One thousand $\mathrm{Ca}^{2+}$ channels open in the plasma membrane of a cell that is $1000 \mu \mathrm{m}^{3}$ in size and has a cytosolic $\mathrm{Ca}^{2+}$ concentration of $100 \mathrm{nM}$. For how long would the channels need to stay open in order for the cytosolic $\mathrm{Ca}^{2+}$ concentration to rise to $5 \mu \mathrm{M} ?$ There is virtually unlimited $\mathrm{Ca}^{2+}$ available in the outside medium (the extracellular $\mathrm{Ca}^{2+}$ concentration in which most animal cells live is a few millimolar), and each channel passes $10^{6} \mathrm{Ca}^{2+}$ ions per second.