What substance takes part in an enzymatic reaction, but is unchanged by the reaction?

The answer to this question would be: enzyme 

Enzyme is a kind of catalyst that will take part in the chemical reaction but it won't be reacted. Enzyme only helps by reducing the energy needed to start the reaction.
Theoretically, the enzyme is not mandatory needed but if you have an enzyme the reaction will be easier, thus increasing reaction speed. But in most case, the reaction might be too slow without enzyme, making enzyme vital in living organism.

Both models are effective because they show the valence electrons, and these valence electrons indicate the reactivity of an element.