Substrate and reactant are essentially the same thing in the context of chemical reactions. They both refer to the molecules that participate in a chemical reaction and undergo a change.

However, the term "substrate" is more commonly used in the context of enzyme-catalyzed reactions. In this case, the substrate is the specific molecule upon which the enzyme acts.

Here's a breakdown:

* Reactant: A general term for any molecule that participates in a chemical reaction.

* Substrate: A more specific term used for the molecule that is acted upon by an enzyme.

In summary:

* All substrates are reactants, but not all reactants are substrates.

* Substrate implies a specific interaction with an enzyme.

It's important to note that the terms are often used interchangeably, and the context will usually make it clear which meaning is intended.