The importance of salt

A nice article in nature chemistry discusses salt (NaCl) and why it is so important,

It is an interesting read, but one aspect that I found disappointing was that it doesn't discuss the role of salt in biology. It points out that salt is essential for life, surely its most important property, but never explains why.

The reason he doesn't go into it may be because once salt enters the body it becomes dissolved in water and the positively charge sodium ion and the negatively charged chloride ion break apart and float freely in water, i.e., they form an electrolyte solution. Whether this is still salt then becomes a matter of definition, but I would argue that the fact we refer to it as "salt water" strongly implies it is.

The reason salt water is necessary for life is because of the central role ions play in a vast range of biological processes. Ions dissolved in water are charged and hence they can carry electrical currents and exert large electrical forces. The flow of ions in and out of cells is responsible for the contraction of muscles and the signals passed between neurons in your brain that make up consciousness. These ions also interact directly with the proteins affecting all sorts of biological functions.

The other fascinating aspect of these salt solutions is that because it is the charge which gives them their central role, the charge should be all that matters, i.e., you should be able to replace sodium chloride with lithium chloride for instance and it will have the same effect.

This could not be further from the truth though, as proven by the fact that that lithium chloride is an effective treatment for bipolar disorder, and is toxic even at low doses, which is worrying considering it used to be in 7up.

                                                        (From the Bonkers Institute)

The body also has some fascinating machinery, called ion pumps, designed to maintain strict concentrations of different ions in different parts of the body.

I am researching the mechanisms for this dramatic variation in the properties of electrolyte solutions for my PhD. The area is called "specific ion effects", and the goal is the modelling of electrolyte solutions in order to explain their properties, especially the dependence on the specific salt.