Academic journal article The American Biology Teacher

Red Onions, Elodea, or Decalcified Chicken Eggs? Selecting & Sequencing Representations for Teaching Diffusion & Osmosis

Academic journal article The American Biology Teacher

Red Onions, Elodea, or Decalcified Chicken Eggs? Selecting & Sequencing Representations for Teaching Diffusion & Osmosis

Article excerpt

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. …

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