Of the twelve well-known atomic constituents of our body eleven elements have specific properties obviously relevant to their rôle, making them indispensable. Oxygen (electronegativity and valence), carbon and hydrogen (both elements have virtually unique properties), nitrogen (amino group), calcium (calcium(II) is necessary for bones in vertebrates), phosphorus (use of phosphate in nucleotides and ATP), sulfur (–S–S– bridges), sodium (Na+ is a small cation), potassium (K+ is somewhat larger, together with its companion and presumed substitute Rb+), iron (used in hemoglobin because of properties of a transition metal), magnesium (Mg2+ takes part in the ATP cycle, if not anything else).

What about chlorine? A search resulted in

Chloride is involved in the exchange of oxygen and carbon dioxide in the red cells. In this, chloride passes into the plasma (chloride shift) and bicarbonate into the cells.

(https://www.sciencedirect.com/topics/biochemistry-genetics-and-molecular-biology/chloride), certain use by muscle cells and also some mention of ligand-gated chloride ion channels in neurons. Chloride in stomach secretion may be dismissed because is used as a generic acid (that is, to complement H+), not for its own special properties. And no mention of biological chlorine compounds other than ionic (such as, having a C–Cl bound) can be quickly found.

Which cells absolutely need specifically Cl to function? Is the anion mandatory for animals to live?

  • $\begingroup$ I am not sure I am clear what you are asking, but mutations in the chloride channel are known to cause cystic fibrosis (a major breakthrough in the understanding of this disease) $\endgroup$
    – user338907
    Apr 15 at 19:08
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    $\begingroup$ @user338907 relevant, but again: so named “chloride channels” are not necessarily selective. Whereas a sodium channel hardly can pump anything but Na⁺ (the rare Li⁺, perhaps?), anion channels conduct mostly Cl⁻ because of abundance, but generally who-knows-what. $\endgroup$ Apr 20 at 10:07
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    $\begingroup$ OK. As regards your second-last sentence, there is Gribble, and chloramphenicol comes to mind. (Extending things to the halogens, there is fluroacetate) $\endgroup$
    – user338907
    Apr 20 at 10:28

My answer may be a little basic, but I think it is comprehensive:

Many elements are special; for instance, many metal ions such as iron or cobalt are required as cofactors or in catalytic sites due to their chemistry. Oxygen is highly electronegative, that also grants it some of its functions. But... without delving too much into things... chloride is the most basic constituent of saline, without which it would not be saline. Here are 3 very, very important and yet

  • It works well with alkali and alkaline earth metals, hydrogen, and many compounds... it's a natural pairing!
  • You cannot separate positive and negative charges in a solution at a macroscopic level. It's not a feasible thing to avoid harboring chlorine atoms as part of biochemistry.
  • It accounts for a large portion of extracellular fluid tonicity.
  • etc...

Thus, most inter- and extra-cellular fluid is relatively chloride ion rich, across the entire bush of life. To my knowledge there is no life, tissue or cell on this planet that does not contain chloride ions as part of its makeup by necessity. It is simply heavily constrained in evolution as a basic component. It's just not practical to obtain K+ or Na+ or Ca2+ ions without Cl-, it is so common in nature.

If you want specific examples, you can always mention its indispensable roles in neuron firing (transmission of axon potentials) and general cellular homeostasis, which is dependent on electrochemistry and osmolarity and stuff like that which is heavily influenced and dependent on the presence of a regulated concentration of solvated Cl-.

  • $\begingroup$ Ī̲ didn’t discuss abundance of elements on Earth (or elsewhere in the known universe), nor hinted that natural abundance of nutrients has something to do with the question. Please, keep your stuff to the topic. $\endgroup$ Apr 13 at 9:32
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    $\begingroup$ Neither did I, as I'm sure you will agree. I see 3 posed questions, perhaps I can restate the answers more tersely. Is chloride necessary for animals? Which cells absolutely need specifically Cl- to function? Is it mandatory? The answers are yes, all, and yes. All animals can and do suffer chloride deficiency. I think the answer can also be extended to most microbes. This does not rule out the possibility that one could engineer a synthetic life to use bromine or fluorine, but I presume you know why neither would be a good elemental substitute. They are very different physicochemical anions. $\endgroup$
    – S Pr
    Apr 13 at 9:40
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    $\begingroup$ That Wikipedia article states that so named chloride channels may conduct many different ions. An empty-worded statement about necessity of Cl⁻ may not count to qualify an answer. $\endgroup$ Apr 13 at 9:44
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    $\begingroup$ There are very many membrane channels that are partially or entirely non-selective, including anion and cation channels. I don't really understand how that is relevant, but I wish you luck on getting an answer that you are looking for! Though I would caution you against looking for answers at bio.SE where the lack of chlorine is a strained, imagined hypothetical. Along those lines, we can speculate and opine all day about silicon-based lifeforms. But that's all fantasy and science fiction until we see a proof of concept. Best of luck! $\endgroup$
    – S Pr
    Apr 13 at 9:58

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