I really have no answer, nor even a clue really, about this question.
I was looking at something else entirely and it just jumped out at me.
The ocean is mostly NaCl so very sodium rich. Our bodies (and plants and…) are very potassium (K) rich.
One would expect that life, evolving in a primitive ocean, would have evolved in balance with the natural mineral levels in that ocean. So what is with the high K level in cytoplasm? Hmmm?
We do know that sodium chloride levels in the ocean are higher now than when fish evolved, as many fish species have active sodium chloride transport systems to keep their overall salinity in balance, so one can guess that over all of the duration of life sodium levels have risen. So was the ancient sea potassium rich? Or not? What do the oldest ocean sediments say, if anything?
Or was this the only way complex life could work, making voltage across nerve cell walls being important to us, bacteria not so much?
Somehow it just seems wrong. We ought to be more sodium tolerant.
OK, some kicking around gives a few interesting links, but nothing that seems to answer the question of “why so much potassium”. Then again, I didn’t “dig here!” much. Just enough to get frustrated at all the OTHER things it leads to…
These folks look at bacterial composition, but ignore K vs Na near as I can tell:
This article inspects the sodium potassium pump, finding it necessary for nerves to function and with high K inside and high Na outside the cell, but doesn’t bother to ask about the overall high need for K in life:
ACTIVE TRANSPORT: THE SODIUM-POTASSIUM PUMP
Since the plasma membrane of the neuron is highly permeable to K+ and slightly permeable to Na+, and since neither of these ions is in a state of equilibrium (Na+ being at higher concentration outside the cell than inside and K+ at higher concentration inside the cell), then a natural occurrence should be the diffusion of both ions down their electrochemical gradients—K+ out of the cell and Na+ into the cell. However, the concentrations of these ions are maintained at constant disequilibrium, indicating that there is a compensatory mechanism moving Na+ outward against its concentration gradient and K+ inward. This mechanism is the sodium-potassium pump.
We spend a lot of energy on Na pumping:
The Na+-K+-ATPase is a highly-conserved integral membrane protein that is expressed in virtually all cells of higher organisms. As one measure of their importance, it has been estimated that roughly 25% of all cytoplasmic ATP is hydrolyzed by sodium pumps in resting humans. In nerve cells, approximately 70% of the ATP is consumed to fuel sodium pumps.
Physiologic and Pathologic Significance
The ionic transport conducted by sodium pumps creates both an electrical and chemical gradient across the plasma membrane. This is critical not only for that cell but, in many cases, for directional fluid and electrolyte movement across epithelial sheets. Some key examples include:
The cell’s resting membrane potential is a manifestation of the electrical gradient, and the gradient is the basis for excitability in nerve and muscle cells.
Export of sodium from the cell provides the driving force for several facilitated transporters, which import glucose, amino acids and other nutrients into the cell.
Translocation of sodium from one side of an epithelium to the other side creates an osmostic gradient that drives absorption of water. Important instances of this phenomenon can be found in the absorption of water from the lumen of the small intestine and in the kidney.
Depending on cell type, there are between 800,000 and 30 million pumps on the surface of cells. They may be distributed fairly evenly, or clustered in certain membrane domains, as in the basolateral membranes of polarized epithelial cells in the kidney and intestine.
Clearly it is very important to “virtually all cells of higher organisms”. Though that just makes me wish for a similar look at ‘lower organisms’… IFF we find that bacteria are closer to seawater Na/K ratio, the question changes to ‘why is higher life’ this way…
but my search-foo was not working well on bacterial Na/K cytoplasm ratios… so I’m taking a break from that search series while I reconsider search words.
So why do we end up with about a 1.6 : 1 ratio of K : Na while sea water has a 0.037 : 1 ratio?
Something just seems terribly strange about that…
Perhaps K is much more concentrated near Black Smokers, or in primordial mud, or wherever life started. It seems like a clue to something, but I can’t see “what?”… Maybe it’s time to swap over from tea to wine and move from analytical to insightful ;-)