### Capsule 1

## How big is a coronavirus?

#### by Julian Venables and Philippe Fort

A human is made up of cells that are like little discrete beings with their external barrier, much like their skin. This ‘skin’ is called the membrane. A 74 kg person is made up of 37 trillion cells! So we are like a cube of cells with 33,300 X 33,000 X 33,300 cells on each side. This 74 kg person has a volume of about 74 litres because as swimmers know well our bodies have about the same density as water. So in every kilo, or litre, of their body they have, 37 trillion/ divided by 74, cells. That makes 500 billion cells in each litre. A millilitre is one thousandth of a litre. Therefore there are about 500 million cells in each millilitre, or cubic centimetre, of our body. As each centimetre cubed has a height, width and breadth of 1cm, you have to pile up 800 cells per centimetre in each direction to fit in the 500 million cells. Work out 800 X 800 X 800 and you will see that is about right. Each cell has a diameter of 1/ 800^{th} of a centimetre or 12.5 µm (micrometre, millionths of a metre). That is the average size of a cell, bearing in mind that the smallest cell in the human body is the spermatozoid (5µm) and the largest is the oocyte, with a diameter of 120 µm (a hundred thousand times bigger in volume than a sperm then).

There you go. You understand now that our cells are miniscule, right? Not if you’re a virus! The coronavirus has a diameter of only 120 __n__m (nanometres = billionths of a meter!), so it has just one hundredth of the diameter of our cells. How many viruses can you put in a cell? More than the number of giraffes in a Mini Cooper obviously but how many roughly? As with the calculation above the volume is the product of 3 dimensions. Therefore about a million (100 x 100 x 100) viruses can fit in a cell.

If you like, draw a funny picture that illustrates what you have just read. Looking forward to hearing your questions and seeing your drawings!