Here's a breakdown:

* Enzymes are biological catalysts that speed up chemical reactions in living organisms.

* Substrates are the specific molecules that enzymes bind to and act upon.

* The enzyme's active site interacts with the substrate, facilitating the chemical reaction.

Think of it like this:

* Enzyme: A lock

* Substrate: A key

* Reaction: Opening the lock

Only the right key (substrate) will fit into the lock (enzyme) and allow the lock to be opened (reaction to occur).