of substances can be tracked along concentration gradients (e.g., water, iodine, and glucose all diffuse from regions of greater concentration to regions of lesser concentration); (c) the direction of osmosis is clearly evident as the dialysis tubing or baggie gain mass and volume; (d) the diffusion of iodine is emphasized by color change; and (e) glucose is detectable in the distilled water outside the artificial cell only at the close of the investigation. It is important to note that dialysis tubing and baggies can be difficult to tie off, potentially resulting in a false positive for starch in the beaker solution. We suggest that students twist and fold over the tubing or baggie before tying off to prevent leakage.
Virtual representations of osmosis allow students to visualize it at the molecular level (see Table 7). MoLo is open-source computer software and offers several free osmosis simulations (Concord Consortium, 2001). We recommend the MoLo activity "Osmosis," which allows students to manipulate the solute concentrate inside and outside the cell and observe the results (Figure 3). The representation shows (a) random movement of particles on either side of the cell membrane, (b) a graph of pressure inside and outside the cell, and (c) movement of molecules through the cell membrane (http://molo.concord.org/database/activities/233.html). Virtual representations require an in-class computer and projector or a classroom set of student computers. Virtual representations are powerful tools for engaging students in visualizing osmosis at the molecular level.
Formative assessments are critical tools for gauging the effectiveness of representations. To assess student thinking, challenge students to make and test predictions when manipulating representations. Student predictions can reveal misconceptions that will need to be addressed. Student predictions also provide insight into conceptual understanding during instruction. Assess student understanding of each representation and have students identify common features across representations. For example, baggies and plant cells are both semipermeable structures, allowing certain materials to pass through.
It is important to critically select representations to support student learning of abstract concepts, such as diffusion and osmosis. We recommend that teachers initially engage students with concrete examples of diffusion and then move to simulations that allow students to see the movement of individual molecules. Focus on rates of diffusion within environments with varying levels of kinetic energy to emphasize random molecular motion along a conc0entration gradient as driving forces for diffusion. Virtual representations help students visualize diffusion at the molecular level while manipulating available kinetic energy to observe resulting changes in the rate of diffusion.
[FIGURE 3 OMITTED]
Progressing from diffusion to osmosis builds upon students' knowledge and experience with the concept of diffusion. Engage students initially with osmosis through observations of familiar examples, such as loss of turgor pressure in plants (lettuce leaves or potato slices). Next, challenge students to collect data and formulate explanations through explorations of osmosis within microscopic and macroscopic cellular representations. Virtual representations allow students to visualize the movement of molecules through cell membranes. Effective molecular-level representations allow students to manipulate the solute concentration within the environment while making and testing predictions of the resulting direction of osmosis into or out of the cell (Sanger et al., 2001). Using models to teach osmosis illustrates how scientists generate, test, and modify models in an attempt to understand how the natural world works (Bogiages & Lotter, 2011). Teach the concept of osmosis through manipulation of models (e.g., living and artificial cells) before introducing vocabulary terms (e.g., hypertonic, hypotonic, and isotonic). In closing, we recommend the use of multiple representations to teach diffusion and osmosis, with careful attention to the sequencing of representations from concrete (macroscopic) to abstract (virtual representations at the molecular level).
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DEANNA LANKFORD is a Research Associate at the University of Missouri Science Education center, 321 Townsend Hall, Golumbia, MO 65211; e-mail: email@example.com. PATRICIA FRIEDRICHSEN is Associate Professor of Learning, Teaching, & curriculum at the University of Missouri, 321-E Townsend Hall, Columbia, MO 65211; e-mail: firstname.lastname@example.org.
