Intuiting Biological Scales using Human Scales

Figure 1. Human Scale Key: Aircraft Carrier (200m), 3-story House (20m), Human (2m), Liver/Gerbil (200mm), Lymph Node/Mosquito (20mm), Skin/Flee (2mm), Hair thickness (200um)
Biological Scale Key: Flee (2mm), Amoeba (200um), Eukaryotic Cell (20um), Mitochondria/Bacteria (2um), Centriol/Large Virus (200nm), Cell Membrane/Small Virus (20nm), dsDNA thickness (2nm)

The length scale of molecular and cellular biology (Figure 1 Biological Scales) covers approximately 5 orders of magnitude, with sizes ranging from 1 nanometer (approximate size of small molecules) to 100 micrometers (approximate size of the largest cells). Unfortunately, it can be difficult to intuit these relative sizes because we cannot directly observe these scales.

Luckily, length scales in the human perspective (Figure 1 Human Scale) also span about 5 orders of magnitude, with sizes ranging from 100 micrometers (width of human hair) to 20 meters (height of 3-4 story building). By simply imagining the sizes of the objects in the Human Scales relative to you (who are presumably human), you can intuit the relative sizes of biological organisms and molecules relative to a cell. This leads to some natural analogies where, for example, the relative sizes of small animals to you are approximately similar to the relative sizes between bacteria and a cell, and viruses “look” to a cell like a mosquito looks to you.

Bio-scale Sheet PDF for mobile devices
Creative Commons License
This work by Eugene Douglass and Chad Miller is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License.

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