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.

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.

Table 1.

Representation continuum.

RepresentationMacroscopicMicroscopicVirtual
DIFFUSION Food dye in hot or cold water
• Represents rates of diffusion in relation to energy within the solvent 
  
 Cologne in a latex balloon
• Particles of cologne diffuse through balloon 
  
 India ink in a drop of water
• Diffusion observed with microscope
• Rate of diffusion changes as slide warms 
  
 Molecular Logic
• Computer program simulates diffusion under conditions manipulated by students 
  
OSMOSIS MACRO Potato slices
• Observe direction of osmosis
• Potato slices placed in saline or distilled water 
  
  Lettuce leaves
• Observe direction of osmosis
• Lettuce leaves placed in distilled water or a saline solution 
  
OSMOSIS CELLULAR LEVEL Elodea leaves or red onion cells
• Osmosis observed in cells within Elodea leaf or red onion
• Leaf/onion peel is exposed to a 20% sucrose solution or distilled water 
  
  Decalcified chicken eggs
• Osmosis observed in decalcified chicken eggs
• Eggs exposed to corn syrup or distilled water 
  
  Dialysis tubing
• Osmosis and diffusion observed
• Semipermeable membrane observed 
  
  Baggies
• Osmosis and diffusion observed
• Semipermeable membrane observed 
  
  Molecular Logic
• Computer simulation of osmosis in virtual cells
• Virtual environment manipulated
• Observation of osmosis at molecular level 
  
RepresentationMacroscopicMicroscopicVirtual
DIFFUSION Food dye in hot or cold water
• Represents rates of diffusion in relation to energy within the solvent 
  
 Cologne in a latex balloon
• Particles of cologne diffuse through balloon 
  
 India ink in a drop of water
• Diffusion observed with microscope
• Rate of diffusion changes as slide warms 
  
 Molecular Logic
• Computer program simulates diffusion under conditions manipulated by students 
  
OSMOSIS MACRO Potato slices
• Observe direction of osmosis
• Potato slices placed in saline or distilled water 
  
  Lettuce leaves
• Observe direction of osmosis
• Lettuce leaves placed in distilled water or a saline solution 
  
OSMOSIS CELLULAR LEVEL Elodea leaves or red onion cells
• Osmosis observed in cells within Elodea leaf or red onion
• Leaf/onion peel is exposed to a 20% sucrose solution or distilled water 
  
  Decalcified chicken eggs
• Osmosis observed in decalcified chicken eggs
• Eggs exposed to corn syrup or distilled water 
  
  Dialysis tubing
• Osmosis and diffusion observed
• Semipermeable membrane observed 
  
  Baggies
• Osmosis and diffusion observed
• Semipermeable membrane observed 
  
  Molecular Logic
• Computer simulation of osmosis in virtual cells
• Virtual environment manipulated
• Observation of osmosis at molecular level 
  

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).

Table 2.

Representations for diffusion.