Table 1. Representation continuum. Representation Macroscopic Microscopic Virtual DIFFUSION Food dye in hot or cold water * Represents rates of X diffusion in relation to energy within the solvent Cologne in a latex balloon * Particles of cologne X diffuse through balloon India ink in a drop of water * Diffusion observed X with microscope * Rate of diffusion changes as slide warms Molecular Logic * Computer program X simulates diffusion under conditions manipulated by students OSMOSIS MACRO Potato slices * Observe direction X of osmosis * Potato slices placed in saline or distilled water Lettuce leaves * Observe direction of X osmosis * Lettuce leaves placed in distilled water or a saline solution Elodea leaves or red onion cells * Osmosis observed in cells within Elodea leaf or red onion * Leaf/onion peel is X exposed to a 20% sucrose solution or distilled water Decalcified chicken eggs * Osmosis observed in X decalcified chicken eggs * Eggs exposed to corn syrup or distilled water OSMOSIS CELLULAR LEVEL Dialysis tubing * Osmosis and X diffusion observed * Semipermeable membrane observed Baggies * Osmosis and X diffusion observed * Semipermeable membrane observed Molecular Logic * Computer simulation X of osmosis in virtual cells * Virtual environment manipulated * Observation of osmosis at molecular level Table 2. Representations for diffusion. Representations for Diffusion Representation Description Evidence of Diffusion MACROSCOPIC Food dye Drops of food dye Slow diffusion of in water are placed in a food dye into cold beaker of very cold water. Rapid water. Drops of food diffusion of food dye are placed in a dye into hot water. beaker of very hot water. Balloon Several drops of Cologne diffuses and cologne are placed through the balloon cologne in a balloon, which into the classroom. is inflated and Students detect the passed among the scent while passing students. the balloon. This representation includes diffusion of a substance through a semipermeable membrane. MICROSCOPIC India ink A single drop of Heat from the and water India ink is place microscope bulb in several drops of increases the water on a kinetic energy of microscope slide. the system, resulting in increasingly rapid molecular motion and diffusion of the ink in the water. VIRTUAL Molecular Teachers and Manipulation of Logic students access computer software to Project Molecular Logic visualize diffusion database through at molecular level URL: http:// and impact of molo.concord.org/ kinetic energy on (database of the rate of biological diffusion. representations at the molecular level). Representations for Diffusion Representation Pros Cons MACROSCOPIC Food dye Minimal teacher Students infer in water preparation; explanation for materials are easily rates of diffusion. accessible. Balloon Minimal teacher Emphasizes diffusion and preparation; of gases rather than cologne accessible liquids. materials. Emphasizes diffusion Students infer through a explanation of semipermeable diffusion through a membrane. semipermeable membrane. MICROSCOPIC India ink Movement of carbon Prepare students to and water particles in India use microscopes. ink model diffusion. Expense and availability of India ink. VIRTUAL Molecular Visualization of Requires Internet Logic diffusion at connection and Project molecular level. computers. Table 3. Plant structures as macroscopic representations. External Representation Environment Osmosis Potato slices or Hypertonic, hypotonic, Observed through lettuce leaves (placed or isotonic changes in turgidity; in solutions of environments individual cells are varying not observed. concentrations) Representation Evidence Pro Potato slices or Lettuce leaves/potato Changes in direction lettuce leaves (placed slices become flaccid and rate of osmosis in solutions of in the saline solution are linked to changes varying and turgid in in cellular concentrations) distilled water. environment. Representation Con Potato slices or Individual cells are lettuce leaves (placed not visible. Students in solutions of must infer changes at varying the cellular level. concentrations) Table 4. Microscopic representations for osmosis in living cells. External Representation Environment Osmosis Evidence Elodea leaf Distilled water Water moves Enlargement of cells or red into cells. central vacuole onion cells 20% sucrose Water moves Contraction of solution out of cells. central vacuoles; chloroplasts clustered tightly together within Elodea cells Representation Pro Con Elodea leaf Elodea is available at Microscopes are needed. cells or red pet stores. Red onions Chloroplasts in Elodea onion cells are sold at grocery cells may be a stores. distraction for students. Table 5. Macroscopic observation of osmosis in decalcified chicken eggs. External Representation Environment Osmosis Evidence Decalcified Distilled Water moves Egg volume chicken eggs water through the increases; egg membrane of appears the egg. significantly larger; mass increases. Corn syrup Water moves Egg volume through the decreases; egg membrane out appears of the egg. shriveled; mass decreases. Representation Pro Con Decalcified Increase or decrease in Eggs are delicate and may chicken eggs egg volume and mass are break. easy for students to observe. Eggs are inexpensive and Only corn syrup should be easily obtained. used; saline will denature membrane. Table 6. Representations for osmosis in artificial cells. External Contents Environment Dialysis Water, glucose, Iodine and Tubing and starch distilled water solution Baggie Water, glucose, Iodine and and starch distilled water solution Contents Diffusion Osmosis Direction Evidence Dialysis Water, glucose, Glucose Out of Positive Tubing and starch cell glucose test solution Iodine Into cell Color change Water Into cell Mass increase Baggie Water, glucose, Glucose Out of Positive and starch cell glucose test solution Iodine Into cell Color change Water Into cell Mass increase Contents Pro Dialysis Water, glucose, Dialysis tubing is Tubing and starch selectively permeable. solution Baggie Water, glucose, Baggies are selectively and starch permeable, easily solution accessible and in- expensive. Contents Con Dialysis Water, glucose, Dialysis tubing must be Tubing and starch ordered and is costly. solution Baggie Water, glucose, Use only thin, store- and starch brand baggies. solution Table 7. Virtual representations of osmosis. Cellular External Representation Contents Environment Osmosis A virtual cell in Cellular Virtual solute Direction of a solution can contents concentration osmosis varies be manipulated remain within external with changes to include constant. environment to the external greater or lesser can be environment. concentrations of manipulated. solute. Representation Evidence Pro Con A virtual cell in Illustrates Visualization Computer and a solution can direction of of osmosis at data be manipulated osmosis as the molecular projector are to include students level. required for greater or lesser manipulate demonstration. concentrations of solute Manipulation solute. concentration of virtual Laptops are of the environment required external to test for student environment. predictions investigation. of the direction of osmosis.
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