Quoting:

OK...well, they're going to be mainly water anyway, so if you calculate a volume of 0.5 x 0.5 x 0.5 micron (=0.125 x 10 exp -18 cubic metres) with a density of 10 exp 3 kg/cubic metre...MW water is 18, Avogadro's number is 6.0225 x 10 exp 23, so 1.25 x 10 exp -16 kg is 4.2 x 10 exp 9 molecules, x3 for atoms = 13 000 000 000!!

And again: 0.5 micron = 0.5 x 10^-4 cm, volume cell = 12.5 x 10^-14 cm^3, mass (as density=1) = 12.5 x 10^-14 g, no. moles water = (about) 0.7 x 10^-14, multiplied by Avogadro's number (6.0223 x 10²³⁾ = 4 x 10⁹ molecules,

= 12 x 10⁹ atoms.

This is, of course, assuming SMALL cells (0.5 micron is small for a eukaryote) - but if you assume 10 micron cells for a SIMPLE eukaryote, this means 8 000 x more atoms - or 10^14, almost spot on....!

                 Ed Rybicki, PhD