Representations for Diffusion
RepresentationDescriptionEvidence of DiffusionProsCons
MACROSCOPIC Food dye in water Drops of food dye are placed in a beaker of very cold water.
Drops of food dye are placed in a beaker of very hot water. 
Slow diffusion of food dye into cold water.
Rapid diffusion of food dye into hot water. 
Minimal teacher preparation; materials are easily accessible. Students infer explanation for rates of diffusion. 
 Balloon and cologne Several drops of cologne are placed in a balloon, which is inflated and passed among the students. Cologne diffuses through the balloon into the classroom. Students detect the scent while passing the balloon.
This representation includes diffusion of a substance through a semipermeable membrane. 
Minimal teacher preparation; accessible materials.
Emphasizes diffusion through a semipermeable membrane. 
Emphasizes diffusion of gases rather than liquids.
Students infer explanation of diffusion through a semipermeable membrane. 
MICROSCOPIC India ink and water A single drop of India ink is place in several drops of water on a microscope slide. Heat from the microscope bulb increases the kinetic energy of the system, resulting in increasingly rapid molecular motion and diffusion of the ink in the water. Movement of carbon particles in India ink model diffusion. Prepare students to use microscopes.
Expense and availability of India ink. 
VIRTUAL Molecular Logic Project Teachers and students access Molecular Logic database through URL: http://molo.concord.org/ (database of biological representations at the molecular level). Manipulation of computer software to visualize diffusion at molecular level and impact of kinetic energy on the rate of diffusion. Visualization of diffusion at molecular level. Requires Internet connection and computers. 
Representations for Diffusion
RepresentationDescriptionEvidence of DiffusionProsCons
MACROSCOPIC Food dye in water Drops of food dye are placed in a beaker of very cold water.
Drops of food dye are placed in a beaker of very hot water. 
Slow diffusion of food dye into cold water.
Rapid diffusion of food dye into hot water. 
Minimal teacher preparation; materials are easily accessible. Students infer explanation for rates of diffusion. 
 Balloon and cologne Several drops of cologne are placed in a balloon, which is inflated and passed among the students. Cologne diffuses through the balloon into the classroom. Students detect the scent while passing the balloon.
This representation includes diffusion of a substance through a semipermeable membrane. 
Minimal teacher preparation; accessible materials.
Emphasizes diffusion through a semipermeable membrane. 
Emphasizes diffusion of gases rather than liquids.
Students infer explanation of diffusion through a semipermeable membrane. 
MICROSCOPIC India ink and water A single drop of India ink is place in several drops of water on a microscope slide. Heat from the microscope bulb increases the kinetic energy of the system, resulting in increasingly rapid molecular motion and diffusion of the ink in the water. Movement of carbon particles in India ink model diffusion. Prepare students to use microscopes.
Expense and availability of India ink. 
VIRTUAL Molecular Logic Project Teachers and students access Molecular Logic database through URL: http://molo.concord.org/ (database of biological representations at the molecular level). Manipulation of computer software to visualize diffusion at molecular level and impact of kinetic energy on the rate of diffusion. Visualization of diffusion at molecular level. Requires Internet connection and computers. 

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.

Table 3.

Plant structures as macroscopic representations.

RepresentationExternal EnvironmentOsmosisEvidenceProCon
Potato slices or lettuce leaves (placed in solutions of varying concentrations) Hypertonic, hypotonic, or isotonic environments Observed through changes in turgidity; individual cells are not observed. Lettuce leaves/potato slices become flaccid in the saline solution and turgid in distilled water. Changes in direction and rate of osmosis are linked to changes in cellular environment. Individual cells are not visible. Students must infer changes at the cellular level. 
RepresentationExternal EnvironmentOsmosisEvidenceProCon
Potato slices or lettuce leaves (placed in solutions of varying concentrations) Hypertonic, hypotonic, or isotonic environments Observed through changes in turgidity; individual cells are not observed. Lettuce leaves/potato slices become flaccid in the saline solution and turgid in distilled water. Changes in direction and rate of osmosis are linked to changes in cellular environment. Individual cells are not visible. Students must infer changes at the cellular level. 

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.

Table 4.

Microscopic representations for osmosis in living cells.

RepresentationExternal EnvironmentOsmosisEvidenceProCon
Elodea leaf cells or red onion cells Distilled water Water moves into cells. Enlargement of central vacuole Elodea is available at pet stores. Red onions are sold at grocery stores. Microscopes are needed.
Chloroplasts in Elodea cells may be a distraction for students. 
 20% sucrose solution Water moves out of cells. Contraction of central vacuoles; chloroplasts clustered tightly together within Elodea cells   
RepresentationExternal EnvironmentOsmosisEvidenceProCon
Elodea leaf cells or red onion cells Distilled water Water moves into cells. Enlargement of central vacuole Elodea is available at pet stores. Red onions are sold at grocery stores. Microscopes are needed.
Chloroplasts in Elodea cells may be a distraction for students. 
 20% sucrose solution Water moves out of cells. Contraction of central vacuoles; chloroplasts clustered tightly together within Elodea cells   

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.

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.

Table 5.

Macroscopic observation of osmosis in decalcified chicken eggs.

