Substrate

Real World Example

Imagine trying to find the right key to fit into a lock when you have a whole bunch of keys on your keyring. Just like how only one specific key fits perfectly into a particular lock, a substrate is a specific reactant that fits into the active site of an enzyme. In this analogy, the lock represents the enzyme, the correct key is the substrate, and when they fit together properly, it allows the enzyme to perform its function, just like when the right key opens a door.