We do in fact feel many desires like that, e.g. the desire to remove certain unneeded materials from our bodies, and other such things. The reason you don't have a specific desire to extract ammonia is that you don't have a specific feeling for that; if you did have a specific feeling, it would be a desire specifically to extract ammonia, just like you specifically desire the actions I mentioned in the first part of this comment, and just as you can have the specific desire for sex.
I feel we are talking past each other, because reading the comment above I'm totally confused about what question you're answering...
Let me rephrase my question: if I substitute one of the parts of an AI with an inequivalent part, say a kidney with a solar cell, will its desires change or not?
If it's worth saying, but not worth its own post, then it goes here.
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