Here's a breakdown:

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

* Substrates are the molecules that enzymes act upon.

Think of it like this:

* Enzyme: A lock

* Substrate: A key

The enzyme (lock) has a specific shape that fits only a specific substrate (key). When the substrate binds to the enzyme, the enzyme facilitates a chemical reaction, changing the substrate into a new product.

For example, the enzyme lactase breaks down the substrate lactose (a sugar found in milk).

Let me know if you'd like to explore specific enzyme-substrate examples!