Is it permissible to use somebody else's words (without quotations) to describe a concept once their work is cited?
No, it's not permissible.  Some minor overlap in wording can occur naturally (e.g., you couldn't possibly avoid all two-word phrases anyone has ever used before), but deliberately copying text is different.  If the copied text is special or remarkable and says something better than you could have said it, then you should attribute it to its source and not seem to be presenting it as your own words.  Citing the source nearby is not considered enough to indicate a verbatim quote.  On the other hand, if there's nothing special about the copied text, then there's no reason to copy it.  (I suppose it could save you a few minutes of work in writing your own explanation to replace these sentences, but that's not considered a good excuse.)