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.

hi my dude or dudette

hydrocarbons are organic compounds that contain only hydrogens and carbons. they can be saturated (those which contain only carbon-carbon single bonds) or unsaturated (those which contain carbon-carbon double or triple bonds) in nature. hydrocarbons can undergo several reactions like substitution, elimination etc.  

on substituting one or more hydrogen atoms in hydrocarbon it results in the formation of haloalkane. as the halogen atoms are large compared to the carbon and hydrogen atoms, due to which the molecular weight increases and the bond become polar because of presence electronegative halogen atom and results in the increase in boiling point of the haloalkane.

thus, the correct option is (b) that is the boiling point of the new compound increases on substituting a hydrogen atom with a halogen in a hydrocarbon.