91Ó°ÊÓ

Practise radioactivity interconversions. Express the following values in the alternative units indicated, with appropriate prefixes as necessary. Answer to three significant figures. (a) 72000 d.p.m. as Bq; (b) \(20 \mu \mathrm{Ci}\) as d.p.m.; (c) \(44400 \mathrm{~Bq}\) as \(\mu \mathrm{Ci}\); (d) \(6.3 \times 10^{5}\) d.p.m. \(\mathrm{mol}^{-1} \mathrm{as} \mathrm{Bq} \mathrm{g}^{-1}\), for a compound with a relative molecular mass of 350 ; (e) 3108 d.p.m. as pmol, for a sample of a standard where the specific activity is stated as \(50 \mathrm{Ci} \mathrm{mol}^{-1}\).

Use the concept of specific activity in calculations. A researcher wishes to estimate the rate of uptake of the sugar galactose by carrot cells in a suspension culture. She prepares \(250 \mathrm{~mL}\) of the cell culture medium containing \(10^{7}\) cells per \(\mathrm{mL}\) and unlabelled galactose ata concentration of \(5 \mathrm{mmol} \mathrm{L}^{-1}\). She then 'spikes' this with \(5 \mu \mathrm{L}\) (regard this as an insignificant volume) of radioactive standard containing \(55 \mathrm{MBq}\) of \({ }^{14} \mathrm{C}\)-labelled galactose (regard as an insignificant concentration). Answer to two significant figures. (a) Calculate the specific activity of the galactose in the culture solution in \(\mathrm{Bq} \mathrm{mol}^{-1}\). (b) If the total cell sample takes up \(79.2 \times 10^{5} \mathrm{~Bq}\) in a \(2-h\) period, calculate the galactose uptake rate in \(\mathrm{mol} \mathrm{s}^{-1}\) cell \(^{-1}\).