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Middle School    |    Formative Assessment Probe

Cells and Size

By Page Keeley

Assessment Life Science Middle School

Sensemaking Checklist

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.

Cells and Size

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The purpose of this assessment probe is to elicit students’ ideas about the size of cells. The probe can be used to determine whether students recognize how small a cell is relative to other things.

Type of Probe

Justified List

Related Concepts

cell size, micrometer (or micron)


Although some of the choices depend on the size of a small object, the best choices are: thickness of a leaf, grain of salt, eye of an ant, width of a hair, piece of sawdust, tiny seed, bread crumb, larva of a tiny fruit fly, speck of pepper, period at end of a sentence, dust mite, frog embryo, point of a pin, and flea egg. Several of the items in this list are living things or parts of plants and animals and thus are made up of a collection of cells (the cell layers that make up the width of a leaf, sawdust, eye of an ant, tiny seed, larva of a tiny fruit fly, speck of pepper, dust mite, and microscopic frog embryo), which generally makes them larger than a single “typical” animal or plant cell. Chromosomes are organelles found within plant and animal cells, which make them smaller than a cell. Likewise, proteins, DNA, and water are molecules found within a cell and within cell structures, which reasons that they are also smaller than a cell. Bacteria are much smaller than animal and plant cells and viruses are much smaller than bacteria. The atom is the smallest particle of matter on the list. Trying to figure out the number of atoms in a cell is almost like trying to figure out the number of stars in the sky.

Generally, any very small object or particle of matter that can be seen with a handheld magnifying lens or the human eye (which can detect sizes up to about 0.1 mm) is larger than a cell. However, an object or organism does not have to be visible by the unaided eye to be larger than a cell. To get a quantitative sense of scale, cells are typically measured in micrometers (μm; also called microns). There are 1,000 μm in 1 mm. Most types of plant and animal cells generally range between 10 and 100 μm (some cells, like eggs, nerve cells, and muscle cells, are much larger than “average” cells). The point of a pin is about 1,500 μm. A grain of table salt is about 300 μm. The width of a human hair is about 200 μm and a flea’s egg is about 500 μm. Dust mites, bizarre looking multicellular animals, are still larger than typical cells even though they are typically photographed using electron microscopes. Several of these dust mites live at the base of your eyelashes and feed on secretions and dead skin cell debris! Dust mites range in size from 250 to 400 μm.

Typically, a microscope with magnification greater than 10× is needed to see most cells. Magnifications of 100× and more are needed to see things smaller than typical cells. While cells of different tissues vary in size, they are still much smaller than many of the things on the list. For example, a typical animal cheek cell is 60 μm, a red blood cell is about 8 μm, and a small leaf’s cell is about 30 μm. Bacteria are single-celled organisms that are much smaller than a plant or animal cell. E. coli, a common bacterium, measures 2 μm. The typical common cold virus measures 20 nm (nanometer). There are 1,000 nm in 1 μm. Even smaller is a water molecule. It measures about 0.2 nm!

Curricular and Instructional Considerations

Elementary Students

Students in the early elementary school grades use magnifying lenses to observe parts of living things that are too small to see clearly with their naked eye. Upper elementary students are just beginning to learn about cells and use simple microscopes to observe them. However, students’ conceptions of a cell’s very small size are limited by their ability to grasp very small magnitudes of scale.

Middle School Students

Students’ fine motor skills help them become more adept in using compound microscopes at the middle school level to view a variety of cells and small parts of organisms and objects. They can interpret what they see under a microscope, can determine the magnification of their view, and begin to connect the size of cells to numbers that are much smaller than a millimeter. However, small scales are still difficult for them to comprehend.

High School Students

At the high school level, students transition from the whole cell to looking at structures within the cell. They learn about the molecules that make up a cell. Their understandings encompass smaller scales, including a growing awareness of nanoscale and nanoscience. They use more sophisticated microscopes and microscopic techniques that allow them to see bacterial cells.

Administering the Probe

This probe can be used once students understand that all organisms are made up of cells. Remove items on the list that may be unfamiliar to students.

Related Disciplinary Core Ideas (NRC 2012; NGSS Lead States 2013)

6–8 LS1:A Structure and Function

  • All living things are made up of cells, which is the smallest unit that can be said to be alive. An organism may consist of one single cell (unicellular) or many different numbers and types of cells (multicellular).
Related Ideas in National Science Education Standards (NRC 1996)

K–4 Abilities Necessary to Do Scientific Inquiry

  • Employ simple equipment and tools (magnifiers and simple microscopes) to gather data and extend the senses.

5–8 Abilities Necessary to Do Scientific Inquiry

  • Use appropriate tools (microscopes) and techniques to gather, analyze, and interpret data.

5–8 Structure and Function in Living Systems

  • All organisms are composed of cells, the fundamental unit of life.
  • Groups of specialized cells cooperate to form a tissue, such as a muscle. Different tissues are grouped together to form larger functional units, called organs.

9–12 The Cell

  • Cells have particular structures that underlie their functions. Inside the cell is a concentrated mixture of thousands of different molecules that form a variety of specialized structures.
  • Most of the cells in a human contain two copies of each of 22 different chromosomes.
  • Each DNA molecule in a cell forms a single chromosome.
Related Ideas in Benchmarks for Science Literacy (AAAS 1993)

K–2 The Cell

  • Magnifiers help people see things that they would otherwise not be able to see.

