“How many cells are there in a 9-year old tree, in a flower and in an elephant?” – I was asked this question recently by an elementary school teacher, and I, as a biologist, should naturally know this answer. The students found out, by research, that the adult human body contains an estimated 10 trillion cells. Fascinated by this number, they asked the teacher on the number of cells of all sorts of organisms.

Estimating the number of cells should, mathematically, not be too difficult: We assume that an average eukaryotic cell is about 10 micro meters across. Further, we assume that a human cell is a cube. We calculate the volume, and then assume that the density of the cell is about like the density of water. This way we can compute the mass of a cell. You then simply weigh the organism, and multiply this mass by the number of cells in one kg, and voila: you have the number of cells in the body.

- Diameter of a cell: 10 micro meters (microns)
- Volume of a cell: 10x10x10 cubic microns = 1000 cubic microns
- If there are a billion (10⁹) cubic microns in a cubic mm, then this means that there are a million cells in a cubic mm.
- Consequently, there are a million million (10¹²) cells in a cubic decimeter (1dm³ = one liter). This happens to be one trillion cells in one liter of volume.
- We assume that 1 liter is about 1kg, assuming the density of water. There are therefore 10¹² cells in one kg.
- If we assume that the person has a mass of 80kg, then we obtain: 80×10¹² cells, this is 80 trillion cells.

This is a more than the estimated 10 trillion, but considering the wide range of cell sizes, I think it’s still an acceptable answer… But then again, what does a number like this really mean to 8 year-olds? What does 10 trillion mean to me? We simply lack every day experience with numbers of this scale.

Back to the original question: “How many cells are there in a 9-year old tree, in a flower and in an elephant?” The elephant one is easy to answer, if one knows the mass of an elephant. An elephant masses 7.5 tons (7500 kg), and is therefore 100 times heavier than a person. We therefore assume that it also contains 100 times more cells. If a human has 10 trillion cells (depending on the estimate), then this would account for 1000 trillion cells in an elephant.This is one quadrillion cells (10¹⁵). Maybe the kids are more interested in the names of these numbers than in the actual cell count…

But don’t forget that there are more prokaryotes (bacteria) growing on your body and in the digestive system than we have body cells. After all, they are about 1000x smaller in volume. Now things are really starting to become interesting. Just my 2 cents.