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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.

segment of the periodic table

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):

But I'd rather someone present an intuitive explanation for this before or after I attempt to dive in.

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    $\begingroup$ Not what you'd call a useful proxy, but arsenic is poisonous because it's similar enough to phosphorus to act as a substitute in the ADP/ATP cycle, but then fails to provide energy to cells the way ATP does. $\endgroup$ – Guntram Blohm Feb 17 at 13:12
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    $\begingroup$ To be fair, the Wikipedia article does not actually say that rubidium ions are taken up in the same way as potassium BECAUSE they have the same charge. It just says that they have the same charge AND are taken up in the same way, which is true. A similar statement appears at the end of the article. $\endgroup$ – barbecue Feb 17 at 16:20
  • $\begingroup$ @barbecue Yes, I realized this later on. Still, I think that sentence in Wikipedia could have been phrased more clearly. $\endgroup$ – Jonathan Jeffrey Feb 17 at 17:13
  • $\begingroup$ @JonathanJeffrey someone seems to have updated the article to make it clearer, was that you? $\endgroup$ – barbecue Feb 17 at 17:20
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    $\begingroup$ As a side note, we use this property in nuclear medicine. Rb-82 is radioactive and can be used in heart studies - myocardial PET scans - as it enters the myocardial cells in a similar way to potassium. Pertechnetate - TcO^-4 is similar to iodine, and there are a lot of other examples. $\endgroup$ – Stefan Feb 17 at 18:23
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There are multiple biological mechanisms that can be brought to bear for distinguishing between atoms. In addition to binding properties (e.g., ionic charge, electronegativity, bond strength), there is also the size of the atoms and even their vibrational properties.

For one of these mechanisms to actually be used, however, it needs to be evolutionarily selected for in some way. The binding properties are by far the simplest to select for, since they are fairly local and permissive in nature, while the others require much more carefully tuned mechanisms.

Thus, we should expect to see most biological selectivity to be developed in terms of binding properties, as seems to be the case. For same-group covalent-binding such as one encounters with oxygen vs. sulfur, there are major differences in the properties of the bonds, giving ample opportunity for biological selectivity.

For Group 1 alkali metals like sodium, potassium, and rubidium, however, ionic bonding is much more uniform. Thus, we should expect mechanisms that apply to one to apply to all unless strongly selected for distinguishing. In the case of potassium versus sodium, which are both very important in biochemistry, a recently discovered mechanism in sodium-potassium pumps uses binding properties to grab both, then atomic size (via steric hindrance) to distinguish sodium versus potassium as described nicely in the answers to this question.

Presumably, there simply has not been sufficient evolutionary pressure to establish mechanisms for distinguishing rubidium from potassium in the same way. While sodium and potassium have approximately equal abundance, rubidium is approximately two orders of magnitude less abundant than either. Cesium, in turn, is nearly two orders of magnitude less than rubidium, and rapidly decaying francium mostly doesn't exist.

Thus, unless there is a particularly problematic effect of occasional incorporation of rubidium or cesium, then, we should expect these elements to be essentially indistinguishable from potassium biologically, but not from sodium, since sodium/potassium distinguishing mechanisms will reject them all along with potassium into the "not the right size to be sodium" bin.

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    $\begingroup$ @user338907 That's fascinating about lithium... do you have any information about mechanism? To me, that would suggest that the sodium selectivity is not just "small enough" but "just the right size", which would cause lithium to not work right either. It sounds like we're getting to the edge of known science here, however. $\endgroup$ – jakebeal Feb 16 at 14:43
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    $\begingroup$ Excellent answer. I have to say, the pore selectivity in (for example) voltage-gated channels permeable to sodium or potassium but not both is something that I find to be one of the most remarkable feats in evolution. $\endgroup$ – Bryan Krause Feb 16 at 16:28
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    $\begingroup$ Nice answer. Upon looking at things more, it appears the verdict is out on whether rubidium is an essential nutritional element (i.e. a mineral) or not for humans and other animals. How that ongoing research bears with the question of how animals evolved rubidium-potassium compatibility is an interesting question. $\endgroup$ – Jonathan Jeffrey Feb 16 at 23:14
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    $\begingroup$ Although I accepted this answer, I think there are still lots of interesting insights and questions in this area, so I welcome others to write their own answers. (I myself am currently being quite fascinated by this paper that describes how some bacteria require lathanides, but can't tell them apart!) $\endgroup$ – Jonathan Jeffrey Feb 16 at 23:25
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    $\begingroup$ I've just asked Help understanding how "steric effects" are distinct from "electronic effects"? $\endgroup$ – uhoh Feb 17 at 9:15

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