Red onions, Elodea, or decalcified chicken eggs? Selecting & sequencing representations for teaching diffusion & osmosis.
Abstract: 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.
Article Type: Report
Subject: Biological models (Usage)
Osmosis (Models)
Osmosis (Study and teaching)
Water chemistry (Study and teaching)
Dextrose (Properties)
Glucose (Properties)
Birds (Eggs and nests)
Birds (Properties)
Authors: Lankford, Deanna
Friedrichsen, Patricia
Pub Date: 08/01/2012
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
Accession Number: 298172417
Full Text: 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

Potato Slices

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

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 Osmosis

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.

* Assessment

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.

* Summary

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).

DOI: 10.1525/abt.2012.74.6.7

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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: lankfordd@missouri.edu. PATRICIA FRIEDRICHSEN is Associate Professor of Learning, Teaching, & curriculum at the University of Missouri, 321-E Townsend Hall, Columbia, MO 65211; e-mail: friedrichsenp@missouri.edu.
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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