Everyday Science
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Everyday Science

The Cell, The Germ, And The Virus

Looking at the microscopic side of life (and smaller)

Photo by CDC on Unsplash

Coronavirus. Everybody is talking about it, everybody is familiar with words like pandemic, social distancing, etc.

But what is a virus? how small is it? Lets’ figure out.

Starting from the cell

The human body — and also the bodies of the other living beings — are made of cells. A huge amount of cells.

Scientists estimated an average human body is made of around 3x10¹³,
30 trillions, that is: 30 000 000 000 000
By the way, this means there are more cells in your body than stars in the Milky Way.

Cell are the basic building blocks of living things and in multicellular beings (and humans are definitely very multicellular) cells are differentiated in order to carry on specific functions.

Cells of humans, mammals and all of multicellular beings have a nucleus, some organelles inside of them and are called Eukaryotic. Given the size of an average human and the number of cells it is made of, it comes out the cells are very, very small.

Eukaryotic cell’s diameter is between 10 and 100 micrometers, i.e. between 1/100 and 1/10 of a millimeter. To make a comparison, a 100 micrometer cell is as thick as a sheet of paper.

Structure of an eukaryotic cell. | LadyofHats, Public domain, via Wikimedia Commons

Smaller than the small ones

Bacteria are living being made of a single prokaryotic cell. prokaryotic cells differ from eukaryotic ones (the one we are made of) because they do not have a true nucleus, nor membrane-bounded organelles. Prokaryota are simpler and more ancient than eukaryota.

A diagram of a typical prokaryotic cell | LadyofHats, Public domain, via Wikimedia Commons

Bacteria are small, but numerous and even our body contains around 3.8x10¹³ bacteria, more than all the cell we are made of and corresponding to a weight of around 0.2 kilograms. We are used to think of bacteria as pathogens, but lot of them are not harmful and even beneficial or useful.

Typical prokaryotic (bacteria) cells are 0.1–5.0 micrometers in diameter, from half to 1/1000 the size of an eukaryotic cell. For a bacterium, an eukaryotic cell is the same size of a building is for a human.

Size comparison between an eukaryotic cell (the castle) and a bacterium (the knight) | Author elaboration of Pixabay pictures

Are they alive?

Viruses come in lot of different shapes but have one thing in common: they cannot reproduce by themselves but have to infect a cell and use it to replicate their nucleic acid. A virus is in fact nothing more than a chain of genetic material (DNA or RNA) enclosed in a protein envelope.

Viruses are even smaller than bacteria, their typical size is around 100 nanometers, 0.0001 millimeter. So small they cannot be seen using an optical microscope, even smaller than the wavelength of the light we can see.

Compared to a cell of our body a virus is like a mouse to a castle, so it is almost incredible how harm can do a thing as minuscule as this.

Size comparison between an eukaryotic cell, a bacterium and a virus | Author elaboration of Pixabay pictures

If you want to see more about sizes of cells, bacteria and viruses, take a look at this video.

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Diego Tonini

Diego Tonini


Writer and science communication enthusiast, I think science can be fun and everyone can understand it!