3.3 Boffin questions

Estimating large numbers

  1. How many proteins are in a HeLa cell?
A photomicrograph showing a close-up of Hela cells grown as a monolayer. Nuclei and some indistinct subcellular structures are visible. A scale bar indicates the cells are about 40 micrometres in length.
Figure 3.1: Phase contrast image of Hela cells in culture by Paul Anastasiadis, Eike Weiß from Fraunhofer Institute for Biomedical Technology, St. Ingbert, German (‘HELA Cels’ by Fraunhofer Institute for Biomedical Technology, St. Ingbert, Paul Anastasiadis, Eike Weiß from Wikimedia Commons used under CC BY-SA 3.0)

Use the following data to estimate the number of protein molecules in a HeLa cell:

  • Protein mass per volume ≈ 0.3 g. ml-1
  • Mass of a typical amino acid ≈ 100 Da
  • Average length of a protein ≈ 400 amino acids
  • Volume of a HeLa cell ≈ 2,000 µm3
  • Avogadro’s number = 6 × 1023

Do not be afraid of the large numbers here! Work through this methodically and make sure you write down the units at each step.


Side Bar – The symbol, Da, refers to daltons (named after the English chemist and physicist, John Dalton) which is also known as the atomic mass unit. It is defined as 1/12th of the mass of a carbon-12 atom which is approximately equal to 1.660 x 10-27 kg. The mole is a unit of substance that was originally defined so that the mass of one mole of a substance, measured in grams, would be numerically equal to the average mass of one of its constituent particles, measured in daltons. So, carbon-12 (6 neutrons and 6 protons) is 12 Da and therefore the mass of one mole of carbon-12 is approximately 12 g. The molar weight of large molecules such as proteins or protein complexes can be expressed in kilodaltons (kDa) or megadaltons (MDa) as appropriate.

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