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Writing a story in which most alien races digest food physically, not useing any chemical stuff. Is this even possible? I know that some lizards and chickens eat rocks for similar reasons, but they've got stomach acid as well. If they are weak and need to eat all the time, it's a plus, story wise, so.

Edit because I wasn't clear enough: I know that it's not posible for there to be NO chemical reaction, what I mean is that they lack bile. I want to know if such a creature can survive, and what the effects of it's odd digestive system would be.

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    $\begingroup$ Wait, do you mind some kind of grinding or nuclear stuff? $\endgroup$ – PatJ Jan 31 '17 at 21:50
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    $\begingroup$ Chemistry is just applied physics, you know... $\endgroup$ – Mołot Jan 31 '17 at 22:07
  • $\begingroup$ Life is chemistry stuff, so no. $\endgroup$ – John Feb 1 '17 at 0:22
  • $\begingroup$ This seems perfectly on-topic and clear to me now that it has been edited. $\endgroup$ – James Feb 1 '17 at 21:12
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Digestion is just one part of a great chain of steps that convey energy bearing material to where the creature needs it most. If you want to have an exotic digestion like this, you'd need an equally exotic way of conveying the material elsewhere in the body.

You might have a system like this if a creature was originally made up of a bunch of smaller creatures, like the Portuguese Man o War. The bigger creature might physically grind up foodstuffs, and then offer it to the other parts of the body to be chemically processed.

If you don't want chemical processing at all, you may need to start the idea of metabolism over from scratch. Our cells are constantly doing chemical processing, so that'd need to be rewritten as well as the digestive tract.

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  • $\begingroup$ Excellent explanation Cort. $\endgroup$ – James Feb 1 '17 at 21:11
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Well, ok but you need nano-bots all the way down.

The way nano-tech works may be not chemistry by definition, but the different in fact is only because of the definition. If you use molecular assemblers, what you have managed is to replace the molecular assemblers made of living organic molecules with those designed by people or some other intelligence.

Mechanical processes that grind so fine as to result in molecular fragments suitable for cellular use would still need to be sorted, distributed, assembled, etc. just as much as biology achieves.

Living cells are finely tuned living machines. Biology pretty much requires this as random processes would destroy biological viability immediately.

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  • $\begingroup$ the bots/molecular assembler are creating chemical changes in the molecules, It is still chemistry. $\endgroup$ – John Feb 1 '17 at 0:23
  • $\begingroup$ @John which is still applied physics. Since it is impossible to digest without breaking bonds, it's mainly a matter of how one does it - with man made machines or with nature made machines. As Gary pointed out, it's the same thing, it's just defined by humans to be different. $\endgroup$ – Mrkvička Feb 1 '17 at 7:03
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If you want only to get rid of digestive secretion (stomach acid, bile etc.), that is relatively easy. Your creature would grind down everything mechanically, and then every bit will be absorbed into bloodstream. But after that there is no choice but to start chemical metabolism. This mechanism is problematic from the scientific point of view, but not impossible. Simple organisms have no digestive organs, and they are just fine with that.

Eliminating chemical metabolism completely is probably not possible for biological beings.

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The only way I can think of to do this would involve a predatory organism that absorbs others. Rather than breaking them down entirely it would take their cells and subvert them, essentially acting like a virus breaking them down and converting them into new cells of the predator.

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The aliens from Forward’s Rocheworld were blobs that “changed the alligence” of dumbbell-shaped basic cells it absorbed, rather than breaking them down into simple molecular building blocks.

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Reading this question I think of birds that contain a gizzard, such as ducks, which contains sand that they have swallowed to assist them in grinding the food (which they swallow whole) so that it can be absorbed into the body.

Expanding upon this, a previous commenter noted that the entire point of having the food broken down is for other digestive tissues and fluids to break the food down to a molecular level so that nutrients can be absorbed by. Trees do something similar, however since they do not have teeth to chew (at least here on earth) they rely on the process of osmosis, allowing water to do the work for them. Water, by the way, is a very corrosive agent.

Even assuming there is no water on your planet (silica based life forms) other "things" could be ingested and held in a gizzard-like organ to break down, or corrode, such as above hinted nuclear isotopes, sacks of bacteria, vegetative symbiosis, etc.

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