This is the new updated edition of the first book in the bestselling Uncovering Student Ideas in Science series. Like the first edition of volume 1, this book helps pinpoint what your students know (or think they know) so you can monitor their learning and adjust your teaching accordingly. Loaded with classroom-friendly features you can use immediately, the book includes 25 “probes”—brief, easily administered formative assessments designed to understand your students’ thinking about 60 core science concepts.

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The purpose of this assessment probe is to elicit students’ ideas about cell size. The probe is designed to reveal whether students think animal cell size is determined by the size of the animal.

More A-More B

Cells, growth, cell division, mitosis

The best answer is C: The average cell of a blue whale is about the same size as the average cell of a pygmy shrew. The size of average mammal cells (this excludes cells that are unusually large, such as neurons) is similar in all mammal species. Although some body cells can be very large and cells vary, the average body cells of most mammals range in size from 10 to 100 micrometers in diameter. Interestingly, the earliest-stage embryos of the whale and pygmy shrew are also a similar size, even though a whale eventually reaches a mass of 150,000 kg, whereas the average pygmy shrew reaches only about 3 grams—about a 50-million-times difference!

Cells are limited in how large they can be because the surface area-to-volume ratio does not stay the same as the size of a cell increases. Cells need to be able to move materials into and out of a cell, and it is harder for a large cell to pass materials in and out of the membrane and to move materials through the cell. Blue whales are larger than pygmy shrews because they have more cells as a result of cell division, not because their cells are larger.

Elementary Students

In the early elementary grades, the focus is on observable structures—body parts (such as arms, legs, and heads) and specific parts of body (such as eyes, feet, and fingers). In the intermediate grades, students learn about internal structures, such as tissues and organs. Students observe that larger animals have larger body parts (such as legs and teeth) and larger organs (such as heart, lungs, and stomach). This observation can lead to a later preconception that the cells of larger animals are also larger. Students learn about growth at the organism level, not as an increase in the number of cells.

Middle School Students

In middle school, students shift from macroscopic structures to microscopic structures. Students learn that all organisms are made up of cells and that the cell is the basic unit of structure and function. They observe differences between plant and animal cells and extend their observations of cells to comparing similar cell types across animal species. Students develop the idea of similarities among species by examining internal structures as well as cells. They can also begin to recognize the very small size of most cells and that most cells repeatedly divide to make more cells. They know that organisms and the organs they contain generally grow in size from birth until they reach adulthood. Middle school is the time for students to understand that an increase in size is due to an increase in the number of cells and to link cell division to the growth of organisms.

High School Students

High school students have a deeper understanding of types of cells, cell size, and how body cells divide and multiply through mitosis. Mathematically, high school students develop an understanding of the relationship between volume and surface area and the way total surface area decreases with an increase in volume. Through lab experiences with model cells made of gels, they can observe how the surface area-to-volume ratio affects the passage of materials into, around, and out of a cell, thus limiting the size of a cell.

This probe can be used with students in grades 6–12. Emphasize that animals have different types of cells that are different sizes, but students should focus on an average, typical cell that is not unusually large, such as a skin, blood, or bone cell.

• Stavy and Tirosh (2000) asked students in grades 7–12 a question similar to the one in this probe, comparing muscle cells of a mouse to muscle cells of an elephant. The majority of students, especially in grades 7 and 8, thought that larger animals have larger cells. The common justification was that “according to the dimensions of the elephants and those of the mice, it is obvious that the muscle cells of the mice are smaller than those of the elephants” (p. 30). This is an example of the intuitive rule “more A, more B.” Most of the younger students who answered correctly explained the equality in terms of the cells having the same function and therefore being the same size. Most of the high school students who responded correctly used formal biological knowledge of cells and also described the elephant as having more cells.
• Confusion about the differences between mitosis and meiosis, both functionally and in purpose, and where these two forms of cell division occur are common. This confusion may contribute to their failure to recognize that cell division contributes to growth (Flores, Tovar, and Gallegos 2003).
• Available research on cell size conceptions is limited. However, in piloting this probe with more than 100 middle and high school students in 2006, many students chose answer B (the blue whale has larger cells than a shrew). Their reasoning matched Stavy and Tirosh’s results and was based on the idea that whales are much larger and therefore need larger cells.

Aaron, R., B. Hug, and R. G. Duncan. 2017. Core idea LS1: From molecules to organisms: Structures and processes. In Disciplinary core ideas: Reshaping teaching and learning, ed. R. G. Duncan, J. Krajcik, and A. E. Rivet, 123–144. Arlington, VA: NSTA Press.

NGSS Archived Webinar: NGSS Core Ideas—From Molecules to Organisms: Structures and Processes, www.youtube.com/watch?v=aAfzbxbp3go&index=2 &list=PL2pHc_BEFW2JjWYua2_z3ccHEd6x5jIBK.

Rau, G. 2004. How small is a cell? The Science Teacher 71 (8): 38–41.

Williams, M., M. Linn, and G. Hollowell. 2008. Making mitosis visible. Science Scope 31 (7): 42–49.

• A lesson to go with this probe can start with the driving question “Why don’t we see cells the size of basketballs?”
• When students revisit this probe a second time after instruction and have the opportunity to revise their answer choice and explanation, encourage them to include the crosscutting concepts of scale, proportion, and quantity to explain their reasoning.
• When students are examining the same cell types of different organisms, encourage them to look not only at the similarity in the shape of the cells but also at the similarity in size. For example, when comparing the red blood cells of frogs to the red blood cells of humans, notice the similar size.
• Develop the idea that cell size is limited by the ability of molecules to pass in, around, and out of cells. Older students can test this idea by making model cells out of blocks of agar of different surface area-to-volume ratios and measuring the rate and depth of penetration of a dye into the model cell. Calculate the surface area-to-volume ratios of the different cell sizes and compare the results of the diffusion based on the ratios. Connect their results to how materials get into and out of a cell and travel within it.
• Have students investigate the question, “Is bigger always better?” in the context of a cell’s ability to carry out its life functions. Encourage them to develop a way to research and test their idea, and have them share their results.
• Ask students why a single-celled organism, such as a paramecium, can never be the size of a human. Develop the idea of why single-celled organisms must be microscopic to carry out the same life processes carried out by multicellular organisms.
• Have students research the largest and smallest animal cell and explain why there is a variation in size.