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How many cells are there in the human body?

“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.
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