Numbers in Biology


Reading numbers and measurements:


31%                           k^3                                y^2
-70°F^                     x^

^

1,203.4                       10°C                           3a^4                        :


0.002

3.14                 0.631               =                     30.7°


6.7x10^-9km               2376.69            0.735µm     42.9 kmh^-1


How Many Carbon Atoms Are in a Cell? A cell with a volume of 1 µm3 and a density of about 1 g/ml
has a total mass of 10−12 grams. From the formula C[4]H[7]O[2]N and the weights of the elements, we
derive a carbon content of about 12 × 4/(12 × 4 + 7 + 2 × 16 + 14) = 48/101 or about one half of
the dry mass. With 30% dry mass (70% water), we obtain ~10^−13 gm of carbon. Next we transformed
the number of molecules using Avogadro’s constant: 6 × 10^23 × 10^−13/12 = 5 × 109 carbon atoms per
cell. To verify this, we have done the calculation in a different way: assuming there are about 3 ×
106 proteins, each one consisting of about 300 amino acids, we get a total of ?109 amino acids. An
amino acid has about five carbon atoms, so we arrive at a similar value. Both estimates depend
linearly on the cell volume, which can vary significantly based on growth conditions.