direct2d or steam question.

I have a expansion i want to purchase through a downloadble content site like steam or d2d. However, my question is, if i buy it through steam, will the expansion still work if i dont have the original game through steam? and for d2d, would it work if i bought it and didnt have the original game purchased of their system, but still had it on my PC?

depends on the expansion, usually it says on the page if you need the steam version. for example the spore galatic expansion requires the steam version of spore.nutcrackr