What chemical process that can be defined as "metabolic" is the simplest found in any organism, and are there even simpler theoretical metabolic pathways, that are simpler than any found in known organisms?

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    $\begingroup$ How would one objectively identify a metabolic pathway as "simple"? What are the criteria for simplicity? $\endgroup$
    – Bryan Krause
    Commented Sep 15, 2023 at 16:35
  • $\begingroup$ We don’t do definitions much in biology. Certainly we define our terms in any discussion, but generally in our own terms, rather than by appeal to some authority. “Pathway” is a synthetic human concept — we, not Nature —define the first step and the last step, draw a ring round connected reactions of our choice, and call it a pathway. Please go back and finish reading the Tour to find out what questions are acceptable here, then the Help on asking — particularly the need to provide context. What is the biological problem you are facing. And, as Bryan requested, define your terms. $\endgroup$
    – David
    Commented Sep 15, 2023 at 18:32

1 Answer 1


Trivially: the protonation/deprotonation of water.

This is a process that happens spontaneously without any other inputs or outputs or catalysis under physiological conditions.

It feeds into basically every other biochemical process, and cannot be reduced into any of them.

I really agree with the commenters, so this answer goes to show that your definition is more about how you choose to draw the box around your focal point than it is about how biology works.

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    $\begingroup$ I appreciate this answer a lot. While I understand the problem with my intitial framing, I think you were able to see through the inexactitude of my framing and get to the real point of my question. Thank you all for the input on how I can better express and think about my questions, and thank you for trying to see the real question beneath my shortcomings in asking it clearly. $\endgroup$ Commented Sep 22, 2023 at 13:08
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    $\begingroup$ @blacktopshaman don't be too harsh on yourself. this stuff is hard. $\endgroup$ Commented Oct 18, 2023 at 19:07
  • $\begingroup$ This will not wash. The protonation of water is not a metabolic reaction, much less a metabolic pathway. First, metabolic reactions are generally regarded (any hit from a Google search or text book of biochemistry) as enzyme-catalysed reactions reacting on metabolites — chemical intermediates in energy generations, anabolism or elimination of waste products. Water participates in such reactions but is hardly regarded as a metabolite and its ionization does not require catalysis. Similarly, definitions of metabolic pathways refer to reactions in the plural, generally using the word "sequence". $\endgroup$
    – David
    Commented Oct 18, 2023 at 22:57

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