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Life Size

Science Snack
Life Size
Explore the size and scale of microscopic biology.
Life Size
Explore the size and scale of microscopic biology.

Not only is it hard to conceptualize things that are too small to see, it’s often surprising to discover the range of sizes in the microscopic world. This Snack compares the relative sizes of biological objects that can’t be seen by the naked eye.

COVID-19 Learning Note: Viruses are much smaller than human cells; they’re even smaller than the bacteria in our bodies. Their tiny size makes them too small to see under a light microscope and detectable only by other means. It also means they are small enough to catch a ride on a tiny sneeze droplet.

Tools and Materials
  • Printout of this attachment (provides three sets of labels)
  • Scissors (to cut out labels)
  • A large table to work on

Cut out the labels and separate into sets.

To Do and Notice

Guess the relative sizes of each of the objects listed and put them in order, from smallest to largest.

How did you do? See the list below.

What's Going On?

When you try to arrange these items according to size, you get a feel for the point at which things become difficult to visually imagine. You can easily see ants and 24-point periods, for example, but what about a bacterium or the droplet from a sneeze?

Humans only directly interact with things that are of a size within a few orders of magnitude of our length scale, around 1 meter. It’s difficult to grasp the relative sizes of things at length scales much smaller than we can see with our eyes. They just seem “small” or “microscopic.”

The list below puts each of the objects here in order of their approximate size.

  • DNA (diameter of a single strand): 2 nanometers
  • protein: 5 to 50 nanometers
  • antibody (large protein): 15 nanometers
  • ribosome: 25 nanometers
  • virus: 20 to 100 nanometers
  • N95 (professional-grade) mask pore size: 300 nanometers
  • wavelength of blue light: 400 nanometers
  • sneeze droplet: 0.5 to 12 micrometers
  • bacteria: 1 to 5 micrometers
  • human cell: 10 to 100 micrometers
  • period in 24-point font: 1 millimeter
  • ant: 5 millimeters


Metric Measurement of Size

1 meter (m) = 1m

10 decimeters (dm) = 1m

100 centimeters (cm) = 1m

1,000 millimeters (mm) = 1m

1,000,000 micrometers (µm) = 1m

1,000,000,000 nanometers (nm) = 1m

Going Further

Try spacing the labels for each item relative to each other, as if they were representing values on a ruler. What’s the best way to space them so that everything can fit? If we wanted to see what was going on inside a virus, how much would we have to magnify it?

You may notice that lining up these items by size makes them hard to space out on a table. Biological interactions occur on length scales that span several orders of magnitude. When dealing with these size ranges, it can be easier to use a logarithmic scale, where things are spaced over powers of ten.