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7 Structure and Function Which One Will Dry Out Last? Cocci Bacilli Spirilla Bacterial cells have three common shapes—cocci, bacilli, and spirilla. It is important for bacteria to have a moist environment so they do not dry out. Under hot, dry conditions, which of the bacterial cell shapes is most likely to dry out last if each cell has the same volume? Circle your answer. A Cocci B Baccilli C Spirilla D They would all dry out at the same time. Explain your thinking. What “rule” or reason did you use to select your answer? _________________________________________________________________ _________________________________________________________________ _________________________________________________________________ _________________________________________________________________ _________________________________________________________________ Uncovering Student Ideas in Life Science Copyright © 2011 NSTA. All rights reserved. For more information, go to www.nsta.org/permissions. 45 7 Structure and Function Which One Will Dry Out Last? Teacher Notes Purpose The purpose of this assessment probe is to elicit students’ ideas about loss of water through cells. The probe is designed to reveal whether students recognize the relationship between shape, surface area, and volume. Related Concepts Cells, cell shape, bacteria, single-celled organisms, volume, surface area Explanation The best answer is A—cocci. The answer is best explained through geometry. Cocci, sphericalshaped bacteria, have the lowest surface area to volume ratio. Given the same volume, spheres have the smallest surface area of any shape. The low ratio of surface area to volume results in less water loss through the cell. The ratio of surface area to volume is important to cells because more contact with the environment through the surface of a cell relative to its environment increases water loss. Long, thinner cells, like the 46 spirilla, have a greater ratio of surface area to volume and therefore would dry out first. However, bear in mind this is the “best” answer, based on general shape, but shape alone cannot be used to predict the rate of drying for some bacteria. Bacteria have a variety of physical and biochemical defenses against desiccation. For example, two bacilli of the same size and shape can have very different rates of drying. Moreover, many bacteria can form very resistant spores that defy predictions based on shape alone. Curricular and Instructional Considerations Elementary Students In the elementary grades, students observe a variety of plant and animal cells and describe their shapes. They learn about the need that all living things have for water. In mathematics they learn about and distinguish between a variety of three-dimensional shapes. N a t i o n a l S c i e n c e Te a c h e r s A s s o c i a t i o n Copyright © 2011 NSTA. All rights reserved. For more information, go to www.nsta.org/permissions. 7 Structure and Function Middle School Students Middle school students deepen their knowledge of cells, including single-celled organisms. They learn that cells are mostly made of water and that cells must control the passage of water in and outside of the cell. They learn about the basic characteristics of bacteria and develop a notion that bacteria are much smaller than the familiar cells they have observed. In mathematics, they develop an understanding of surface area and volume and apply this to several different phenomena in science. For example, water will evaporate more quickly from an open container that has an opening with a large surface area than from a container of the same volume with a smaller opening. They also deepen their knowledge of ratios at this level and use ratios to describe relationships. High School Students High school students deepen their understanding of the factors that affect the passage of water in and out of a cell. They learn about the cell membrane in more detail. They explore the ratio of surface area to volume in cells, organs, and organisms and use it to explain several important biological ideas. Using mathematics, they can compare surface areas of different shapes and predict what happens when the surface areas increase while the volume stays the same. Administering the Probe This probe is most appropriate for high school students. Show students microscopic photos of the three different shapes of bacteria. Please note that it is difficult to illustrate volume in a two-dimensional drawing. Make sure students know that the volume is the same for all three cells in this example by explicitly stating this to the students. It is the shape that changes. Related Ideas in National Science Education Standards (NRC 1996) K–4 The Characteristics of Organisms • Organisms have basic needs. For example, animals need air, water, and food; plants require air, water, nutrients, and light. Organisms can survive only in environments where their needs can be met. 5–8 Structure and Function in Living Systems • All organisms are composed of cells—the fundamental unit of life. Most organisms are single cells. • Cells carry on the many functions needed to sustain life. 9–12 The Cell • Cells have particular structures that underlie their functions. Every cell is surrounded by a membrane that separates it from the outside world. Related Ideas in Benchmarks for Science Literacy (AAAS 2009) K–2 The Cell • Most living things need water, food, and air. 3–5 The Cell • Some living things consist of a single cell. Like familiar organisms, they need food, water, and air; a way to dispose of waste; and an environment they can live in. 6–8 Scale • Some properties of an object depend on its length, some depend on its area, and some depend on its volume. Uncovering Student Ideas in Life Science Copyright © 2011 NSTA. All rights reserved. For more information, go to www.nsta.org/permissions. 