The molecules on which enzymes act are known as ...........

Explanation

**Step-by-Step Solution:**

1. **Understanding Enzymes**: Enzymes are biological catalysts that speed up chemical reactions in living organisms. They have specific regions called active sites where substrates bind.

2. **Identifying the Role of Substrates**: The molecules that enzymes act upon are known as substrates. These substrates fit into the active site of the enzyme.

3. **Defining Substrate**: A substrate can be defined as a molecule that is acted upon by an enzyme. It is essential for the enzyme to function properly.

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