I know they are small and they get inside cells. But how big are the viruses and the cells? My focus is the current Coronavirus strain, which causes COVID-19.
I have to say these are only my estimates after browsing scientific papers and articles…and in an area I know very little about. However I’m only looking for order-of-magnitude comparisons.
Name and Size
The name of the virus is Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) [1]
It “has round or elliptic and often pleomorphic form, and a diameter of approximately 60–140 nm.” [2] Pleomorphic means that it can assume different forms.
A nm is a nanometre, meaning one billionth of a metre. There about 10 million nm in one cm.
If we say that SARS-CoV-2 is about 100 nm across, about 100,000 of them could line up across your finger (1cm width)
How big are the Cells in our body?
The first thing I discovered was how wildly varying the sizes of the cells in our bodies are. Based on values from [3] it seems they have diameters from 4 to 100 to 200 micrometres.
A micrometre is one millionth of a metre, so is 1000 times bigger than a nanometer. In science we tend to describe things in units of ‘1000’s of the base unit’ (metre, here) or 1/1000s.
But which cells does SARS-CoV-2 invade? Researchers refer to “both of the proteins that SARS-CoV-2 uses to infect cells. In the lungs, they found the RNAs for these proteins mainly in cells called type II pneumocytes. These cells line the alveoli (air sacs) of the lungs and are responsible for keeping them open..” [4]
So, my reading is that I can probably use these type II pneumocytes as my example cell.
SARS-CoV-2 and Type II Pneumocyte Cell Relative Sizes
The cells “have a cubic shape and a diameter of 9 µm” [5] That’s puzzling; a cube with a diameter? A µm is a micrometer, so it’s on the smaller end of the above cellular diameter range.
We can make the maths easy by calling this 10 micrometer.
10 micrometre is 10,000 nm. The SARS-CoV-2 virus is about 100 nm across. So you could fit about 100 viruses across the cell.
Assume all these things are spheres, I’d estimate that 1,000,000 SARS-CoV-2 viruses could fit inside a single Type II Pneumocyte Cell.
This is absolutely to NOT suggest there are 1 million viruses in our cells! It’s just for me to understand the very rough scales of the things involved.
That name again
SARS-CoV-2 is part of Coronaviridae family, which is “named after their corona- or crown-like surface projections seen on electron microscopy” [5].
So there seems to be no way to see the tiny viruses in a ‘normal’ (optical) microscope and so we must use electron microscopes.
References
[1] https://www.thelancet.com/journals/laninf/article/PIIS1473-3099(20)30235-8/fulltext
[2] https://www.ncbi.nlm.nih.gov/books/NBK554776/
[3] http://book.bionumbers.org/how-big-is-a-human-cell/ Values given for volumes, converted to diameters by me. I’m just after estimates.
[4] https://www.sciencedaily.com/releases/2020/04/200422132556.htm
[5] https://www.sciencedirect.com/topics/biochemistry-genetics-and-molecular-biology/type-ii-pneumocyte