RepresentationExternal EnvironmentOsmosisEvidenceProCon
Decalcified chicken eggs Distilled water Water moves through the membrane of the egg. Egg volume increases; egg appears significantly larger; mass increases. Increase or decrease in egg volume and mass are easy for students to observe.
Eggs are inexpensive and easily obtained. 
Eggs are delicate and may break.
Only corn syrup should be used; saline will denature membrane. 
 Corn syrup Water moves through the membrane out of the egg. Egg volume decreases; egg appears shriveled; mass decreases.   
RepresentationExternal EnvironmentOsmosisEvidenceProCon
Decalcified chicken eggs Distilled water Water moves through the membrane of the egg. Egg volume increases; egg appears significantly larger; mass increases. Increase or decrease in egg volume and mass are easy for students to observe.
Eggs are inexpensive and easily obtained. 
Eggs are delicate and may break.
Only corn syrup should be used; saline will denature membrane. 
 Corn syrup Water moves through the membrane out of the egg. Egg volume decreases; egg appears shriveled; mass decreases.   
Figure 1.

Decalcified egg in corn syrup.

Figure 1.

Decalcified egg in corn syrup.

Figure 2.

Egg comparison: (left) hypotonic solution and (right) hypertonic solution.

Figure 2.

Egg comparison: (left) hypotonic solution and (right) hypertonic solution.

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 semipermeable 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.

Table 6.

Representations for osmosis in artificial cells.

ContentsExternal EnvironmentDiffusionOsmosisDirectionEvidenceProCon
Dialysis Tubing Water, glucose, and starch solution Iodine and distilled water Glucose  Out of cell Positive glucose test Dialysis tubing is selectively permeable. Dialysis tubing must be ordered and is costly. 
   Iodine  Into cell Color change   
    Water Into cell Mass increase   
Baggie Water, glucose, and starch solution Iodine and distilled water Glucose  Out of cell Positive glucose test Baggies are selectively permeable, easily accessible and in-expensive. Use only thin, store-brand baggies. 
   Iodine  Into cell Color change   
    Water Into cell Mass increase   
ContentsExternal EnvironmentDiffusionOsmosisDirectionEvidenceProCon
Dialysis Tubing Water, glucose, and starch solution Iodine and distilled water Glucose  Out of cell Positive glucose test Dialysis tubing is selectively permeable. Dialysis tubing must be ordered and is costly. 
   Iodine  Into cell Color change   
    Water Into cell Mass increase   
Baggie Water, glucose, and starch solution Iodine and distilled water Glucose  Out of cell Positive glucose test Baggies are selectively permeable, easily accessible and in-expensive. Use only thin, store-brand baggies. 
   Iodine  Into cell Color change   
    Water Into cell Mass increase   

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.

Table 7.

Virtual representations of osmosis.

RepresentationCellular ContentsExternal EnvironmentOsmosisEvidenceProCon
A virtual cell in a solution can be manipulated to include greater or lesser concentrations of solute. Cellular contents remain constant. Virtual solute concentration within external environment can be manipulated. Direction of osmosis varies with changes to the external environment. Illustrates direction of osmosis as students manipulate solute concentration of the external environment. Visualization of osmosis at the molecular level.
Manipulation of virtual environment to test predictions of the direction of osmosis. 
Computer and data projector are required for demonstration.
Laptops are required for student investigation. 
RepresentationCellular ContentsExternal EnvironmentOsmosisEvidenceProCon
A virtual cell in a solution can be manipulated to include greater or lesser concentrations of solute. Cellular contents remain constant. Virtual solute concentration within external environment can be manipulated. Direction of osmosis varies with changes to the external environment. Illustrates direction of osmosis as students manipulate solute concentration of the external environment. Visualization of osmosis at the molecular level.
Manipulation of virtual environment to test predictions of the direction of osmosis. 
Computer and data projector are required for demonstration.
Laptops are required for student investigation. 
Figure 3.

MoLo Osmosis model.

Figure 3.

MoLo Osmosis model.

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.

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).

References

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