Why do you suppose cells use ca2 (which is kept by ca2 pumps at a cytosolic concentration of 10–7 m) for intracellular signaling and not another ion such as na (which is kept by the na pump at a cytosolic concentration of 10–3 m)

Cell uses Ca2+ for intra cellular signaling but not Na+ because Na+ is a potent regulator of plasma osmolarity as well as blood pressure.

Explanation:

If cell uses Na+ ion for intra cellular signalling then the normal plasma level of Na+ ion wil fluctuate during the occurrence of cell signaling.This fluctuation ultimately result in the disturbance of plasma osmolarity which will be very much harmful for the cell because disturbance in the plasma osmolarity can hamper the normal osmotic pressure that can either swells or shrinks the cell.

well in order to answer this question you have to provide more information < 3

c. 75.7

i think that is the right