Kein takes the place of the article. Compare this to the English construction of "the citizen is no member". Just like you can't use "not" alone with a noun in English so you can't use "nicht" with nouns in German. And German unlike English lacks the "not a" construction.
If it were only so simple. It also depends on the number (plural is keine, regardless of noun gender) and the case (Nominativ, Akkusativ, Dativ, und Genitiv, which could result in keines, keinem, keinen, oder keiner as being the proper inflection).