How Small is a Virus--Setting the Scale

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The student will compare the viruses with other living things.

Big Idea

Ever wonder how small a virus really is? Check this lesson out which compares virus size to common, everyday objects.

What Students Will Learn in this Lesson

1 minutes

It's the start of a new unit. A review of the metric system is in order. In this case, we need to focus on the very small. Students also focus on what it mean to be alive. Here is an overview of what students will learn today.  


3 minutes

Start the lesson by playing the CNN clip, Ebola in the News or this clip from KETV Newswatch 7 from Nebraska.  (I chose the latter and played from 1:48-4:00.




Explain to the students that in light of the recent news, it is important to understand what a virus is and how a virus spreads.  Today they are going to learn about the relative size of a virus.  

Is It Alive--What is a Virus?

7 minutes

Pose the following scenario to the class:  

  • Scientists are currently having a debate about viruses. Some scientists consider viruses to be living things. Other scientists do not think viruses are living things?  
  • In your lab notebook, write the word that you think best describe a virus:  living or nonliving.  Then explain your thinking.  What evidence do you have to support your claim whether viruses are living or nonliving?   


Using this assessment probe, elicit students’ ideas about what they consider to be characteristics of life. Using viruses as a context, have students explore the criteria scientists use to determine if something is considered a living or a nonliving thing. Do not reveal the correct answer to the students at this time.  Allow students to debate their ideas and inform them that you will revisit this assessment probe at the end of the unit to see if students have changed their thinking.

(Note:  There is no absolutely right or wrong answer to this question as scientists are still debating the issue. However, the better answer based on the evidence that will be presented in this unit is that viruses are nonliving. Viruses fall into a gray area between living and nonliving things. A virus is not a cell. It is just a piece of nucleic acid surrounded by a protein coat. It must live within a host cell to reproduce. It does not have membrane bound organelles or ribsomes of its own. It cannot make its own proteins.  A virus does not grow and mature like a living thing does.  Viruses do not respond to stimuli from their environment and they do not maintain homeostasis. The debate occurs because viruses have genes that can mutate and help the viruses have an survival advantage in its environment.)  

Setting the Scale

7 minutes

Using the Setting the Scale worksheet, review conversions in the metric system.  Remind students that it is important to consider which unit will best help explain the size of an object.  Show students a softball, a pencil, a sugar cube, and a human hair.  Ask students the following questions:  

  • What are the four different base units.   

(Answer: meter, gram, liter, Kelvin)

  • For what does the unit ‘m’ stand for?  


  • When measuring the length of following items: a softball, a pencil, a sugar cube, dime, and a human hair, what unit in the metric system would be the best to use? 

(Answer: centimeters for the softball and pencil, millimeters for the dime and sugar cube, micrometers for the human hair)

  • What would be the best scale to use when measuring a virus? 


Use the prefix table on the worksheet and review how to make metric conversions. Remind students that the metric system is a base 10 system.  Give students time to answer the questions on the worksheet.

  • How many centimeters are in a meter?  (100 cm)
  • Relate the number of micrometers to meters. (There are 10^6 micrometers in 1 meter)
  • What is the relationship between nanometers and meters?  (There are 10^9 nanometer in 1 meter)

(Note:  I also encourage students to place a summary of the worksheet in their lab notebook. Students can then refer back to these notes later in the unit to help them support any claims they might make. This is a good practice for them to start early in the year. Despite their best laid plans, students tend to forget important details that may help them support their reasoning as the unit or course progresses.)



Construction of the Scale

10 minutes

Using masking tape, have students make a 2 m long strip and apply it to the wall. Label this strip one. Repeat these instructions to make a second 2 m long strip. Apply the strip to the wall as well and label it strip two. Strip one will be a model that represents 2 meters.  Strip 2 will be a model that represents 2 millimeters.  The scale for strip two is 1m represents 1 mm.  

On strip 1, mark where 2 m, 1 m, 50 cm, 100 cm, 250 mm, 500 mm, 1,000 mm, 1,500 mm and 2,000 mm would be located. 

On strip 2, mark where 2 mm, 1 mm, 250 µm, 500 µm, 750 µm, 1,000 µm, 1,250 µm, 1,500 µm, 1,750 µm and 2,000 µm would be located.  Next mark where you would find 500,000 nm, 1,000,000 nm, 1,500,000 nm and 2,000,000 nm. 

Using sticky notes, label where a typical virus, the length of a rod bacterium, length of a red blood cell, width of a human hair, sugar cube, dime, pencil, and softball would be placed on both scales.  

Have students also complete the data table on the back of the worksheet.  



Putting It All Together: Visualizing Powers of Ten

12 minutes

Start by showing the last three minutes of the Power of Ten video

(I start the video at 5:56 and end at 9:00).  


After viewing the video, then have students construct the following model.  Make a one meter strip out of masking tape.  Label it 10 to the 0 power.  Next make a 10 centimeter strip (1 decimeter).  Label it 10 to the -1 power.  Next make a 1 centimeter strip.  Label it 10 to the -2 power.  Finally, make a one millimeter strip.  Label it 10 to the -3 power. 

In their lab notebook, have students make a number line that shows scale by powers of ten. Have them place a hand, skin cell, lymphocyte (white blood cell), the nucleus, DNA, a typical virus, carbon nucleus, and proton on the scale to show relative size. (The objects are listed correctly in descending order.)

Checking Student Comprehension

6 minutes

Students should complete the comprehension section of the activity. Students can start this section of the worksheet, but the majority of students will need to finish it as homework. It will prepare them for part 2 of the lesson.