47 7 Structure and Function 6–8 The Cell • All living things are composed of cells, from just one to many millions, whose details usually are visible only through a microscope. • About two-thirds of the weight of cells is accounted for by water, which gives cells many of their properties. 9–12 Scale Because different properties are not affected to the same degree by changes in size, large changes in size typically change the way that things work in physical, biological, or social systems. 9–12 The Cell • Every cell is covered by a membrane that controls what can enter and leave the cell. Related Research • This question is similar to one used by Livne (1996) in which high school biology students were asked about the cooling of milk. They were given a spherical baby bottle and a cylindrical baby bottle and told that they each contained the same amount of milk. They were then asked which bottle of milk would cool off fastest, or whether they would cool at the same time. Fewer than 50% predicted the cylindrical bottle would cool faster because of the greater surface area. Most predicted they would cool at the same rate because their volumes were equal. The students were then presented with the question about which type of bacteria are most resistant to drying. About 40% recognized the ratio of surface area to volume and correctly predicted the spherical bacteria (cocci) would be most resistant. However, the majority of students said they would dry out at the same time because they had the same volume. • Stavy and Tirosh (2000) identify the intuitive rule “same A, same B” to explain why students choose that the bacteria would dry out at the same time. Their reasoning is that if the bacteria have the same volume, then they have the same resistance to drying. Suggestions for Instruction and Assessment • To further address the relationship between surface area and volume and issues related to scale, consider combining this probe with “Whale and Shrew” in Uncovering Student Ideas in Science, Vol. 2: 25 More Formative Assessment Probes (Keeley, Eberle, and Tugel 2007). • After students have made their predictions and explained their answers, show 3-D models that have nearly identical volumes and approximately the shapes of the bacteria—for example, a Styrofoam ball, a wooden rod, and a piece of thick rope. Ask students to imagine these are the bacterial cells and that they have the same volume. Ask if they would change their predictions after looking at the models. Alternatively, provide students with modeling clay. Have them create the three shapes from the same volume of clay, then calculate and compare the surface areas of each shape. • For high school students, connect the surface area to volume relationship to the idea of cells needing to diffuse material quickly in and out for gas exchange, food, and so on, in addition to maintaining water content. Because most cells normally exist in an aqueous environment, water loss is less of an issue than the ongoing need to bring in materials for food or to exchange gases. These needs are also related to the relationship between surface area and volume. • Give students a ball of clay and have them mathematically calculate the relationship Indicates a strong match between the ideas elicited by the probe and a national standard’s learning goal. 48 N a t i o n a l S c i e n c e Te a c h e r s A s s o c i a t i o n Copyright © 2011 NSTA. All rights reserved. For more information, go to www.nsta.org/permissions. 7 Structure and Function between surface area and volume. Have them shape the clay into a rectangular solid and perform the same calculation. Then have them reshape the clay into a long ropelike cylinder shape and perform the calculation again. Have them compare all three of the volumes (they are the same), the surface areas, and the ratio of surface area to volume. Have them compare their results to the three bacterial cell shapes in the probe. • As a postassessment, this probe can be modified to ask students to predict and explain the order in which these cells would dry out if exposed to environmental conditions that promote desiccation. • Challenge students to come up with other examples of when a sphere is the best shape for a particular circumstance. Contrast this with examples of when it is better to have a large exposed surface area. Related NSTA Science Store Publications, NSTA Journal Articles, NSTA SciGuides, NSTA SciPacks, and NSTA Science Objects American Association for the Advancement of Science (AAAS). 2001. Atlas of science literacy. Vol. 1. (See “Cells and Organs” map, pp. 74–75.) Washington, DC: AAAS. Konicek-Moran, R. 2008. Dried apples. In Everyday science mysteries: Stories for inquiry-based science teaching, 99–106. Arlington, VA: NSTA Press. Maguire, L., L. Myerowitz, and V. Sampson. 2010. Exploring osmosis and diffusion in cells. The Science Teacher 77 (8): 55–60. SciPack: Cell Structure and Function Related Curriculum Topic Study Guides (in Keeley 2005) “Cells” “Scale” References American Association for the Advancement of Science (AAAS). 2009. Benchmarks for science literacy online. www.project2061.org/publications/ bsl/online. Keeley, P. 2005. Science curriculum topic study: Bridging the gap between standards and practice. Thousand Oaks, CA: Corwin Press and Arlington, VA: NSTA Press. Keeley, P., F. Eberle, and J. Tugel. 2007. Uncovering student ideas in science, vol. 2: 25 more formative assessment probes. Arlington, VA: NSTA Press. Livne, T. 1996. Examination of high school students’ difficulties in understanding the change in surface area, volume, and surface area/volume ratio with the change in size and/or shape of a body. Unpublished master’s thesis, Tel Aviv University, Tel Aviv, Israel. National Research Council (NRC). 1996. National science education standards. Washington, DC: National Academies Press. Stavy, R., and D. Tirosh. 2000. How students (mis-) understand science and mathematics: Intuitive rules. New York: Teachers College Press. Uncovering Student Ideas in Life Science Copyright © 2011 NSTA. All rights reserved. For more information, go to www.nsta.org/permissions. 49