K–2 Scale

  • Things in nature and things people make have very different sizes, weights, ages, and speeds.

3–5 The Cell

  • Microscopes make it possible to see that living things are made mostly of cells. Some organisms are made of a collection of similar cells that benefit from cooperating.*
  • Some living things consist of a single cell.

6–8 Cells

  • All living things are composed of cells, from just one to many millions, whose details usually are visible only through a microscope. Different body tissues and organs are made up of different kinds of cells.*

9–12 Cells

  • Within every cell are specialized parts.
  • The work of the cell is carried out by the many different types of molecules it assembles, mostly proteins.

*Indicates a strong match between the ideas elicited by the probe and a national standard’s learning goal.

Related Research

  • Studies have shown that students have difficulties with orders of magnitude. In a study of 16-year-old Israeli students (Dreyfus and Jungwirth 1988, 1989), students thought that molecules of protein were bigger than the size of a cell. Over a third of students’ responses showed “inadequate” ideas about cells (Driver et al. 1994).
  • Research conducted by Arnold (1983) indicated that students have difficulty differentiating between the concepts of cell and molecule. Students identified any materials encountered in biology class (carbohydrates, proteins, and water) as being made up of smaller units called cells. Arnold coined the term molecell to describe the notion of organic molecules being considered as cells.
  • The range of numbers people can grasp increases with age (AAAS 1993, p. 276).

Related NSTA Resources

American Association for the Advancement of Science (AAAS). 1993. Benchmarks for science literacy. New York: Oxford University Press.

Driver, R., A. Squires, P. Rushworth, and V. Wood- Robinson. 1994. Making sense of secondary science: Research into children’s ideas. London and New York: RoutledgeFalmer.

Jones, G., M. Falvo, A. Taylor, and P. Broadwell. 2007. Nanoscale science: Activities for grades 6–12. Arlington, VA: NSTA Press.

Keeley, P. 2005. Science curriculum topic study: Bridging the gap between standards and practice. Thousand Oaks, CA: Corwin Press.

National Research Council (NRC). 1996. National science education standards. Washington, DC: National Academy Press.

Suggestions for Instruction and Assessment

  • Be aware that describing cells as being “very small” is a relative term to students. Small compared to what? When teaching the concept of smallness of cells, make comparisons to things that are smaller and larger than a cell.
  • Provide students with opportunities to examine and compare very small things that their unaided eyes can detect, such as a grain of salt or width of a hair, to things their unaided eyes cannot detect, such as individual cells on a prepared microscope slide. Using a microscope, have students compare the two differently sized things, noting the difference in relative size under the same magnification.
  • Begin by having students in the early elementary grades use 3×–10× magnification hand lenses to magnify things. Encourage them to wonder what they might see with more powerful lenses (AAAS 1993).
  • By the upper elementary school grades, the magnification that students use should increase to 30×–100× magnification, using more powerful handheld viewers, dissection scopes, or simple microscopes. Students’ observations should include microscopic one-celled organisms, plant and animal cells, and small animals, such as brine shrimp. As they observe different types of cells, they should be encouraged to think about whether those cells could be seen without a microscope.
  • By middle school, students should have developed “magnification sense” and can extend their observations of cells using a microscope to photographs of cells, including bacteria, taken under much greater magnifications than their school microscopes can provide.
  • Middle school students are learning about the fundamental unit of matter (the atom) and molecules made up of atoms as well as the basic unit of life (the cell). When taught separately, there is the potential for misconceptions to develop related to the sizes of atoms, molecules, and cells. Explicitly address the size of a cell in comparison to atoms and molecules because some students think they are similar in size and fail to recognize that cells are made up of atoms and molecules. The Powers of Ten website, at, provides a source of representations to help students distinguish between the magnitudes of scale in observing cells versus atoms and molecules.
  • At the high school level, help students make explicit comparisons of the size of protein and DNA molecules to the size of a cell, recognizing that these molecules fit within cells. Combining this probe with “Is It Made of Cells?” and “Is It Made of Molecules?” from Volume 1 of this series (Keeley, Eberle, and Farrin 2005) may help reveal whether high school students hold the concept of molecell, as is indicated by research studies.

American Association for the Advancement of Science (AAAS). 1993. Benchmarks for science literacy. New York: Oxford University Press.

Arnold, B. 1983. Beware the molecell! Aberdeen College of Education. Biology Newsletter 42: 2–6.

Dreyfus, A., and E. Jungwirth. 1988. The cell concept of 10th graders: Curricular expectations and reality. International Journal of Science Education 10 (2): 221–229.

Dreyfus, A., and E. Jungwirth. 1989. The pupil and the living cell: A taxonomy of dysfunctional ideas about an abstract idea. Journal of Biological Education 23 (1): 49–53.

Driver, R., A. Squires, P. Rushworth, and V. Wood- Robinson. 1994. Making sense of secondary science: Research into children’s ideas. London and New York: RoutledgeFalmer.

Keeley, P. 2005. Science curriculum topic study: Bridging the gap between standards and practice. Thousand Oaks, CA: Corwin Press.

Keeley, P., F. Eberle, and L. Farrin. 2005. Uncovering student ideas in science: 25 formative assessment probes. Vol. 1. Arlington, VA: NSTA Press.

National Research Council (NRC). 1996. National science education standards. Washington, DC: National Academy Press.

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