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Comparing Sizes of Microorganisms

Comparing Sizes of Microorganisms

Use of a microscope reticule allows measurement of a Spirogyra.
Courtesy of David R. Caprette, PhD, Rice University.

  • Grades:
  • Length: 60 Minutes


Students create scale models of microorganisms, compare their relative sizes, and discover that microbes come in many different sizes and shapes.

This activity is from The Science of Microbes Teacher's Guide and is most appropriate for use with students in grades 6–8. Lessons from the guide may be used with other grade levels as deemed appropriate.

The guide is available in print format.

This work was developed in partnership with the Baylor-UT Houston Center for AIDS Research, an NIH-funded program.

Teacher Background

Microbes are organisms too small to be seen with the naked eye. Even so, there are enormous variations in size and type among microbes. This activity allows students to compare the sizes of various microorganisms relative to an object with a standard size (0.5 mm) that is visible without magnification. Students will compare microbes (listed on the Microbe Scaling Chart), which range from an amoeba—measuring 300 micrometers (equivalent to 0.3 millimeters) in diameter or larger—to the polio virus, which is only 0.03 micrometers in length.

Students will use metric measurements for their calculations.

Objectives and Standards


  • Think critically and logically to make the relationships between evidence and explanations.

  • Communicate scientific procedures and explanations.

  • Use mathematics in all aspects of scientific inquiry.

  • Develop descriptions, explanations, predictions, and models using evidence.

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

Life Science

  • Living systems at all levels of organization demonstrate the complementary nature of structure and function.

  • Millions of species of animals, plants, and microorganisms are alive today. Although different species might look dissimilar, the unity among organisms becomes apparent from an analysis of internal structures, the similarity of their chemical processes, and the evidence of common ancestry.

  • All organisms are composed of cells—the fundamental unit of life. Most organisms are single cells; other organisms, including humans, are multicellular.

Materials and Setup

Teacher Materials (see Setup)

  • paper square (2.5 m x 2.5 m)

Materials per Group of Students

  • set of 4 prepared text strips

  • 4 hand lenses

  • 4 metric rulers marked in millimeters

  • 4 pairs of scissors

  • assorted markers or colored pencils

  • meter stick

  • paper (or science notebook)

  • several sheets of colored or plain paper, or a roll of chart or kraft paper

  • tape or glue

  • copy of student sheet (see Lesson pdf)

  • group concept map (ongoing)


  1. Use a word processing program to type the text passage below on one line (using 12-point Helvetica font).

  2. “The period at the end of this sentence is larger than a/an                          .”

  3. Create a total of 24 rows of text with this same phrase. Print the page and cut into strips so that each strip contains one sentence (one strip per student). Do not photocopy the page, as this will reduce the sharpness of the printed text, particularly the period at the end of the sentence. Create a model of the period character by making one 2.5-m x 2.5-m (250-cm x 250-cm) paper square. The square represents the period character enlarged 5,000 times. Obtain several sheets of plain or colored paper, or a roll of chart or kraft paper, for students to use to make large microbe models.

  4. Make copies of the student sheet. Place materials for each group on trays in a central location.

Procedure and Extensions

  1. Call students’ attention to the prepared strips of paper.

  2. Ask students to examine the periods at the end of the phrase, first with their eyes only, and then with a hand lens. Tell students to draw what the period looked like in each case. Discuss their observations. Ask, Did the period appear the same when it was magnified as when you observed it with a naked eye? (When magnified, the periods are square.)

  3. Have students record their observations in science notebooks or on sheets of paper.

  4. Ask, What can you say about the size of the period? Tell students the period is about 0.5 millimeters, or 500 micrometers, in length and width. Have students identify the centimeter and millimeter markings on a centimeter ruler. Ask, How many periods could be lined up, end-to-end, within a meter? (2,000)

  5. Before continuing, you may wish to review the metric system. Explain that the meter is the fundamental unit of length in the metric system. At 39.37 inches, a meter is slightly longer than a yard (36 inches). A centimeter is approximately the width of an average fingernail (0.3937 inches). Ask students, How many centimeters make a meter? Hopefully, they will say “100” (the prefix “centi” is Latin for one hundred). Ask, How many millimeters make a meter? (1,000; the prefix “milli” signifies one thousand.) Thus, one centimeter (cm) is equivalent to 10 millimeters (mm).

  6. Introduce students to an even smaller measure, the micrometer (µm), or micron, which is one millionth (or 10-6) of a meter. Mention that a micrometer is a measure too small for the naked eye to see and that one centimeter contains 10,000 micrometers. Ask, What is the size, in micrometers, of the period you observed? (500 µm) Follow by asking, Why is the ruler not divided into micrometers? (markings would be too small)

  7. Ask students, What do you know about scale models? For example, you might mention a road map or a model of the solar system. Ask, Why do we make scale models? (To understand the relative position, size, or distance of objects.)

  8. Next, tell students that they are going to make a scale model of microbes, called a “Microbial Mural,” using the size of the period as the scale standard. Ask, If I increased the length and width of the period by 5,000 times, what shape would it have? (square) How large do you think it would be? (If the size of the period is 0.5 mm, multiply 0.5 mm x 5,000. Answer: 2,500 mm x 2,500 mm, which is equivalent to 2.5 m x 2.5 m.)

  9. Bring out the prepared square of paper (period model) and display it on the wall. Explain that the sheet represents the size of the period enlarged 5,000 times. Ask, If we enlarged most microbes 5,000 times, do you think they would be larger or smaller than the period? (Even when enlarged 5,000 times, each of the microbe models will fit on the period.)

  10. Distribute the student sheets and assign each group several microbes. Instruct students to make scale drawings or artwork of each of their assigned microbes, based on the line drawings and sizes provided on the chart. Depending on students’ ages and experience, you may want to give them only the information from one or both of the “approximate actual size” columns and have each group calculate the scale sizes of their organism models.

    Note: Make certain every group is assigned at least one microbe large enough to draw (organisms 1–7). The remaining microbes are so small that some will be represented only by a tiny dot on the mural. The microbe sizes described on the "Microbe Scaling Chart" page represent typical measurements within the normal size range for each kind of organism. Students may find references to different sizes if they are conducting additional research about the organisms. In addition, some organisms named on the chart actually represent relatively large groups of related species or forms. For example, there are approximately 150 different known species of Euglena.

  11. Have students place their models on the large paper square. This is an effective way for students to self-check.

  12. Discuss the mural with students. Ask students if they could use the names of the any of the microbes on the mural to complete the sentence on their sentence strips. Revisit the concept maps and have students add information from this activity.


  • Have students research other microbes and create additional scale models to add to the Microbial Mural.

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Science Education Partnership Award, NIH

Science Education Partnership Award, NIH

Grant Number: 5R25RR01860