How big are cells? Ask most middle school students that question, and they’ll tell you that cells are very small – even microscopic! But what if you asked them to compare the size of cells in a whale and a shrew?

That’s the engaging scenario laid out in “Whale and Shrew,” a formative assessment probe from *Uncovering Student Ideas in Science, Volume 2: 25 More Formative Assessment Probes* (NSTA Press, 2007). The probe unearths a subtle, yet fundamental concept of cell theory – that there are natural constraints that necessarily limit cell size. How can you help your students reach this conclusion?

An inquiry-based activity provides an opportunity for students to answer the question, “How does the size of a cell affect its ability to exchange materials with its environment?” Or, more simply stated, “Why are cells small?”

These two activities could be used as is, or they could be adapted for a guided inquiry experience. Both involve analyzing how the surface area to volume ratio affects the rate of diffusion in cubes of various sizes. Each uses different materials and a slightly different procedure.

Cell Size and Division

Students test”cells” made of agar and phenolphtalein with sodium hydroxide. They observe the rate of diffusion (evident by color change) in 1x1x1, 2x2x2, and 3x3x3 cubes.

Experiment on Cell Surface Area and Volume

In this lesson, students test “cells” made of potatoes with Lugol’s solution. They observe the rate of diffusion (evidenced by color change) in cubes with a length of 0.5 cm, 1 cm, 1.5 cm, 2 cm, 2.5 cm, and 3 cm. They also calculate the surface area, volume, and ratio of surface area to volume of all cubes.

How can you turn these activities into an inquiry-based lesson? We recommend starting with the probe to assess student thinking and spark interest. Next, you may ask students to generate a testable question based on the probe, or you may choose to provide the question for them. Students can then plan and conduct an investigation using the materials specified in either one of the lessons. Prompt students to generate claims and draw conclusions based on evidence from the investigation.

Teachers of younger students (grades 5 and 6) may wish to use the potatoes and Lugol’s solution, while an 8th grade teacher may have access to agar, phenolpthalein, and sodium hydroxide. Teachers may also modify this activity by removing the surface area and volume calcuations and instead relying on the qualitative observations made during the activity. Either way, students will gain new insight into why cells in any organism are small.

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