The specific reactants that an enzyme acts on are called

substrates

Explanation:

Substrates -

It is one of the biochemical species that is involved in a biochemical reaction , that reacts with the enzyme , in order to give the product , is known as a substrate  .

In the very first step , the enzyme selectively binds with the substrate , to give an enzyme - substrate complex .

And , in the last step , complex further gives the final product of the biochemical reaction .

stratosphere, is the answer for (apex)