The word constitutive describes an essential part of a whole, especially physical makeup. Without its constitutive engine, steering wheel, and tires, you don’t really have a car; it's just a box with seats.
Just like a government may be based on its constitution, a whole is based on its constitutive parts. Something that is constitutive may also be necessary for a whole to work correctly. Participate in a 24-hour dance-a-thon, and you’ll likely find your legs are constitutive to your success. Constitutive is one of those words that has both general and scientific uses; you might see other senses of the word in both chemistry and biology books.