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So a human digestive system is generally acidic, especially at the stomach part, when it goes down to even 1.5 pH. Similar things must be happening in other mammals and to an extent, probably in all animals. Is there however some hard evolutionary, biological, chemical or any other block stopping the digestive system from being alkali and having stomach juices to be instead be pH 13 and still do their job?

Of course it doesn't have to be that extreme, my question comes down to having a generally alkaline system.

Also, it's set in a context of an alien biosphere, so the only limitation really is the fact of the life being eukaryotic and carbon-based, everything else is on the table

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    $\begingroup$ Our small intestine - the longest part, is alkali, so is our mouth, so is poop in healthy individuals. The stomach is the exception. Can you clarify your criteria of " having a generally alkali system" that we don't already fulfil? $\endgroup$ Dec 1, 2023 at 13:25
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    $\begingroup$ @Escapeddentalpatient. The alkali parts that you mentioned are, as far as I'm aware, barely above 7 pH, if at all, not really exceeding 8.5, which is for example a pH of a still drinkable water. What I'm looking is the matter of whether chemistry and biology would allow an organism which has an "inverted" digestive system, with strong basic juices in one place surrounded by generally neutral or slightly acidic ones (or alkali, that would still fit) $\endgroup$
    – Yulian
    Dec 1, 2023 at 15:39
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    $\begingroup$ For the record, it's more efficient on this site to make your choice and ask us to help you rationalize it rather than asking what choice you should make. In other words, rather than asking, "could alkaline digestive systems work?" it's easier to ask "My aliens have alkaline digestive systems, can science today be used to rationalize this?" They're not the same thing: one is asking if it can exist in Real Life and the other is simply building a world rule for an imaginary world (which is what we do). $\endgroup$
    – JBH
    Dec 1, 2023 at 17:30
  • $\begingroup$ @JBH In that case, should I rebuild the post, or is it just for future notice? $\endgroup$
    – Yulian
    Dec 1, 2023 at 17:33
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    $\begingroup$ @Yulian Nah... keep it for future reference. $\endgroup$
    – JBH
    Dec 1, 2023 at 18:03

2 Answers 2

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Your stomach is acid, because that's the best sort of environment for the functioning of pepsin, a protein-chopping enzyme. It is very definitely not the only such enzyme in your body, or even your digestive system... there are other similar things that work best at different pH and appear elswhere. Trypsin appears in pancreatic juice which is alkaline and gets secreted into the top of the small intestine, for example.

You ask if there's a "hard evolutionary block", which depends on exactly what you mean. Things could probably have gone either way in the distant past, but it would be difficult to evolve an alkaline stomach from the starting point of an acidic one. Could you have a non-acidic stomach if you started from scratch? sure, nothing in chemistry or biology requires it be acid. It just happened to work out that way. It may be there are some advantages, but they needn't be overwhelming or even existent in your fictional setting... don't go into too much detail and you'll be fine.

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  • $\begingroup$ That's about what I was looking for with just one clarification question: Would biology of a potential pray or vegetation be in any way different than excepted in an environment where acids do digestion? $\endgroup$
    – Yulian
    Dec 1, 2023 at 15:41
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    $\begingroup$ @Yulian acids don't do digestion, enzymes do digestion. Enzymes have ideal operating conditions... specific pH and temperature ranges, and your stomach acid is there to help pepsin do the best job... not to dissolve what you eat! That your world has things with different operating conditions doesn't change the fact that it is still enzymes doing the digesting, so in theory nothing else need change at all. $\endgroup$ Dec 1, 2023 at 16:35
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    $\begingroup$ @Yulian there'd be differences in what the intermediate stages of digested food look like, and some things will end up being digested differently due to reactions of the base with whatever they're made of, but that can be quietly filed away under "details", I suspect. Maybe you'd get soapier poop and foamy farts, but only if you felt that added to your setting... $\endgroup$ Dec 1, 2023 at 16:39
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Just a wild guess, can we assume if start consuming acid say of very low pH as only food then maybe our stomach will adjust to produce alkaline to react with acid to produce certain salts which in turn help the organism to survive.

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