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 ```100fmoles of insulin receptors displayed by 1 million beta cells in binding assay. How do you calculate the number of receptors on 1 cell?```
 ```Dear Val, Thanks for your question. A mole is an Avagadro's number of particles. Avagadro's number is 6.02x10^23. The significance of this number is that for this number of particles of any homogeneous substance (atoms, molecules), the weight in grams will equal the substance's atomic weight in Daltons. An example: If you have 6.02x10^23 hydrogen atoms, each of which has a mass of 1 Dalton, then the mass of this large number of hydrogen atoms is 1 gram. Now to your question. 100 fmoles is 100 femtomoles of insulin receptors. You don't need to know the mass of the insulin receptors to solve the problem. 100 femtomoles = 100 x (10^-15) moles [femto means 10^-15] So, we have 100 x 10^-15 moles of insulin receptors on 10^6 beta cells. If we assume that the receptors are distributed evenly (a big assumption in vivo), then we have 100 x 10^-15 x 6.02 x 10^23 receptors / 10^6 beta cells = 6.02 x 10^4 receptors / cell = 60,200 receptors / cell. Final answer: 60,200 receptors / cell Here's a page on moles and Avogadro's number for more details: http://en.wikipedia.org/wiki/Mole_%28unit%29 If you're interested, here is a page on Avogadro himself: http://en.wikipedia.org/wiki/Avogadro I hope this information was helpful. Feel free to ask for clarification. -welte-ga```