‘lock and key’ theory is used to explain how an enzyme works

The 'lock and key' theory is a model for how enzymes work. Here's a step-by-step explanation:

1. Enzymes are proteins that speed up chemical reactions in the body. They do this by lowering the energy needed for the reaction to occur.

2. The 'lock and key' theory suggests that each enzyme has a specific shape, like a lock, and can only be activated by a substance with the right shape to fit into it, like a key.

3. The substance that fits into the enzyme is called the substrate. The place on the enzyme where the substrate fits is called the active site.

4. When the substrate enters the active site, it forms a temporary structure called an enzyme-substrate complex. This is like the lock being opened by the key.

5. While the substrate is attached to the enzyme, the reaction takes place quickly and easily. The substrate is converted into one or more products.

6. Once the reaction is complete, the products leave the active site, freeing it up for another substrate molecule to enter.

7. The enzyme is not changed or used up in the reaction, so it can continue to catalyse the same reaction with other substrate molecules.

This 'lock and key' model helps to explain why enzymes are so specific in their action - each one can only catalyse a specific reaction because only a specific substrate (or substrates) can fit into its active site.