So, I was browsing the Wikipedia article for rubidium, and came across this interesting tidbit:
Rubidium is not a known nutrient for any living organisms. However, rubidium ions have the same charge as potassium ions and are actively taken up and treated by animal cells in similar ways.
This immediately struck me as interesting, and a completely wrong explanation, as all singly-ionized ions have the same net charge.
Now, from my basic understanding of chemistry, elements of the same group have the same valence electron structure, which accounts for much of the element's chemical effects. And if you look at the periodic table, sure enough, rubidium falls below potassium, in the Group 1 alkali metals.
But, from my old memories of basic biology, even saying "these elements are vertically adjacent on the periodic table" is an incomplete explanation, because, for example, the body has no problem "telling apart" sodium from potassium, or plenty of other vertically adjacent elements.
A survey of other potentially relevant features reveals the calculated radius of rubidium (265 pm) is relatively close to the radius of potassium (243 pm), and the first ionization energy of rubidium (403.0 kJ/mol) is also pretty close to that of potassium (418.8 kJ/mol). So they do seem fairly similar, but question still remains, how "similar" does an element need to be to become a biological proxy?
This leads to me to the following questions:
- Why does rubidium act as a biological proxy for potassium? Also:
- Why is cesium also a decent, but less good, biological proxy for potassium?
- Why is potassium not a biological proxy for sodium?
- If rubidium acts as a biological proxy, why is it still not a "nutrient" and not considered to play a biological "role"?
- Which other sets of elements can act as biological proxies for each other? (I presume expanding this question to molecules yields too many answers to be useful.)
- Is this rubidium-potassium proxy effect specific to animals or to all life?
Some good references I've found, skimming the literature (I started by looking at some references in this article):
- Relman AS. The physiological behavior of rubidium and cesium in relation to that of potassium. Yale J Biol Med. 1956;29(3):248–62.
- Relman notes that this phenomenon was first noted by Sydney Ringer in experiments begun 1882, published in the Journal of Physiology in the year 1884 (or so).
- Fieve RR, Meltzer H, Dunner DL, Levitt M, Mendlewicz J, Thomas A. Rubidium: biochemical, behavioral, and metabolic studies in humans. Am J Psychiatry. 1973;130(1):55–61.
But I'd rather someone present an intuitive explanation for this before or after I attempt to dive in.